Does anyone make a hybrid inverter that is normally grid tied but also has a battery system. My thinking is you could use your batteries for your own usage at night or when the grid was down but still give the grid back some power during the day after you topped up your batteries. I suppose I could do it with 2 inverters, a load management scheme and transfer equipment but I was trying to keep it simple.
I am thinking of two DC busses. A variable DC buss coming of of the PV array which feeds a battery charger and is the normal source for the grid tie inverter. The other buss is the battery pack buss. This feeds any DC only loads like swimming pool pumps. It is the alternate feed for the grid tie inverter.
The output of the inverter is connected to the load side of a isolation switch. When the isolation switch is opened, this would signal the inverter that it is OK to put out AC because the inverter is disconnected from the grid.
Obviously the loads you want to power when the PoCo goes away would be also on the Load side of this isolation switch.
"night usage" would also be the things I want to use in an outage. I was thinking at least one fridge, the well pumps and a general lighting circuit. I am still in the process of mapping all my breakers but this could end up being 2 circuits. My fridge in the house is on a general lighting circuit (JFK era construction). I have a couple of Watt Hour meters that I can use to see what I am really talking about once I split these out.
Depending on the size and type of battery bank, a special battery area may be required.
As of right now, it sounds like a transfer switch, sub panel, and a grid tie inverter that can switch to free running mode. The charge controller would be connected to the PV panel output and possibly the PoCo fed panel to charge the batteries if the PV panels are disconnected.
I was thinking about 6 golf cart batteries, that will be 1200-1300 A/H @ 36v. Maybe something like 40kwh usable if I am thinking right. That will require a 36v inverter setup. Plan B is I could go to 48v.
The inverter that does both grid tie and independant AC power output is called a 'multi-function' inverter. They connect to battery pack, PV array, PoCo, AC load, and standby generator. Basically they are everything in one box. I believe Trace is one brand, there are probably others.
I believe battery pack voltage and PV output voltage have to be the same. Therefore the PV array will dictate battery pack voltage. Lead acid batteries are not the best as far as storage efficiency but they can be significantly cheaper. If possible, use deep cycle batteries because they have better grids for the lead to plate back onto. You may also have to check with the local fire department if a significant amount of sulphuric acid is going to be onsite.
Do you need that complexity? 'Tied to the grid' means you can take what you want, [within limits], including charging back ups. Plus you get paid when the sun's out. The downside, I take to be, if the poco dies so do you. The cost of batteries / inverter over service life might prove more expensive than a generator, a can of gas and regular switching, especially if non-recorded use eats into a bigger % of your standing charges. How often and long are the outages? Ten years ago, EDF seemed to go phut at the drop of a hat. Now I can't recall the last outage that wasn't notified with a postcard.