On Sunday morning June 30th, we experienced our first prolonged power outage since moving in to our new passive house in Connecticut. A tree came down on one of the power lines around the corner and power was out for about six hours. It was time to see our inverter in action — the inverter that we bought from Converdant Vehicles to turn our Prius into a backup generator.
The inverter works by taking energy from the big hybrid battery in the Prius and converting it to a pure sine wave AC current, no different from what we typically get from the power grid. (For more information on the Converdant inverter, see Backup Electrical Power for a Passivhaus Project.)
We placed the inverter on the porch and connected it to the generator sub-panel outlet in the garage. We then parked the car in front of the garage and connected the car to the inverter with a cable we had professionally installed into the hybrid battery.
We turned on the car, turned on the inverter, and flipped the switch on the generator sub-panel. (The sub-panel controls our critical circuits.) Instantly, our well pump and ventilation system were back up running.
You won’t need much gasoline, and it’s quiet
When I turned on the car, I noticed that it was almost out of gas, down to the last bar on the display. Fortunately, this setup is very efficient. The inverter takes as input the DC current from the hybrid battery. As the hybrid battery loses its charge, the Prius’s gas engine turns on to recharge the hybrid battery.
If there is only a small appliance load on the inverter, the gas engine turns on infrequently. We only had a few hundred watts of power being drawn, so we used little gasoline.
Another cool thing is that the only noise this setup makes is the sound of the Prius idling. We could hear our neighbors’ loud gas generators from hundreds of feet away, but most of the time we were pulling electricity from the Prius in silence because there was plenty of charge in the hybrid battery without the car even idling.
The sine-wave electricity can run electronic devices
Perhaps the most impressive aspect of this setup is that the inverter generates 240V/120V split-phase pure sine wave AC power. With it, we can operate both 240 volt appliances (well pump, HRV) and 120 volt appliances (fridge, lights, computers).
Because it’s pure sine wave power, we don’t have to worry about the generator frying our computers, TVs, etc. I was able to watch recorded TV on an LED flat panel using a signal from our Windows Media Center PC we use as a DVR while the family took hot showers (complements of the sunny day prior to the outage).
All this while the Prius either sat silently or idled in the driveway.
Paul Honig lives in a passive house in the northwest corner of Connecticut with his wife Diane and two boys. He enjoys teaching math and playing roller hockey. Paul and Diane are the authors of a blog called Our Connecticut Passive House.