Considering solar with battery storage

[pinefall]

With the new 30% tax credit I’m considering solar (PV) again. We get our electricity from the local rural electric co-op who gets it from the Tennessee Valley Authority (TVA). Our power is relatively cheap, currently $0.11427 per kWh so the break even point, even with the credit, is many years out for a solar installation. TVA does not offer net metering and options for selling power back are very limited and pay much less than retail cost. So there are really two advantages: it’s the right thing to do; and, with a battery storage system can provide emergency power in an outage (which is preferable to a whole house gas/diesel generator).

Our electric co-op has smart metering and I can look at our last five years of power usage with detail to the to the hour. Over the past five years, we have only had 6 months with >1500 kWh usage. In the spring and fall, our usage drops to a monthly low of a bit over 500 kWh. (All appliances are electric, HVAC is heat pump with addition of wood stove.) Last year, the highest month was 1570 kWh, lowest 610 kWh, average 950 kWh.

With this usage, what is a reasonable size for generation and for battery storage? We don’t have expectation of selling excess power back. But would like enough storage to be comfortable during an extended utility outage. Outages rarely last long but did have one 14 hour outage a few years ago.

I’m thinking generation should be sized to the base load, i.e., cover power usage 100% during the lowest usage months of 500 - 600 kWh so there is no excess capacity, but that is pretty small. If I look at daily or hourly usage, it gets even more complicated when considering how much storage is reasonable.

Comments?

 

[ABMax24]

By my math you need about 11,400 kwh per year, that equates to roughly an 8kw PV array to cover 100% of your usage on an annual average. At a nominal $3/watt that system will be $24,000. To run bare bones appliances you're looking at a single Tesla Powerwall at $11,500 (plus install), to actually run your house "as normal" for any duration during an outage you will need at least 2, and really you want 4 to try and avoid buying/selling from the grid.

Basically you are spending $70k+ to buy an almost offgrid solar PV system due to your utilities draconian views on net-metering. It's not worth installing a system that ships power back to the grid because your provider takes that electricity while paying you nothing.

Here's what I would do, get an electrician to wire up a transfer switch and plug outside the house and buy a 7000 watt portable generator to plug into it for grid outages. Write letters to both the utility, and your state representatives asking for net-metering or a similar system to be adopted in your area.

 

[woodgeek]

I appreciate wanting to do the right thing. I will assume that you primarily are interested in reducing CO2.

You can look at your carbon footprint, and find better/cheaper ways to reduce CO2.
This is a decent calculator: https://www.carbonfootprint.com/calculator.aspx

I looked it up, and TVA power is relatively green compared to the rest of the US: 0.56 lbs CO2/kWh, versus 0.85 lbs CO2/kWh for the US national average.

Your 11,400 kWh annual consumption works out to only 2.9 metric tons CO2/year. The US average TOTAL carbon footprint is currently 16 tons per person. Assuming you don't live alone, your home electric usage is probably a small contribution to your overall family's carbon footprint.

What can you do? Well, let the carbon footprint thingy be your guide.

Some ideas:
1. If you have a conventional electric HWH you can switch to a HPWH and shave off a few kWh, and there are great rebates for that.

2. You can get a cheap, long range BEV or PHEV. Burning gasoline releases about 1 metric ton of CO2 for every 80 gallons used (80% out the tailpipe, and 20% during extraction and refining). So your 2.9 MT from electricity is equivalent to only 232 gallons of gasoline per year. If you family is driving 10,000 miles/year, you are likely emitting more CO2 from your vehicles. With your green TVA electricity you would emit only 0.56/4 = 0.14 lbs CO2/mile in an EV getting 4 kWh/mile.

3. You could reduce dairy and red meat consumption for your family. This would probably reduce your carbon footprint as much as your solar plan, and would have the advantage of reducing habitat loss due to agricultural activity, and making you healthier and live longer too.

For backup power... just get a cheap genny (like a couple kW), some nice extension cords and a metal can for gasoline (and add stabil to the gas for multi year storage). The whole thing will be under $500. Then during an outage you can run your fridge, your stove blower, your wifi, some fans and keep your phones charged. No problem.

 

[pinefall]

I’m looking for encouragement to do this! Over the past 15 years I’ve run the numbers a few times and each time came to the conclusion not to invest in a solar installation. But in the past few years battery technology has improved, inverter technology has improved, and solar panel efficiency has improved. And now there is a federal incentive offering a 30% credit on the panels, batteries, installation, even related electrical upgrades. It seems like it should work now.

Yes, sizing the system for 100% of usage without net metering doesn’t make sense since there would be excess capacity for much of the year. But I was thinking maybe a small system, perhaps 5kW which in principle could zero out the bill in low usage months and cut the cost the rest of the year. Storage is harder to optimize. A 10kWh battery system could potentially address the intermittent power outage, covering basic needs. An extended outage would require management, particularly if there wasn’t enough sun to recharge. But those long outages are rare enough. The bigger question for storage though is for leveling generation and usage. Looking at the details from our smart meter, most of the time when the sun is shining we are using much less than 5kW. So we would need enough storage to make this available in the evening, nights, morning when power usage may be higher. Which seems to be pushing it to >20kWh of storage to effectively utilize the 5kW of generation capacity. But that additional storage really increases the price and makes it all less compelling.

 

[EatenByLimestone]

If you have to try that hard to convince yourself it’s a good idea it probably isn’t.

Can you up your insulation or change to a heat pump or something similar to decrease energy usage?

 

[DBoon]

If you want to do it, just do it (if there is the money to do so). A 5 kW system output could (probably) reasonably be used on-site if you timed your activities and peak power use to coincide with peak sun generation. That means you would have to change your lifestyle a bit and invest in some timers for hot water heaters, air conditioners, heat pumps, etc. I get the desire to have a battery backup instead of a generator - this is in my plans as well but battery prices are just too high for me at this time. You could do something pretty good and get started with $10-15k (after tax) and then add more later.

Think about an EV as well. That could consume some of that 5 kW array output as well before it goes back to the utility at less than retail rates (if the vehicle is home during the day to recharge).

And reducing demand is also a good first step - e.g., HPWH to replace an electric resistance one.

 

[NoGoodAtScreenNames]

I know this is reviving an older thread but I wonder what the net metering rules actually are for the OP. Having bad net metering could mean either 1) that you are paid back for any net excess solar you at the end of the month and are paid back the fuel charge rate ie less than retail or 2) every watt you send into the grid is sold at the fuel charge rate and any watt entering from the grid is bought at the retail rate.

I have option 2 (but I got generous upfront rebates) and financially it makes the most sense to only buy a solar array that can handle my base load during the daytime i.e. try not to sell anything back to the grid. At night you pull from the grid. Of course you could add a battery but as others have said that will increase the cost and extend the payoff period.

My solar is a bit over sized for my net metering rules but I’m doing my best to electrify and try to shift my consumption to the daylight hours. I try to charge the PHEVs, run the appliances etc. when the sun is out. Also if / when I get a heat pump that will help create a new base load in the heating months that will reduce the amount I’m selling back to the grid at a discounted rate. For me that’s $0.18 as a buyer and $0.05 as a seller. The difference in the costs is an average of about $30 per month for me.

 

[pinefall]

I know this is reviving an older thread but I wonder what the net metering rules actually are for the OP. Having bad net metering could mean either 1) that you are paid back for any net excess solar you at the end of the month and are paid back the fuel charge rate ie less than retail or 2) every watt you send into the grid is sold at the fuel charge rate and any watt entering from the grid is bought at the retail rate.

I have option 2 (but I got generous upfront rebates) and financially it makes the most sense to only buy a solar array that can handle my base load during the daytime i.e. try not to sell anything back to the grid. At night you pull from the grid. Of course you could add a battery but as others have said that will increase the cost and extend the payoff period.

My solar is a bit over sized for my net metering rules but I’m doing my best to electrify and try to shift my consumption to the daylight hours. I try to charge the PHEVs, run the appliances etc. when the sun is out. Also if / when I get a heat pump that will help create a new base load in the heating months that will reduce the amount I’m selling back to the grid at a discounted rate. For me that’s $0.18 as a buyer and $0.05 as a seller. The difference in the costs is an average of about $30 per month for me.

My available metering option is similar to your option 2, that is based on instantaneous usage not net usage over the month. Full retail cost for any power I use but payment for only the power company’s “avoided cost” for any excess power sold back. The value of the “avoided cost” can change, but the local electric co-op says to expect about 1/6 of the retail rate, so worse than your ratio.

Plus there is a $250 application fee and a $500 interconnect fee for the program. And the agreement is for only 5 years.

So lots of disincentives from the power company on this.

 

[peakbagger]

Got 15K available? Go on Ebay and look up solar trailers or DC solar. For 10 to 20K you can get a solar trailer which is a complete microgrid in a box with about 50 KWh of forklift batteries. It has two Sunny Island inverters so it can put out 240 VAC at 50 amps continuous with various surge amperages available. Some of the trailers have a backup diesel generator (usually 20k). With the right internal settings, they will grid tie but in your case, I see it staying off grid. They have around 2.3KW of panels. An additional charge controller would need to be installed to add more panels. I would pour some footings, lift the equipment off the trailer and permanently install the panels then sell the trailer at is dual axle with trailer brakes with a GVW just under 5 tons.

The Sunny Island units are about as flexible as every made.

 

[begreen]

Got 15K available? Go on Ebay and look up solar trailers or DC solar.

All you have to do is figure out how to get it home.

 

[peakbagger]

They got sold all over the US but a lot in the southeast (anywhere where there was NASCAR track). The alternative is I have seen the electronics package in an outdoor enclosure for sale.

 

[begreen]

They got sold all over the US but a lot in the southeast (anywhere where there was NASCAR track). The alternative is I have seen the electronics package in an outdoor enclosure for sale.

They're cool systems. California is the closest I found to me. The genset has only 2.5 hrs on it. Another caveat is that the trailer requires a pintel hitch.

 

[NoGoodAtScreenNames]

My available metering option is similar to your option 2, that is based on instantaneous usage not net usage over the month. Full retail cost for any power I use but payment for only the power company’s “avoided cost” for any excess power sold back. The value of the “avoided cost” can change, but the local electric co-op says to expect about 1/6 of the retail rate, so worse than your ratio.

Plus there is a $250 application fee and a $500 interconnect fee for the program. And the agreement is for only 5 years.

So lots of disincentives from the power company on this.

Yeah with bad net metering rules you really can’t compare you monthly or yearly electricity use to the yearly output of solar panels. Solar panels only work when it’s sunny - duh we all know that. But don’t think of the sun lasting from sun up to sun down. My best production happens in April and on a good day I can get a little more than 60kwh. However most of that is occurring between 9:00 AM and 2:00pm. At that levels it’s hard to load shift to self consume everything. A battery would solve that somewhat but for me it still didn’t make sense. Maybe if / when battery costs come down it will. I don’t lose power often so I don’t have that pushing me - the pure economics didn’t make sense.

 

[pinefall]

Yeah with bad net metering rules you really can’t compare you monthly or yearly electricity use to the yearly output of solar panels. Solar panels only work when it’s sunny - duh we all know that. But don’t think of the sun lasting from sun up to sun down. My best production happens in April and on a good day I can get a little more than 60kwh. However most of that is occurring between 9:00 AM and 2:00pm. At that levels it’s hard to load shift to self consume everything. A battery would solve that somewhat but for me it still didn’t make sense. Maybe if / when battery costs come down it will. I don’t lose power often so I don’t have that pushing me - the pure economics didn’t make sense.

Right, I've come to the same conclusion. All of our home energy is electric: heat pump for heating (+ plus wood stove) and cooling, electric oven and cooktop, electric clothes dryer, electric hot water, etc. Natural gas is not available here and we don't use propane. Our electric rates are low compared nationally and our power usage is relatively modest. Our rural electric co-op's smart metering lets me see historic power usage by the hour. Excluding heating and cooling, the biggest usage (cooking, laundry, etc.) is generally in the evening when solar would not be producing. Heating typically peaks in early morning, also with no or limited solar. Cooling has the most overlap with solar production potential but it is by no means complete. The amount of our power usage that could potentially be switched to times when solar would be producing is very limited. (If we added plug-in electric vehicles, they would likely get charged overnight.) So it comes down to net metering or battery storage as a way to time-flex production vs usage. With a more reasonable net metering policy as is available in other regions, I think it would work for us. But with the very poor option we have available, it is not economically viable. In principle, battery storage would work (with the added benefit of back-up power) but the cost needs to come down more for it to make financial sense for us. Battery technology continues to improve and costs should continue to come down. So we'll revisit it again.

 

[Rusty18]

If you really want to get your feet wet you could do a grid tied but sized to offset your average through the day when the solar production would be at its peak. It might not be that economical but it should be small enough that you could swing the cost to see how it works and give you a better idea for designing a bigger system.