Generator runtime calculator

Pick your generator (or type its spec sheet numbers), enter the fuel you have and what you'll run, and see how long the power lasts.

36.4hours of power

About 0.137 gal an hour at 33% load

One tank fill lasts 6.9 h · 3.3 gal a day · 21.9 kWh delivered

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Carbon monoxide kills. Run a generator outside only, at least 20 feet from doors, windows and vents (CDC).

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Outage runtime planner, fully offline

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How it's calculated

  1. Makers publish two runtimes on one tank: at 1/4 load and at rated load. From those we get two burn rates: tank size ÷ hours at 1/4 load, and tank size ÷ hours at rated load.
  2. Your load as a share of the rated watts places you between those two points. We draw a straight line between them to estimate gallons per hour at your load. Below 1/4 load we keep the 1/4-load burn rate, so the runtime shown is never longer than the published one.
  3. Runtime = fuel on hand ÷ gallons per hour. One tank lasts tank size ÷ gallons per hour; fuel per day = gallons per hour × 24.
  4. Real burn depends on temperature, altitude, fuel age and how the load cycles (a fridge compressor starts and stops). Treat the answer as a planning estimate and keep spare fuel.
  5. Run a generator outside only, at least 20 feet from doors, windows and vents (CDC).

Worked example

A Honda EU2200i is rated 1,800 W with a 0.95 gallon tank: 8.1 h at 1/4 load (0.117 gal/h) and 3.2 h at rated load (0.297 gal/h). A 600 W load is 33% of rated, 1/9 of the way from 1/4 load to full load, so it burns about 0.117 + (0.297 − 0.117) × 0.111 = 0.137 gal/h. Five gallons last 5 ÷ 0.137 = 36.4 hours, and each tank fill lasts about 6.9 hours.

Sources

Questions

My generator isn't in the list. What do I enter?

Pick Custom and type four numbers from its manual or spec sheet: rated (running) watts, tank size, runtime at 1/4 load and runtime at rated load.

How do I work out my load in watts?

Add up the running watts on each appliance's label, or read them with a plug-in watt meter. Motors such as fridges and pumps also draw a short surge when they start; the generator must be able to supply it, but this calculator uses running watts.

Is it safe to run a generator in the garage with the door open?

No. The CDC says to keep it outside and at least 20 feet from doors, windows and vents, because of carbon monoxide.

Does it do batteries and solar?

This page covers fuel generators. The full BackupHours app also plans home batteries, solar with storage and EVs that can power the house.

By the All Things AI team · Last tested · Questions or a wrong number? support@allthingsn.com

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