Fuel-Free Back Up Power
Updated: Nov 11, 2021
Usually PV systems are backed up with generators, but they are loud, burn fuel, and are inconvenient and expensive to buy and to run.
We needed to upgrade our electrical system to 240V to run electric appliances like a stove, and we needed more energy storage capacity to get through a few cloudy days. We added another battery bank (more about that in another post) and an 8 kW 240V Outback inverter.
Our solar contractor Zeke Yewdall of Dark Forest Solar offered to find buyers for the old 120V equipment, but I had a crazy idea that just might work. Why not use the old solar PV system to back up the new one, I asked?
We hired our neighbors Robert and Max of Max Electric to rewire our electrical panel for 240V, and relocate the breakers for the circuits we wanted to back up (well pump, essential lights, electronics) to one leg of the panel, putting less important circuits on the second leg.
When the new solar system is running, we have 240V and can power the stove, as well as all the 120V circuits on both legs of the panel. When the energy in the batteries falls below a setpoint (which is pretty low since the new batteries are not lead acid and can tolerate being deep-discharged without reducing their lifespan), the old 120V system takes over and we have another 10 kWh of capacity on backup power. All without running a generator or using fuel!