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Why hybrids do it better

February 24, 2021

After the recent power outage, some people have asked how we used our Prius to provide backup power to our house. That is something we have done several times at this point, living in a storm-prone area. Our setup pretty much follows Frank Perkins’s description. I will point the curious to that, because there are some subtleties that deserve a bit of thought before using a hybrid car in this fashion: 1) Whether this will supply enough power for your needs. 2) Whether to run the power through your house’s circuit panel using a cut-over switch, or just run extension cords. 3) If the first, how to acquire or make a one-phase to two-phase adapter that lights up the correct side of your house’s electrical panel, and which of your home circuits can be run safely from that. 4) What inverter to use, and why you likely want a pure sine-wave inverter for this purpose. If, for example, you will be powering circuits with arc-fault circuit breakers. 5) How much current you can draw from your car without frying anything, and where to hook the inverter. 6) How to hook everything up, and how to set up your car for this purpose. It seems quite easy and straightforward to me. But I’m a nerd born and bred. Someone who does something dumb could electrocute themselves. I haven’t read about people doing this with hybrids other than the Prius, so don’t know how well this works with other hybrids or what pitfalls to avoid.

That said, hybrid cars make good backup and field generators, better than the small gasoline generators custom built for that purpose. Let me explain why.

Any planning for a power system should begin by thinking about what needs to be powered. After a hurricane knocks down the local power, we want 1) to keep our refrigerator running, since we like to eat, 2) to keep our lights on, since we like to see, and 3) to charge computers and phones, since we like to communicate. The refrigerator is the large draw. Most of the time, it is just an insulated box consuming only a few watts for control and to light the front panel. Then, its thermostat decides some cooling is needed, and turns on the compressor, spiking its consumption to near 800 watts. Most households will exhibit that kind of variation in load.

The large problem with getting electrical power from an internal combustion engine driving a device to generate current is that the engine runs all the time. Most cars will burn around four gallons a day, at idle. The typical 2kW generator will burn three gallons a day, even under light load. My dutiful brother-in-law, in a small town in northeast Texas, was busy scrounging for gasoline last week to keep the lights on for my sister during the recent blackout. Their little generator is a hungry beast. He gets full credit: the power is back on and my sister is happy.

A hybrid vehicle brings two resources that significantly improve things. First, it has a large capacity battery, which acts as its alternate energy source. Second, it has some power control smarts. When I hook my Prius up to serve as a power source for the house, here is what happens: For a few of hours, it will sit quietly, powering the house through an inverter, draining its lithium battery. In that state, the energy flow is all electric and quite efficient. Eventually, the car’s control system will detect that the lithium battery is low. It then runs the engine for a short period — around 20 or 30 minutes — to charge the battery. Once the battery is adequately charged, it turns the engine off. Which returns the car to the first state, where the lithium battery is providing power. The car will go through that cycle for days on end. How often the engine runs depends on the power demand. Regardless, the engine is running a) only for short periods of time, and b) only under load, while charging the battery. This last blackout, we consumed around a gallon a day. That is just an estimate — we started the blackout with 3/4 tank of fuel, and three days later when grid power returned, had just under a 1/2 tank.

Ford has realized that serving as a field generator is a significant feature for its new hybrid trucks, and includes power outlets in some of them. I hope more hybrid manufacturers move in that direction. Living in a hurricane area, I know I would consider paying for an option or trim upgrade that provides 120V PSW AC power, from a hybrid vehicle I otherwise was looking to buy. I think, today, my sister would, too. I can cobble the system together. But this is an important use case that should be engineered into the vehicle. And yes, please provide numerical watts or horsepower on the energy flow display.

Mules are the ancient hybrid that powered the world for millennia. They are known for their efficiency: they do more work than horses on less food. Muleskinning was a common job with armies and construction companies and other organizations that had the need to pack significant material over ground. George Washington paraded female donkeys to help his royal Spanish stud with the horses. (That sentence previous is one of those that one never dreams one would write until having done so.)

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