We are about to sign a contract for a significant solar Photo Voltaic (PV) energy system for our home and I was reviewing the research & decision making process we just went through.
It started about 9 months ago with online research into complete systems being sold by national vendors and research on the DTE website about the DTE programs.
Going from the offerings of complete systems I tried to understand the individual component choice and the important specifications for the panels, inverters, and interconnects.
I originally focused on the Enphase microinverter technology because I’d be very comfortable doing that kind of installation myself as the voltage is limited to 230VAC just like any home electric panel. However, I found out that to get the DTE credits and the tax credits I have to have a licensed electrician do the install.
No problem, I thought, I could work with an Ann Arbor electrician and I know quite a few from my “day job” in the real estate industry. However when I started talking to electricians nobody seemed too excited about solar installations.
It was also about this time that the wife said she really wanted the solar panels to be able to produce some kind of energy even if the grid was down in an emergency situation. (We’ve had a back-up generator for 12 years now but I understand and agree with her logic. It would be a shame to have 7 or 8 thousand watts of generating capability on the roof during a blackout and not be able to use any of it.)
That decision removed the Enphase microinverter option from the table. So more study.
It turns out that if you want to have one of the popular battery back-up systems you end up spending a lot of money on batteries. Even if you only need it for emergencies. These systems take the solar energy and pump it into batteries, then take the battery voltage and convert it into AC for your home.
So you need two electronic products, a charge controller to charge the batteries, and an inverter to convert the 12 to 48 volt DC output of your battery bank back to 240AC for your home use.
Sounds easy enough, except that those systems involve more compromises on the rest of the design.
In fact, too many compromises based on the idea that we’ d only use it during emergencies.
So here is the compromise I came up with:
Buy a normal grid-tie system, but leave some “hooks” in the design so I could install a small charge controller, battery bank, and inverter system later for emergencies.
The only challenge left was that the charge controller products are normally designed for a maximum solar array voltage of 200 to 300 volts. (The grid tie optimum is a maximum of 600 volts.)
I proposed to have the roof “strings” of panels divided into say three 200V strings that would be tied in series at an accessible combiner box before being sent to the grid tie inverter. But I got some confused looks from the vendors I was talking to. About this time (October 2010 actually) one company announce that they were introducing a high voltage charge controller that could work with voltages up to 600 volts. That kind of solved the problem. Assuming we can afford one of those when the go into production early in 2011 we should have a fairly straight path to an emergency back-up system without loosing the optimization of the grid tie system.
At that point then I was ready to get quotes and move ahead.
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