How generator runtime is calculated
Runtime is simply the fuel you have divided by how fast the generator burns it: runtime = fuel available ÷ fuel use per hour. The hard part is the second number. Fuel use depends on the generator's size, engine, design, and — most of all — how much electricity you're drawing.
In Quick Estimate mode we use a generic fuel-use curve for your fuel type and generator design, and show a deliberately wide range because we don't know your exact model. In Find My Generator mode you pick your model and we use specifications checked against the manufacturer's published sources; when your load and fuel match a published test point, that runtime is the center of the result, and anything else is clearly labeled as estimated. In Manufacturer Data mode we start from the runtime your manual publishes and adjust it to your load, which is usually much closer to reality than a generic curve. The full formulas are on the How It Works page.
Why generator load affects runtime
The more watts you draw, the harder the engine works and the more fuel it burns. But the relationship isn't proportional: an engine idling with a light load still consumes a substantial amount of fuel. That's why a generator at 25% load doesn't run four times longer than at 100% — often closer to two or three times longer. Keeping the load steady and switching off what you don't need is the easiest way to stretch fuel.
Running watts vs starting watts
Running watts are what an appliance draws continuously. Starting watts (or surge watts) are the short burst electric motors need to get going. A refrigerator might run at about 700 watts but need over 2,000 watts for a moment when its compressor starts. Runtime is driven by running watts; starting watts determine whether the generator can start the appliance at all. The calculator assumes one motor starts at a time while everything else keeps running.
Gasoline generator runtime
Gasoline is the most common portable generator fuel. Typical portable generators burn somewhere between 0.1 and 1.2 gallons per hour depending on size and load. Gasoline goes stale in storage — usually within a few months without stabilizer — so rotate stored fuel and never refuel a hot, running generator.
Propane generator runtime
Propane contains less energy per gallon than gasoline (about 91,500 BTU versus 114,000), so a generator burns more of it for the same output, and dual-fuel generators are usually rated at lower wattage on propane. Cylinders are sold by weight: a 20 lb cylinder holds about 4.7 gallons. Bulk tanks are sized in gallons and filled to about 80%. Propane stores for years without degrading, which makes it popular for emergency preparedness.
Diesel generator runtime
Diesel engines extract more work from each gallon than small gasoline engines, so diesel generators usually run longer on the same amount of fuel, especially at moderate to high loads. Diesel also stores better than gasoline, though it still benefits from treatment and rotation.
Why manufacturer runtime specifications are useful
Manufacturers typically publish a runtime such as "10 hours at 50% load on a full 6.6-gallon tank." That single figure captures the real efficiency of your specific engine far better than any generic model. Enter it in Manufacturer Data mode and we'll scale it to your expected load. Note which load the spec was measured at — a 25% load figure looks much better than a 50% one.
Why real-world runtime can differ from estimates
- Engine efficiency and design vary between models, even at the same wattage.
- Loads rise and fall as appliances cycle on and off.
- Heat, cold, and high altitude reduce engine performance.
- Stale fuel, dirty air filters, and overdue maintenance increase fuel use.
- Appliance wattages vary widely; nameplate values are the best source.
How to plan fuel for a power outage
- List what you actually need to power — food storage, heat, water, medical devices, communications.
- Build that list in the calculator and check the generator can handle the running and starting load.
- Look at the 24-, 72-hour, and 7-day fuel amounts, and plan toward the high end of the range.
- Consider running in intervals (for example, a few hours on, a few off) to stretch fuel.
- Store fuel safely in approved containers, away from the home, and follow local regulations.
- Always operate the generator outdoors, away from doors, windows, and vents.
Frequently asked questions
›How long will a generator run on 5 gallons of gas?
It depends heavily on the generator and the load. As a rough planning figure, a conventional 7,500-watt gasoline generator at half load burns around 0.6–0.9 gallons per hour, so 5 gallons lasts roughly 5½–9 hours. A small 3,000-watt inverter generator at light load can stretch 5 gallons past 24 hours. Your owner's manual runtime spec is the best starting point — enter it in Manufacturer Data mode.
›How long will a generator run on a 20 lb propane tank?
A full 20 lb cylinder holds about 4.7 gallons of propane. A mid-size generator producing around 2,500 watts on propane typically uses roughly 2–3 lb per hour, giving about 6–10 hours. Larger loads can empty a 20 lb cylinder in 4–5 hours. Cold weather can also reduce how fast a small cylinder can vaporize propane.
›Does a generator use less fuel at lower loads?
Yes, but not proportionally. A generator still burns a meaningful amount of fuel even with nothing plugged in, so halving the load does not double the runtime. Inverter generators that throttle the engine down at light loads save the most.
›How much fuel do I need for a 3-day outage?
Multiply your hourly fuel use by 72 hours, then add a safety margin. For example, a conventional 7,500-watt gasoline generator at half load may need roughly 40–70 gallons for 72 hours of continuous running. Running only when needed (for example, a few hours at a time to keep the fridge cold) can cut that dramatically. The calculator shows 8-hour, 24-hour, 48-hour, 72-hour, and 7-day amounts for your inputs.
›What is the difference between running watts and starting watts?
Running watts are the continuous power an appliance needs once it's on. Starting (surge) watts are the brief extra burst a motor needs to start — refrigerators, pumps, and air conditioners can need two to three times their running watts for a second or two. Your generator must handle the total running load plus the largest single starting surge.
›Why is my actual generator runtime different from the estimate?
Real runtime depends on your specific engine, whether it's an inverter, how the load changes over time, temperature, altitude, fuel age and quality, and maintenance. Appliance wattages also vary widely by model. That's why results are shown as ranges and should be used for planning, not as a guarantee.