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Winter Outage Simulator

A gas furnace still needs electricity for its blower and controls. Pick a power station, set the weather and your thermostat, and see roughly how long it keeps the heat on. It is a simulation, not a test, and every assumption is on the page.

1Power station

2Weather and thermostat

The average over the hours you care about.
Lower saves battery: the furnace runs less.
The cold the furnace was sized for. Ask your HVAC contractor if you don’t know it.

3Furnace and other devices

Our planning figure is 600 W; newer furnaces often draw less. Check the rating plate.
Simulated runtimeDetails

How the simulation works

Heating load calculations treat a house’s heat loss as growing with the gap between indoor and outdoor temperature. ACCA Manual S sizes a furnace at 100% to 140% of the load at the local design temperature, with 70 °F indoors. So the share of each hour your furnace runs is roughly your thermostat minus the outdoor temperature, divided by 70 °F minus the design temperature, divided again by how oversized the furnace is. The simulation draws that oversize factor between 1 and 1.4, which is where the likely range comes from.

The battery side is the same model as our real-world runtime simulator. It steps the battery down every 2 minutes, 200 times, and charges it for the inverter’s idle draw and conversion losses as well as the devices. The furnace cycles on and off, as do the fridge’s compressor, and the inverter keeps drawing idle power in between.

What it leaves out

Ways to stretch the battery

Questions

Is this a test?
No. It is a computer simulation with assumed inputs. Nobody ran a furnace on these units. The assumptions and their sources are listed on this page.
Why does the outdoor temperature matter so much?
A house loses heat faster the colder it is outside, so the furnace runs a bigger share of each hour. The blower draws the same watts whenever it runs, so more running time means a shorter battery.
What is the design temperature?
The cold outdoor temperature a heating system is sized for. Many load calculations use the 99% value: the temperature a place stays above for 99% of the hours in a year. Your HVAC contractor or local building department can tell you the figure for your area.
Can I plug my furnace into a power station?
Only if it has a plug or an electrician has installed a furnace power inlet or transfer switch. Most furnaces are hardwired. See our furnace page.
How do I find my blower’s watts?
Look at the rating plate inside the furnace door. Volts times the blower motor’s amps gives an upper bound. Furnaces made since mid-2019 usually have more efficient blower motors and draw less than our 600 W planning figure.

Sources

  1. resnet.us/blog/wp-content/uploads/formidable/RESNET-Basics-of-Manu…
  2. energyvanguard.com/blog/we-are-the-99-design-temperatures-oversized-hvac-sy…
  3. powerequipment.honda.com/generators/generator-wattage-estimation-guide
  4. energy.gov/sites/default/files/2024-10/furnace fans ecs fd.pdf

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