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  1. When electricity goes down, oil furnace and gas water heater will not be able to cycle on and off, correct?

  2. Is there an alternative to a generator for these appliance, preferable battery backup?

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    Think a small generator needed for a furnace would be much cheaper than having enough batteries to run it. Only need a 1500 or 2000w generator and probably have enough extra power to run a light and LED TV. A battery system needs a charger, an inverter, plus probably a bank of batteries(for extended time, day or two)
    – crip659
    Commented Aug 15, 2021 at 22:28

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Impossible for me to say for an oil furnace.

But a typical gas water heater (at least "old style" with a pilot light always on) is totally independent of electric power. If a gas water heater has an electrical connection then you have to figure out if it is only for communication (monitoring and/or remote control) or if it is actually involved in basic functoning (electronic ignition or other key features).

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  • Found from @gil comment below. I believe due to EPA, there is now intermittent pilot ignition. This unit requires electricity. And thermostat is certainly electric.
    – paulj
    Commented Aug 16, 2021 at 13:29
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They sell UPS units, some are quite large in capacity. If your hot water has a blower on it it will not light until the system detects enough air flow to exhaust the fumes from the combustion chamber. Generally they do not use much current. The oil furnace would have an ignition system and a oil pumping system, you need to supply enough power to not only keep them going but to also start them. You will have to determine how much power this is going to require. Generally the controls do not take much power but you need to check and be sure. With this information look for a UPS with at least 125% of what you need, more would be good. Next you need to determine how long this needs to operate on battery, many of the USP system systems are sized to operate about 20 minutes with good batteries. Many come with charts showing how long it will operate with a given load. I know of some installations that used deep discharge marine batteries to power them. You can also get commercial type of UPS systems that have external batteries. This is not an inexpensive solution but it will work; for how long depends on the capacity.

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    Forgot about the furnace fan, probably main power draw on a furnace plus the motor driving the injector pump.
    – crip659
    Commented Aug 15, 2021 at 22:38
  • Good catch, I was thinking of a hot water system and ignored the blower.
    – Gil
    Commented Aug 15, 2021 at 22:49
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    With a hot water system you will also need to power the circulator pumps and the thermostat.
    – mikes
    Commented Aug 15, 2021 at 23:57

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