Can I Run a Pellet Stove on a UPS, and How Long Will It Last?

Yes, a pellet stove will run on a UPS, but only for about a quarter of an hour, and the stove makers who approve UPS units approve them for one purpose: shutting the fire down cleanly. Harman's battery backup brochure puts it plainly: "UPSs simply provide enough power for quick shut down of the unit without emitting smoke in the home." If you want heat through an outage, the tool is an inverter/charger on a deep-cycle battery, not a bigger computer UPS.

Why a pellet stove needs backup power at all

A wood stove keeps burning in an outage. A pellet stove does not, and the reason is not heat, it is smoke. The stove relies on a combustion blower to pull exhaust out through the vent. Harman's manuals spell out what happens without it: "A power failure will cause the combustion blower to stop. This may lead to exhaust seeping into the room." Vertical rise in the venting may give some natural draft, which Harman says is "no guarantee against leakage."

That is why Harman "strongly recommends installing battery back-up," and why the first job of any backup is keeping the combustion blower turning until the fire in the burn pot is out. Continued heating is a second, much larger job.

What a stove actually draws, from the manuals

Forum threads commonly quote 300 to 500 W for ignition and 50 to 150 W for running. The first range roughly matches the manuals. The second is low: every manual rating gathered below puts normal operation at 180 W or more.

Two things stand out. First, the nameplate amps overstate real draw: 2.8 A x 120 V is 336 W, while the same P43 manual lists 200 W standard use. Size from the watt figures, or better, from a measurement. Pellet stoves plug in, so a plug-in wattmeter works; our guide to measuring power draw covers the method. Watch a full startup, because the igniter phase is the peak.

Second, the igniter is not a brief spike. The US Stove 5660 manual says the igniter "will stay on for at least 10 and up to 12 minutes, depending on when Proof of Fire" is reached. That is very different from a gas furnace igniter, which glows for under a minute (see UPS for a gas furnace).

Worked example: a computer UPS

Take a typical 750 VA desktop UPS with one 12 V 9 Ah battery, about 108 Wh nominal, feeding a stove at its 200 W standard use. With inverter efficiency around 0.85, the battery must supply about 235 W, or roughly 20 A at 12 V. That is near a 9 Ah battery's 10 to 15 minute rate, where it delivers only about 45% of its rating (the reason is in why UPS runtime is not linear):

That is in the same range as SEC America's figure of "approximately 15 minutes" of stove operation on a UPS. If the outage hits during ignition the picture is worse: 440 to 460 W is close to or above the watt rating of many 750 VA units, so check the watt figure, not the VA (see VA vs watts).

Fifteen minutes is enough for what the UPS is meant to do, as long as the stove knows to shut down. Harman stoves with Rev E or later software (from November 2010) handle this themselves: on a UPS the stove "will pulse the blower every few seconds to clear exhaust until the fire is out." SEC America describes the general case: the stove "can't communicate with the UPS, so the shut-down process will not begin automatically." On a stove without that logic, if the outage comes at night and nobody wakes up, the stove runs for a few minutes and then shuts off abruptly, which is the smoke scenario the UPS was bought to prevent.

Worked example: an inverter/charger on a deep-cycle battery

For continued heat, Harman approves three Tripp Lite inverter/chargers, each running from a 12 V deep-cycle battery and each quoted to run the appliance "for up to eight hours." An inverter/charger sits between the wall and the stove, keeps the battery charged while power is on, and switches to battery automatically.

Check the eight hours against the manual ratings. A common 12 V 100 Ah deep-cycle battery holds about 1,200 Wh nominal:

At the same 20 A draw, a 100 Ah battery is near its 5-hour rate, where deep-cycle lead-acid typically delivers about 80% of its rating.

So "up to eight hours" on one battery implies the stove averaging roughly half its rated standard use, which is plausible on a low heat setting and not at full output. Drawing lead-acid that deep regularly also shortens its life; stopping at about 50% (about 510 Wh delivered) gives roughly 2.5 hours at 200 W. To stretch runtime, add batteries: Harman notes the APS750 and APS1012SW can charge multiple deep-cycle batteries, while the APS700HF "cannot use multiple batteries." Plug your own measured draw into the runtime calculator.

Do not try to get the same result by wiring a big battery to a desktop UPS. Its charger and cooling are designed for its small internal battery; extended runtime battery packs explains why that modification goes badly.

The waveform question, and the setting most owners miss

Of Harman's three approved inverter/chargers, two have modified sine output and one is pure sine, and that difference changes setup:

• APS1012SW (1000 W, pure sine): "No changes needed on pellet stove due to pure sine wave." Connect it and it takes over automatically.
• APS700HF (700 W) and APS750 (750 W), modified sine: on touch-control stoves, the dealer goes into the Authorized Dealer Area, touches Power Failure and presses Continue Operation. On knob-control boards, turn on dip switch #4.

Harman's reasoning is in the brochure: its pellet controls "are pre-set for quick shutdown," and since these two units "are not pure sine wave, they should be set to Continue Operation to allow stove to continue heating during power failure." Skip the change and a modified sine unit can still give you a safe shutdown, but not the hours of heat you paid for. For the general difference between the two outputs, see pure sine vs simulated sine.

Which backup fits your goal

A power station can carry the energy, but it is not on any stove maker's approved list cited here, and its handling of a power cut is the open question. UPS vs portable power station covers how they differ, and UPS transfer time explains why a few milliseconds can matter to electronics. For outages lasting days, battery runtime stops making sense; UPS vs generator walks through the alternative.

Where this answer stops applying

• Brands without a backup program. The approvals and settings above are Harman's. The US Stove 5660 manual says the heater "will not operate during a power outage" and requires a surge protector. The Englander 25-PDVC manual gives outage instructions instead of backup advice: if power is off for more than about three minutes, the stove is off when power returns.
• Older Harman controls. Plugging directly into an approved UPS applies to Rev E or later software. Harman's P-Series manual also warns that "some generator or battery back-up systems may not be compatible with the micro-processor electronics."
• Runtime beyond the battery. Harman notes that if the outage outlasts the battery, "smoke leakage may still occur unless your stove has been safely shut down."