Ask Bob – December 2022

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Bob Pierce answers questions about energy efficiency, consumer products, and cooperative governance. He is Clearwater Power’s Chief Operating Officer and welcomes your questions on our Ask Bob page.

Q: I have a Marathon electric water heater. During a recent power outage, I tried to operate the water heater from my generator but it wouldn’t work. My generator has a nameplate capacity of 13,000 watts and the water heater only uses 4,500 watts. Any idea why this won’t work?

A: From a capacity perspective, you are correct: The Marathon draws 4,500 watts at 240 volts, so it should run easily on a 13,000-watt generator.

How are you hooking the generator up? Since the water heater is hard-wired, the only legal way to do it, would be to have a transfer switch installed that prevents back feed onto the power line.

A typical transfer switch energizes your entire breaker panel with generator power. This means your generator will be trying to power anything in your house that happens to be switched on. In one way or another I suspect there is some other load being powered at the same time as the water heater and the combination is drawing your generator down.

For example, some places have a separate main breaker for the well pump on the meter pole. Depending on how you hooked up, you could be powering the well pump and the water heater at the same time and that could bog down even a 13,000-watt generator.