Project goals: Lower utility bill No grid connection Emergency power during extended outage Currently have a 16 kW, natural gas fueled, automatic, emergency generator My proposed requirements: island operation (aka., no grid connection) the power company can approach me with their plan 10 kWh usable storage - enough to handle 40 miles of EV range. 240 VAC output to derated NEMA 14-50, maximum 6.5-7.5 kW output SOC level, triggered independent circuit: kitchen, hot water, mini-split AC, hall way lights, and furnace fan circuits ~2 kW peak solar array - enough to 80% charge the storage battery on a sunny day Implimentation thoughts: Used (salvage) EV for battery and backup for primary EV Candidates: Bolt, BMW i3, Leaf Repurpose vehicle power electronics for inverter COTS solar, battery, 240 VAC controller Solar array with mini-inverters to handle shadow and failure isolation Bob Wilson
Have you played with NREL's PVWatts? This calculator is local-climate-aware, accounting for typical losses from cloud cover and other weather at your particular location. Putting in for a 2kW-dc system at Huntsville with default parameters, I get this result, just half of what I'd need for my smaller all-electric house, after major energy conservation work, and without an EV: Your winter minimum is 55% of your summer maximum, not too shaby. In my climate zone, winter minimum is only 18% of summer max, making the winter capacity for off-grid too expensive, so I must go grid-tied. If I did have enough winter capacity to go off-grid, I'd be foregoing a large amount of potential revenue from the summer surplus. Surplus that ought to go help offset someone else's inability to install solar. Island operation is now possible on modern grid-tie systems with battery storage and appropriate controller (Powerwall and others) and system architecture. My current microinverters were not set up for this mode, but with new firmware and $$$ for batteries, controller, and service entrance re-configuration, could now do so. With you already having a backup generator with transfer switch, you might be part way there already. My system has 7kW-dc, 6kW-ac, and produces just over 6000 kWh/year. NREL thinks I should get 7,600, but doesn't account for the shadowing of a neighboring wall of trees. If you do have tree shadow impairments, then I do agree with using micro-inverters instead of a string inverter. ============== P.S. You mentioned hot water. If electric, do you have a heat pump water heater already? If not, this would be a good upgrade independent of any solar system.
