I’m providing an update to what I posted previously about using an AC compressor with an LRA of 135 with 2 Powerwalls. After the Powerwall installation, the AC compressor wouldn’t even start spinning during a simulated grid outage. Other backup circuits experienced low voltage causing disruption to electronic devices, including some electronic device damage. The contractor installed a Sure Start device on the AC compressor. With that device installed, the compressor rotor will spin up but there is still a very low voltage condition on the other circuits causing electronic devices to reset, including the thermostat connected to the HVAC system. I did a little digging on the Sure Start to see if it should have worked in my situation and this is what I found. Electrical Engineers out there, please verify my calculations.
Below is the guidance provided by the company that makes the Sure Start device. It says that the “generator surge capacity should exceed 50% of the Locked Rotor Amperage of the compressor in use.” The generator in this case is two Powerwalls. The
surge capacity of two Powerwalls is 14 kWs (7 kWs each). The AC compressor has an LRA of 135 amps. Converting 135 amps to kWs at 240 volts is 32.4 kWs. Reducing 32.4 kWs by 50 percent still is 16.2 kWs to spin the rotor. That is more than the surge capacity of two PWs so the Sure Start will not work. Even if the Sure Start device could achieve a 60 percent reduction in required amps as mentioned in the literature, this would lower the kW draw to about 13 kWs.
If my math is correct (or if it gets corrected by other people much smarter than me
) others could use this to determine if there is any hope of backing up their home’s AC with Powerwalls.