Any mature solution for solar Air Conditioning?

[7acres]

We are finishing our upstairs and are financing it by refinancing our mortgage and getting some cash out of the deal. Our heat pump is getting up there in age and will need to be replaced in a couple years. The wife and I would be thrilled to take the AC off the grid and off the power bill. From some minimal research I've done I estimated we could upgrade to solar powered AC for around $15,000. We've got the re-fi approved from the bank so now I'm trying to verify in more detail how much the AC install will cost and exactly what vendor and configuration we will need.

After more research I'm starting to wonder whether companies providing solutions today are too immature. First of all there don't seem to be any solutions based on a central unit. They all seem to be re-packaging their solutions for RV, Bus, Cab, Forklift, Crane, etc. air conditioning. Nothing wrong with that as those were designed to run off of DC. But it also means we will need some combination of about 5 units to cool the whole house. That and I worry that the blowers kicking on and off will end up being unacceptably noisy.

I know I'm not the first one to propose such a project. But I thought I'd stop in here for a sanity check. What's the smart thing to do here? Also, did I miss any companies selling a central DC powered unit designed around solar power?

 

[iamlucky13]

In my opinion, the smart thing to do is stay on-grid. The added costs of batteries and over-capacity for extended low production periods makes it very unlikely to be economical. I think few people truly appreciate how high the value of net metering is.

If you really do want to go off-grid, I'd probably stick with an alternating current AC system (AC AC ). You will have some slightly reduced efficiency for the inverter, but there are far more and far more refined options available for AC HVAC than for large DC HVAC. I'd bet the availability of high-efficiency variable speed AC HVAC systems, especially some of the newest ductless systems, more than makes up for the what you'll lose in your inverter.

It's not that you're the first to propose such a concept. It's that such a small number of people currently are interested in large DC HVAC systems that there's little investment being made in developing them.

One more thing to consider if you do go off-grid is having a generator to supplement your solar production. This will reduce the degree you need to oversize your panels and batteries to get through any extended low-production periods. I'm sure you'd prefer not to ever burn fossil fuel when you've got solar panels, but theoretically you'd only be using the generator a few days a year if you see your batteries straying below the preferred charge levels.

 

[EatenByLimestone]

You can add a cupola. Its low tech, but is a solar poiwered whole house air conditioner.

 

[peakbagger]

This is going to sound familiar

You are asking several questions that are interrelated and based on a fundamental premise that many people don't understand. Barring a major battery discovery or your own private hydroelectric generator with substantial storage pond., the usual cost for off grid power over the long run is 5 to 7 times the cost of on grid power. If you are willing to trade ongoing extra cost for philosophy I still expect that an AC coupled solution is the way to go as there is little demand for DC powered equipment. The reason there is little demand for DC equipment is its more costly to build, low voltage means lots of copper and copper is expensive.

If you just plain want to go off the grid, forgetting the initial and ongoing cost, the first thing to know is most off gridders don't run AC off their batteries, they can but it means the entire system is more expensive. You are far better off spending the money on super insulation and shading techniques to reduce the AC load.

The far better approach is look at what the structure is for net metering from your utility and then build an on grid system that will take advantage of it.

There is no big new cooling technology the biggest improvement for cooling is refinements to current technology to make them more efficient. As much as folks like central cooling, the best efficiency least cost are mini split type units that only cool the occupied spaces.

 

[7acres]

Very good feedback. Kind of confirmed what I was afraid of.

I can see now that sticking with an AC powered unit is going to be the way to go. Back to the drawing board.

 

[iamlucky13]

You can add a cupola. Its low tech, but is a solar poiwered whole house air conditioner.

I apologize if there's some specific product called a cupola you're referring to that I'm not aware of, but an ordinary vented cupola is a passive ventilation device, not an air conditioner. It draws fresh air in without fans or need for a breeze, but does not cool or dehumidify it. It definitely can improve comfort compared to having no air conditioning, and do so without needing any power, but especially in the often-humid SE, won't offer nearly the same level of comfort as air conditioning.

 

[EatenByLimestone]

It works off heat rising, like our chimneys. So solar heat does power it.

When I lived in Houston, the open air bus stops used large venturis to cool them. It may not work as well as a regular air conditioner, but it does work.

It just occured to me that ammonia based refrigeration could be easily powered bythe sun too.

 

[Where2]

If you have net metering, just stick a PV array (1.33kW times your A/C tonnage) on the house, and poof, there goes your A/C expense. The 4kW array I have basically runs my 3-ton central A/C unit for free in my South Florida climate where I need to run the A/C 10 months out of the year. Just the other day, my neighbor was asking me how high my electric bill had been this summer, complaining that his bill had been above $300 for more than one month this summer. My bills: August: $63.87; July: $63.58; June: $51.54; May: $37.86; April $27.99. Several of my coworkers complained to me this summer about their electric bills in the $300-$400 range. (Running total for the last 12 months, my electric bill was $636.43, for an all electric house where we use hot water, a dishwasher and an electric dryer!)

The really amazing part is that my neighbor entirely rebuilt his house in 2008 (concrete block from the slab up). My house is 1962 architecture with a concrete block first story and 2x4 stick built second story. I added new windows and 2" polyiso in the stick walls in 2010. The second floor ceilings have R25 batts, but minimal air sealing. My first story windows are at least 20 years old, and probably average close to 40 years old (I know two are original). My 4kW PV array generates ~6MWh per year, and basically runs my 13SEER 18 year old central A/C for free.

 

[EatenByLimestone]

If you have net metering, just stick a PV array (1.33kW times your A/C tonnage) on the house, and poof, there goes your A/C expense. The 4kW array I have basically runs my 3-ton central A/C unit for free in my South Florida climate where I need to run the A/C 10 months out of the year. Just the other day, my neighbor was asking me how high my electric bill had been this summer, complaining that his bill had been above $300 for more than one month this summer. My bills: August: $63.87; July: $63.58; June: $51.54; May: $37.86; April $27.99. Several of my coworkers complained to me this summer about their electric bills in the $300-$400 range. (Running total for the last 12 months, my electric bill was $636.43, for an all electric house where we use hot water, a dishwasher and an electric dryer!)

The really amazing part is that my neighbor entirely rebuilt his house in 2008 (concrete block from the slab up). My house is 1962 architecture with a concrete block first story and 2x4 stick built second story. I added new windows and 2" polyiso in the stick walls in 2010. The second floor ceilings have R25 batts, but minimal air sealing. My first story windows are at least 20 years old, and probably average close to 40 years old (I know two are original). My 4kW PV array generates ~6MWh per year, and basically runs my 13SEER 18 year old central A/C for free.

And what did the monthly savings cost? What is the opportunity cost?

 

[7acres]

If you have net metering, just stick a PV array (1.33kW times your A/C tonnage) on the house, and poof, there goes your A/C expense. The 4kW array I have basically runs my 3-ton central A/C unit for free in my South Florida climate where I need to run the A/C 10 months out of the year. Just the other day, my neighbor was asking me how high my electric bill had been this summer, complaining that his bill had been above $300 for more than one month this summer. My bills: August: $63.87; July: $63.58; June: $51.54; May: $37.86; April $27.99. Several of my coworkers complained to me this summer about their electric bills in the $300-$400 range. (Running total for the last 12 months, my electric bill was $636.43, for an all electric house where we use hot water, a dishwasher and an electric dryer!)

The really amazing part is that my neighbor entirely rebuilt his house in 2008 (concrete block from the slab up). My house is 1962 architecture with a concrete block first story and 2x4 stick built second story. I added new windows and 2" polyiso in the stick walls in 2010. The second floor ceilings have R25 batts, but minimal air sealing. My first story windows are at least 20 years old, and probably average close to 40 years old (I know two are original). My 4kW PV array generates ~6MWh per year, and basically runs my 13SEER 18 year old central A/C for free.

That is awesome. Thats the dream right there. It is such a huge up front expense. But ultimately it lowers your overall cost of living expense. That's why we want solar so bad.

 

[peakbagger]

Solar is not that much of "huge expense". A 4 KW array is going to run around $3 to 3.50 an installed watt for a total of $14,000, subtract the federal rebate of $4,200 and the remainder is $9,800. There are most likely are state incentives to drop the cost even further. I would definitely suggest checking this database fro your state it has all the incentives http://www.dsireusa.org/. The prior poster didn't give enough info to really analyze his situation but 4KW looks small for an older inefficient house with full time AC. I would expect a 6 to 8 KW array for typical a lot of the 2000 to 2500 square foot homes being built these days.What you really need to do is figure out you past years electrical usage and size a system to match it.

SInce you didn't say what state you were in, I picked Raleigh NC for spot to run PV Watts (an easy to use solar calculator). I used the default power rate of $0.10 per KW delivered, that comes out to $583 per year electrical savings. That's a 6% simple rate of return. 6% is not bad considering that its a fairly secure investment (you are selling power to yourself). Odds are an equivalent investment of similar safety is at best returning 1%. That's worse case, many states have incentives and possibly low interest loans. which would drive down the installed cost and increase the payback. They also may have rebates on high efficiency heating and cooling systems. The state may also be in the SREC market and then you have another sources of yearly revenue to offset the cost possibly reducing the payback by a couple of years.

One option that we didn't discuss is going ground source geothermal for cooling and heating. Depending on what you have underground it can be competitive if you already have an air handling system and your house isn't on rock. It too is a large upfront expense which may be offset by incentives..

The one thing I do have to caution you is you are potentially a ripe target for a slick salesman selling you on the concept of leasing a PV system or some other variant where they rent your roof and sell you power at a claimed rate lower than what your utility is charging. Its tempting and for a few folks might make sense, but for the vast majority of folks they are far better off buying as the leasing company will "skim the crème" of the payback. There are also long term issues with a leased system. The lease is secured by lien on the house, if you go to sell, it will complicate the transaction and the leasing firm may elect to force you to buy out the lease at a high cost before selling. Generally they contractually have the right to review the credit rating of the potential buyer and due to the way they finance the systems, its to their incentive to force the current buyer to buy it out. (the scam is they install the system at a very inflated cost (that they pay to themselves) and then take a higher rebate from the government, they then write off the high installation cost over the life of the project. If an owner wants a buyout, they sell it at the inflated cost they have on the books.)

 

[7acres]

Solar is not that much of "huge expense". A 4 KW array is going to run around $3 to 3.50 an installed watt for a total of $14,000, subtract the federal rebate of $4,200 and the remainder is $9,800. There are most likely are state incentives to drop the cost even further. I would definitely suggest checking this database fro your state it has all the incentives http://www.dsireusa.org/. The prior poster didn't give enough info to really analyze his situation but 4KW looks small for an older inefficient house with full time AC. I would expect a 6 to 8 KW array for typical a lot of the 2000 to 2500 square foot homes being built these days.What you really need to do is figure out you past years electrical usage and size a system to match it.

SInce you didn't say what state you were in, I picked Raleigh NC for spot to run PV Watts (an easy to use solar calculator). I used the default power rate of $0.10 per KW delivered, that comes out to $583 per year electrical savings. That's a 6% simple rate of return. 6% is not bad considering that its a fairly secure investment (you are selling power to yourself). Odds are an equivalent investment of similar safety is at best returning 1%. That's worse case, many states have incentives and possibly low interest loans. which would drive down the installed cost and increase the payback. They also may have rebates on high efficiency heating and cooling systems. The state may also be in the SREC market and then you have another sources of yearly revenue to offset the cost possibly reducing the payback by a couple of years.

One option that we didn't discuss is going ground source geothermal for cooling and heating. Depending on what you have underground it can be competitive if you already have an air handling system and your house isn't on rock. It too is a large upfront expense which may be offset by incentives..

The one thing I do have to caution you is you are potentially a ripe target for a slick salesman selling you on the concept of leasing a PV system or some other variant where they rent your roof and sell you power at a claimed rate lower than what your utility is charging. Its tempting and for a few folks might make sense, but for the vast majority of folks they are far better off buying as the leasing company will "skim the crème" of the payback. There are also long term issues with a leased system. The lease is secured by lien on the house, if you go to sell, it will complicate the transaction and the leasing firm may elect to force you to buy out the lease at a high cost before selling. Generally they contractually have the right to review the credit rating of the potential buyer and due to the way they finance the systems, its to their incentive to force the current buyer to buy it out. (the scam is they install the system at a very inflated cost (that they pay to themselves) and then take a higher rebate from the government, they then write off the high installation cost over the life of the project. If an owner wants a buyout, they sell it at the inflated cost they have on the books.)

 

[7acres]

Well, I think we just went off the deep end (in a possibly good way). Peakbagger's assessment that it doesn't have to cost a fortune looks to be what we're seeing now. We just got off the phone with a local solar company. It looks like there are a lot of incentives in my area, Upstate SC. A rough quote for an 8kw grid tied array with net metering would be $27,000 up front. But the power company, Duke Energy, would pay us about $8k (which gets taxed) after just 7 weeks of the system being installed. On top of that we ultimately get back about $8k in federal rebates and another $6,500 in rebates from the state. Basically the net cost after all the rebates we're looking at $5,750. Way cheaper than we thought possible. Of course this is all based on a phone call.

Goodness, I feel so ignorant about all of this. Ignorant but optimistic. We've got them scheduled for an on-site assessment next Wednesday. I continue to appreciate any words of wisdom as my wife and I are really interested in this now.

 

[iamlucky13]

I have a hard time believing an 8 kW system could be had for that little. That's a lot of subsidies. I guess the advice to read the fine print always applies, but if they're really offering a large system like that for so cheap, and your roof can support it (something they'll check in the onsite assessment), then this is a very easy decision to make.

Just the other day, my neighbor was asking me how high my electric bill had been this summer, complaining that his bill had been above $300 for more than one month this summer.

Weird. Does he keep his windows open when running the AC? Since a 4 kW array should make 700-900 kWh per month in the Florida summer, he's clearly using a lot more energy than you unless your electricity costs a lot more than the national average. Playing with a few numbers and making a few assumptions, that seems like a 4 ton AC unit running 16 hours a day, which sounds crazy to me.

 

[peakbagger]

Sounds like you are in a great state for solar, do it sooner than later as there is an overall pull back on incentives for solar in some states (like Arizona and Ohio ) due to lobbying by the Koch brothers. Most states grandfather existing installations if they reduce the incentives.

A minor item when you are totaling up the savings is the federal rebate is on net cost, you need to subtract all the other rebates and incentives and then take 30% off. More than a few folks add in the cost to replace the roofing under the shingles if its deteriorated, it up to you and your tax adviser on this one You want to try to get new roof user the panels as its bear to have to remove them to replace the roof later on.

Ideally you want to make the energy savings first and then size the array to your future use but most folks just use their current usage. 8 KW is a bit large, you may need to do something called a line sized tap on your power panel but depends on how large. This is not a deal breaker just something that may need to be looked at. If you have 200 AMP panel you may not. I replied to thread on preparation for solar in the green room that might be worth reading.

Definitely ask if there is a SREC market in the state and if so how do you sign up. You basically sell the right to "brag" you are being green in exchange for a check every year. Could be $500 or could be $5,000 depends on the market. It may require extra paperwork and best to discuss up front who does the paperwork.

Ultimately if you do not understand anything do not sign it. Ask questions and get a couple of contractors to make proposals after you have checked their references.

 

[7acres]

Sounds like you are in a great state for solar, do it sooner than later as there is an overall pull back on incentives for solar in some states (like Arizona and Ohio ) due to lobbying by the Koch brothers. Most states grandfather existing installations if they reduce the incentives.

A minor item when you are totaling up the savings is the federal rebate is on net cost, you need to subtract all the other rebates and incentives and then take 30% off. More than a few folks add in the cost to replace the roofing under the shingles if its deteriorated, it up to you and your tax adviser on this one You want to try to get new roof user the panels as its bear to have to remove them to replace the roof later on.

Ideally you want to make the energy savings first and then size the array to your future use but most folks just use their current usage. 8 KW is a bit large, you may need to do something called a line sized tap on your power panel but depends on how large. This is not a deal breaker just something that may need to be looked at. If you have 200 AMP panel you may not. I replied to thread on preparation for solar in the green room that might be worth reading.

Definitely ask if there is a SREC market in the state and if so how do you sign up. You basically sell the right to "brag" you are being green in exchange for a check every year. Could be $500 or could be $5,000 depends on the market. It may require extra paperwork and best to discuss up front who does the paperwork.

Ultimately if you do not understand anything do not sign it. Ask questions and get a couple of contractors to make proposals after you have checked their references.

Fantastic advice. I'm all ears. The roof replacement is definitely something to look into and decide on first. The shingles are in decent shape.

As far as making savings first we've thought of that but the timing might be a little off. Since moving here in 2010 one of the first things I did was install a Generac 17kw whole house generator that runs off propane. As part of this overall project we want to upgrade the stove and water heater to run off propane. But I'm not sure if the timing is going to let us know exactly what that will save us before the solar install though. I'm also a little sad wondering if the generator will be made obsolete by the solar install.

We do have a 200 AMP panel connected to utility power (and a 60AMP remote transfer panel for the generator). From a bit of research SC doesn't appear to have a proper SREC market.

I'll look up your post on solar preparation and give it a read.

 

[iamlucky13]

Propane is good for cooking and a backup generator. It's about on par for cost with electricity as a heat source, so I don't see much incentive to switch to a propane water heater, unless you want faster recovery time. Especially in your climate, I'd suggest considering a heat pump water heater if you want to reduce water heating costs.

Unfortunately, most solar PV systems do not replace a backup generator. Normal solar systems depend on the grid to regulate voltage and frequency. Doing so for backup requires a grid-interactive inverter that can run either way (modest extra cost), and should be paired with a battery bank to absorb excess power and provide surge power for high loads, in addition to keeping your lights on overnight (large extra cost).

You can ask the installer about the option of backup power, but I don't think all of them offer the option. Depending on your expectations for how long you want to be able to run, whether you're willing to cut your power use during that time, and the resulting costs, you may very well prefer to just keep your generator, since it's already paid for and has a very high capacity.

 

[7acres]

Propane is good for cooking and a backup generator. It's about on par for cost with electricity as a heat source, so I don't see much incentive to switch to a propane water heater, unless you want faster recovery time. Especially in your climate, I'd suggest considering a heat pump water heater if you want to reduce water heating costs.

Unfortunately, most solar PV systems do not replace a backup generator. Normal solar systems depend on the grid to regulate voltage and frequency. Doing so for backup requires a grid-interactive inverter that can run either way (modest extra cost), and should be paired with a battery bank to absorb excess power and provide surge power for high loads, in addition to keeping your lights on overnight (large extra cost).

You can ask the installer about the option of backup power, but I don't think all of them offer the option. Depending on your expectations for how long you want to be able to run, whether you're willing to cut your power use during that time, and the resulting costs, you may very well prefer to just keep your generator, since it's already paid for and has a very high capacity.

I just did a little reading up on the grid-interactive inverter.

Currently when the power goes down the generator's remote transfer panel fires up the generator and transfers the load directly to it with no batteries in the mix.

Does someone make a grid-interactive inverter that don't necessarily need a battery bank?

 

[peakbagger]

I think I I replied in the other thread that Outback may make a inverter/charge controller that does what you need and it may (no guarantees have the capability). You are getting into fairly sophisticated territory once you start messing with grid interactive inverters as they have to be designed to kick off when the utility goes away. I would suspect its far easier to install a standard grid tied inverter and then let it shut off if the grid goes down. The UL requirements for grid tied inverters are once they trip, they need a minimum of 5 minutes of clean continuous grid power before they will synch up again. In theory your auto generator start could be programmed to swap an automated transfer switch and start up in that wait period. The grid tied inverter would need to be on the utility side of the transfer switch, otherwise it may never synch to the grid as most generators put out power that is too dirty to fool the grid inverter. I expect your generator has a minimum run time that it runs before it shuts down so if the grid comes back I expect that the grid inverter will already be synched to the grid so that when you shut the generator down the transfer back to utility is pretty quick. The problem is if you go to a hybrid design the number of contractors qualified to install and support a system goes way down and the prices go up. The $3 to $3.5 a watt may go up to $5 a watt. They can do grid tie cheap as the buy containers of panels, racking and inverters and every installation is standard. Once you go with "weird" they have to design the system, program it and deal with the inevitable teething issues. Most generators I am aware of are auto transfer on loss of utility but manual transfer back as otherwise the generator might switch back when you don't want it.

Generally unless you are using all the waste heat the cost to generate power with propane is higher than the cost of utility power. I design large systems that do that but it gets real expensive down below about 1 MW of power.

 

[Where2]

And what did the monthly savings cost? What is the opportunity cost?

Peak nailed the number pretty well below. I have no intentions to sell the house before the PV array has more than paid for itself. I've been here 18 years already. The 13 SEER central A/C unit I had installed in 1998 has certainly saved me plenty.

Solar is not that much of "huge expense". A 4 KW array is going to run around $3 to 3.50 an installed watt for a total of $14,000, subtract the federal rebate of $4,200 and the remainder is $9,800. There are most likely are state incentives to drop the cost even further. I would definitely suggest checking this database fro your state it has all the incentives http://www.dsireusa.org/. The prior poster didn't give enough info to really analyze his situation but 4KW looks small for an older inefficient house with full time AC. I would expect a 6 to 8 KW array for typical a lot of the 2000 to 2500 square foot homes being built these days. What you really need to do is figure out you past years electrical usage and size a system to match it.

My 4kW array was never intended to zero my power bill, 4.4kW was literally all I could fit on the second story facet of my roof that faces south. It just conveniently generates enough energy to run the central A/C which was what the OP originally was proposing to use solar for. Certainly, if the OP can get an installed, no strings attached, price of $5,750 for an 8kW array, it beats his original concept of a $15k solar A/C option by leaps and bounds because you can use things like electric dryers, heat pump water heaters and plug-in vehicles with a grid-tied PV house.

The generator will NOT be made obsolete by the PV array. I still keep a 5,500W unit in my garage for hurricane season, because hurricanes tend to knock out my power for 7-10 days after a storm (I'm 2 miles from the ocean). When the grid is down, my PV setup is UL 1741 compliant, and goes down with the grid protecting the linemen who come from as far away as Canada to fix the grid. I think what you will find as you look more closely into the PV concept is that hooking your water heater and stove to propane may cost you more in the long run than the free sunshine that falls on your house costs. Shortly after my PV array went up, the local gas company offered to bring natural gas down our street past our house. When I did the math, it made no sense to add a minimum $20/mo gas bill to my expenses and take my electric dryer, electric water heater, and electric stove off electric power at $0.11/kWh. The purchase costs of changing over relatively new appliances while adding another bill to my monthly expenses to lower the electric bill didn't lower my ultimate cost of living. If you don't presently own it, you may wish to inquire with your propane supplier about the cost to own your propane tank. One of my neighbors owns their tank, another pays a monthly fee to the local gas company to meter read, and automatically refuel the gas company's propane tank.

I'm already working on the design of the PV system that will go on our "summer home". At 45.8°N, the 4.6kW array will generate ~5MWh per year. That's plenty of energy I can use to feed an electric dryer, electric oven, heat pump water heater, and one or two mini-splits to heat and cool the house. As others have mentioned, conservation is cheaper than oversizing a PV array. I'm already working on doubling the thickness of the 2x4 walls in the summer home to add more insulation. I still saw snow falling twice in May when I was there. Not needing to fire up the oil furnace to heat the place or heat water will be a great cost savings, even at the bargain $1.969/gallon I paid to refill the oil tank this year.

 

[peakbagger]

My 3 arrays total 4.6 KW. I generate all my power plus have enough surplus to heat my smaller but fairly energy efficient home in the shoulder seasons in northern NH with a mini split. Last winter I burned about 2.5 cord in addition to a mini split. I have an electric stove and oven. I also have a solar thermal system that takes care of the hot water for about 7 months a year and then turns into a preheater. I wouldn't recommend solar thermal for anyone now that PV panels are so cheap, far better off putting in a few extra panels and a Heat pump hot water heater.

The one big caveat is that I have a good net metering plan available in NH, I can build up a surplus and let it keep building up with no yearly reset. The only annoying downside is the state of NH charges a sales tax on all the power I am buying back from myself. Its minimal but still bugs me. Other states don't have it as good for net metering, many have a yearly reset where the surplus is taken away or paid off at some very low rate. In these states it may not make sense to oversize a system to build up big surplus. That's why its important to design the system around the net metering rules in your area.

I do have a generator, I have never used it to replace utility power. I don't lose it very often and when I do its not for very long. I think its important to have one. Its a heck of lot cheaper than paying for batteries.

 

[Highbeam]

After more research I'm starting to wonder whether companies providing solutions today are too immature. First of all there don't seem to be any solutions based on a central unit. They all seem to be re-packaging their solutions for RV, Bus, Cab, Forklift, Crane, etc. air conditioning. Nothing wrong with that as those were designed to run off of DC.

Not sure about the others but an RV air conditioner runs off of 120 volt AC power and not DC as you wrote. I've spent many many hours running a genset to get that 120 volt power to cool off my RV. The standard units are 13.5k-15k btu and require a 3000 watt genset to run with a lot of that being startup current.

 

[7acres]

My 3 arrays total 4.6 KW. I generate all my power plus have enough surplus to heat my smaller but fairly energy efficient home in the shoulder seasons in northern NH with a mini split. Last winter I burned about 2.5 cord in addition to a mini split. I have an electric stove and oven. I also have a solar thermal system that takes care of the hot water for about 7 months a year and then turns into a preheater. I wouldn't recommend solar thermal for anyone now that PV panels are so cheap, far better off putting in a few extra panels and a Heat pump hot water heater.

The one big caveat is that I have a good net metering plan available in NH, I can build up a surplus and let it keep building up with no yearly reset. The only annoying downside is the state of NH charges a sales tax on all the power I am buying back from myself. Its minimal but still bugs me. Other states don't have it as good for net metering, many have a yearly reset where the surplus is taken away or paid off at some very low rate. In these states it may not make sense to oversize a system to build up big surplus. That's why its important to design the system around the net metering rules in your area.

I do have a generator, I have never used it to replace utility power. I don't lose it very often and when I do its not for very long. I think its important to have one. Its heck of lot cheaper than paying for batteries.

I think what is otherwise a pretty easy decision for us will get messy real fast if we try to integrate the generator into this.

 

[peakbagger]

I like experimenting but when it comes to an investment like this, I really prefer to stick to tried and true proven methods and that's a straight grid tie. The only place where you get to decide is go with a couple of 4 KW SMA string inverters with the Secure Power Supply or go with microinverters. microinverters are better with shading but more expensive up front while the SMA string inverters have the SPS which give you some limited power during a power outage (as long as the sun is out). String inverters normally get installed indoors in nice cool basement while microinverters are bolted to the back of the panels and get roasted. Installers like microinverters as they are very close to plug and play. String inverters need slightly more design time.

 

[7acres]

Peak nailed the number pretty well below. I have no intentions to sell the house before the PV array has more than paid for itself. I've been here 18 years already. The 13 SEER central A/C unit I had installed in 1998 has certainly saved me plenty.



My 4kW array was never intended to zero my power bill, 4.4kW was literally all I could fit on the second story facet of my roof that faces south. It just conveniently generates enough energy to run the central A/C which was what the OP originally was proposing to use solar for. Certainly, if the OP can get an installed, no strings attached, price of $5,750 for an 8kW array, it beats his original concept of a $15k solar A/C option by leaps and bounds because you can use things like electric dryers, heat pump water heaters and plug-in vehicles with a grid-tied PV house.

The generator will NOT be made obsolete by the PV array. I still keep a 5,500W unit in my garage for hurricane season, because hurricanes tend to knock out my power for 7-10 days after a storm (I'm 2 miles from the ocean). When the grid is down, my PV setup is UL 1741 compliant, and goes down with the grid protecting the linemen who come from as far away as Canada to fix the grid. I think what you will find as you look more closely into the PV concept is that hooking your water heater and stove to propane may cost you more in the long run than the free sunshine that falls on your house costs. Shortly after my PV array went up, the local gas company offered to bring natural gas down our street past our house. When I did the math, it made no sense to add a minimum $20/mo gas bill to my expenses and take my electric dryer, electric water heater, and electric stove off electric power at $0.11/kWh. The purchase costs of changing over relatively new appliances while adding another bill to my monthly expenses to lower the electric bill didn't lower my ultimate cost of living. If you don't presently own it, you may wish to inquire with your propane supplier about the cost to own your propane tank. One of my neighbors owns their tank, another pays a monthly fee to the local gas company to meter read, and automatically refuel the gas company's propane tank.

I'm already working on the design of the PV system that will go on our "summer home". At 45.8°N, the 4.6kW array will generate ~5MWh per year. That's plenty of energy I can use to feed an electric dryer, electric oven, heat pump water heater, and one or two mini-splits to heat and cool the house. As others have mentioned, conservation is cheaper than oversizing a PV array. I'm already working on doubling the thickness of the 2x4 walls in the summer home to add more insulation. I still saw snow falling twice in May when I was there. Not needing to fire up the oil furnace to heat the place or heat water will be a great cost savings, even at the bargain $1.969/gallon I paid to refill the oil tank this year.

When the power goes down do you run extension cords from appliances to your 5,500W generator?

Also, my propane situation is kind of unique. I pay $72 per year to rent the 500 gallon tank. No monthly bill. No requirement that I buy X amount of fuel from them per year. I've had the tank 5 years now and all I pay is the $72/yr. Haven't needed them to refill it this whole time since it's only my generator hooked up to it. I'm at about 71% full now.