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Size the generator to your critical loads

A generator sized for critical loads uses far less fuel than one sized for the whole house. Use the running and surge figures from continuity-3.

How

1. Add up the running watts of every critical load you will run at the same time.
2. Find the single load with the highest extra starting watts (usually a pump, fridge or compressor) and add only that one surge, as Eugene Water & Electric Board's sizing method shows.
3. Use the generator sizing calculator below. Example EWEB figures: refrigerator 700 W running plus 2,200 W extra starting; 1/2 HP well pump about 1,050 W running plus 2,200 W extra starting.
4. If your generator charges batteries through an off-grid inverter/charger, size it to the charger input as described in power-15.
5. Allow for altitude. Honda's manual for one inverter generator says output drops about 3.5 percent for each 1,000 ft (300 m) of altitude even with the high-altitude carburettor jet, and more without it. Check your own model's manual for altitude and heat derating.
6. Take fuel use from the generator's own data. Generac lists consumption at half and full load for each model; its 10 kW unit is shown using about 0.97 US gal of propane per hour at half load and 1.48 at full load.
7. Feed that consumption figure into the generator fuel calculator to see how much fuel each autonomy tier needs.

Do these first

• power-15

Sources

1. Generator sizing considerations. Eugene Water & Electric Board, accessed 2026-11
2. How much fuel does my home standby generator use. Generac, accessed 2026-11
3. Ten Steps of Resilient Power. US Cybersecurity and Infrastructure Security Agency (CISA), accessed 2026-11
4. EG2800i generator owner's manual (high altitude operation). Honda Power Equipment (manufacturer), accessed 2026-11

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