Garden Generator runtime & fuel cost calculator
Generator runtime is always quoted at a convenient load — usually 25% or 50% — and rarely at whatever you are actually running. Working from your real load tells you how often you will be refuelling at 3am, and how much fuel to have standing by before a storm.
How this is calculated
Fuel burn (gal/hr) = load in kW ÷ the electrical energy a gallon yields in a generator. Runtime = tank capacity ÷ burn rate. Fuel for an outage = burn rate × hours.
Source: Energy per gallon derived from fuel heating values and typical small-generator efficiency (roughly 25-30%)
Worth knowing: The kWh-per-gallon figures are approximations for typical portable generators; inverter models running at part load do appreciably better, and old or heavily loaded units worse. Check your own generator's published runtime at a stated load and adjust. Propane is quoted per liquid gallon — a standard 20 lb barbecue bottle holds about 4.6 gallons.
Common questions
How many watts do I actually need?
Add the running watts of what you must keep going — a fridge is around 150 W running, a furnace blower 500 W, a well pump 1,000 W, lights and phones very little. Then allow headroom for starting surges: motors can draw two to three times their running figure for a second or two when they kick in.
Why does propane give fewer hours per gallon?
A gallon of liquid propane holds about 91,500 BTU against roughly 114,000 for gasoline, so the same volume carries less energy. Propane compensates by storing indefinitely without degrading, which gasoline does not — it is the better choice for a generator that sits unused for years.
Can I run a generator in the rain?
Not unprotected. Use a purpose-made generator cover or an open canopy that keeps rain off while leaving it fully outdoors and well ventilated. Never move it into a garage or shed to keep it dry — that is the single most common way people are poisoned by carbon monoxide during an outage.
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