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Generator size and fuel runtime
Enter running watts, surge watts, and a tank of gasoline, diesel, or propane. See the generator rating to shop for and how long that tank lasts.
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Running watts and surge watts
Running watts are what the generator must supply continuously. Surge watts are the brief peak when a motor starts: a fridge, a pump, or a power tool. Shop for a generator whose running rating covers the first number and whose surge rating covers the second. If the surge is more than twice the running load, the surge number is the one that matters.
Fuel is estimated per kilowatt-hour at the load you pick: about 0.5 litres of gasoline, 0.32 litres of diesel, or 0.4 kilograms of propane. A small engine’s manual will differ. A standard 20 lb propane cylinder holds about 4.5 kg of propane, not 20 lb of burn time.
Half-load is included because a generator sized for a pump’s startup spends most of its time well below that peak, and that is when you want the fuel figure.
How to read a tank
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Step 1
Add up the loads you will run together
Do not add every appliance in the house unless they really run at the same time.
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Step 2
Put the motor’s surge on its own line
The higher number is what the generator must survive for a few seconds.
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Step 3
Match the tank unit to the fuel
Litres for gasoline and diesel. Kilograms for propane.
Generator questions
Why not a single litres-per-hour number from the box?
Box figures assume one load. This page scales a simple per-kilowatt-hour rate so you can change the watts and see the hours move.
Is propane cleaner to store?
A sealed cylinder stores more calmly than a can of gasoline. The calculator only converts weight into hours.
Where should it run?
Outside, away from windows. Carbon monoxide is the hazard this page does not calculate, and it is the one that matters most.
Related
Runtime only matters for the loads you actually intend to run.