geek said:
My system will be a 5.5kw system with a Sunny Boy 5000US 240w inverter.
Down the road I may be looking for battery backup but not for the whole house, mainly for heat purposes and my new pellet stove, hopefully just a couple batteries will be good enough.
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I'm wondering what your montly electric useage is? I got mine down to 300 KWh per month before going to solar. And, my 5250 watt system make 5-10% more power per year then I use. I doubt you're going to do much better, but perhaps youve got a little more sun if you're close to the coast. My point is - if your montly useage tends to be 500 or 600 KWh, then your system is likely to be too small if you wish to make all your power.
I got looking in my inverter database. You'd never be able to hook that Sunnyboy inverter to a battery backup system. It requires way too much voltage and needs a DC solar array voltage of at least 300 volts. The specs on it are similar to a Fronius inverter that several installers tried to sell me and I'm glad I refused.
Battery backup isn't even the main issue. If all you wanted was a battery backup, you could just keep a battery bank around hooked to an AC charger. Then, run it through a cheap inverter to run your house in an emergency. You can buy a rugged and cheap modified wave inverter - 4000 watts for less than $200.
To me, the big issue is - with a system like you're installing - if the grid is down - so is your entire system. The power could be out for a week and the sun shining brightly - and you're screwed. All those panels sitting there doing nothing. That because conventional grid-tie solar systems are engineered to shut down when the grid is out. Does that make sense to you? With a battery backup type system, it CAN be made to work with the grid down - from the batteries or direct from the solar panels. Note that with some grid-tie inverters, there are ways to bypass the system and force them to work - but I doubt your installer will tell you how.
As far as I'm concerned, there are only two reasons to install a high-voltage input inverter like you're getting. To eliminate problems with a very long wire run - or - just laziness and ease of installation. With a high-voltage inverter, many of the panels can simply be hooked together in series - instead of parallel. Also keep in mind that most installers don't "think out of the box" , they just install pre-designed kits.
If you ever want the ability to incorporate battery back-up. or "legal" ability to use your solar panels with the grid down, you need an inverter than can run with a 12, 24, 48, or 60 volt DC input. Whatever the input is set at, that's what the battery bank voltage must be, and that's what the controller/charger must be set at for battery charging. With a 300 volt input inverter like you're getting - first - it would be pretty hard to have a 300 volt battery bank to match. It would take fifty 6 volt batteries. And, you'd never find a charger/controller capable of charging at a 300 volt rate. The highest on the market that I know of is 48 volts.
I'm curious what your system is costing you? Mine - with 5400 watts in panels, dual 3000 watt Outback inverters, dual Outback MX60 controller/chargers, and a six Rolls/Surette batteries, etc. cost $41,700. I got back $21,000 from the State incentive, $2000 from the Federal government, and $5000 back from the State government. So, my out-of-pocket expense was $13,700.
Basic equipment is:
30 Kyocera KC175GT Panels
Outback 3048-2 power panel
MX-60 Charge Controllers, breakers and disconnects
8 Surette S-460 Batteries
Unirac Aluminum racking with stainless steel mounting hardware