Generator Runtime & Fuel Consumption Calculator

Quick answer

Generator runtime is not just tank size ÷ a universal fuel number. Fuel burn changes with engine design and electrical load, and multi-fuel generators can have different continuous watt ratings on gasoline, propane, and natural gas. The most defensible estimate starts with your generator's own published runtime or consumption data.

Enter the manufacturer's load point plus the fuel quantity/runtime or consumption rate. If you have two published load points, the calculator can interpolate between them. It will not extrapolate above your highest verified point just to produce a confident-looking number.

How generator fuel use changes with electrical load

Fuel consumption usually rises as electrical load rises, but not in perfect proportion. An engine still consumes fuel just to keep itself turning, cooling, and regulating voltage/frequency at light load. Inverter generators can reduce engine speed at lighter loads when their economy mode allows it, which changes the curve again.

Real manufacturer data show the problem. Champion's current 201602 tri-fuel manual publishes propane consumption at 0%, 25%, 50%, and 100% load, and natural-gas consumption at those same points. The increments are not linear. That is much better evidence than pretending every extra kilowatt adds the same amount of fuel.

Why the selected fuel's running-watt rating matters

Load percentage is calculated as average load watts ÷ continuous running watts on the fuel actually being used. Starting/peak watts are not the denominator.

For example, Champion currently rates model 201602 at 5,000 running watts on gasoline, 4,500 W on propane, and 4,050 W on natural gas. The same 2,250 W household load is therefore 45% of the gasoline rating, 50% of the propane rating, and about 56% of the natural-gas rating. There is no responsible universal “propane loses 10%” switch to apply across every model; use the actual fuel-specific rating.

Gasoline generator runtime

Gasoline manufacturers commonly publish a tank capacity plus runtime at a stated load. That gives a model-specific fuel rate:

fuel consumption per hour = published fuel quantity ÷ published runtime

If your generator says 7.7 US gal and 12 hours at 50% load, the published test point works out to about 0.64 US gal/h or 2.4 L/h at that load. The calculator can then scale a different amount of gasoline at the same load directly, or interpolate when you supply a second manufacturer point.

We do not automatically subtract 5% from gasoline tank capacity. The deep-research report proposed that as a generic “unusable fuel” allowance, but we did not find a defensible manufacturer or Canadian authority basis for applying it to every portable generator. If your manual specifies usable capacity, use it. Otherwise use the quantity you actually expect to have available and leave practical margin yourself.

Propane generator runtime

A “20 lb propane cylinder” is named for its nominal propane capacity, not its total filled-cylinder weight. Current Worthington portable-cylinder specifications list 20, 30, 40, and 100 lb LPG capacities as distinct cylinder sizes. The calculator therefore uses propane mass directly instead of pretending a second 80% haircut must be applied to the advertised 20 lb capacity.

A retail exchange cylinder may contain less propane than the cylinder's nominal capacity. Use the actual pounds or kilograms supplied on the exchange label or receipt rather than assuming every exchanged 20 lb cylinder contains 20 lb.

Natural-gas generator consumption

Utility natural gas normally does not have a finite runtime while the utility supply remains available. The useful outputs are m³/h, CFH, planned daily consumption, total consumption, and optional variable fuel cost.

Natural-gas flow is model-specific too. Champion's 201602 manual, for example, publishes 26.8 CFH at 0% load, 36.9 CFH at 25%, 50.9 CFH at 50%, and 73.5 CFH at 100%. That same manual specifies a 5–7 inch water-column supply-pressure range for that generator. Those are model-specific numbers, not a sizing rule for every gas generator.

Gasoline vs propane vs natural gas

How much fuel for a 24-, 48-, or 72-hour outage?

Do not automatically multiply by 24 hours/day unless you genuinely plan to run the generator continuously. The calculator asks for hours of generator operation per day because many households deliberately shut down or rotate loads to conserve fuel.

The planning math is straightforward once you have a defensible hourly rate:

• fuel/day = fuel/hour × planned run hours/day
• total outage fuel = fuel/day × outage days
• variable fuel cost = total fuel × your local unit price

An 8-hours/day × 3-day plan and a 12-hours/day × 2-day plan both contain 24 generator operating hours. If load and fuel rate are the same, their calculated fuel requirement is the same. The household experience is not: refrigeration, heating, sump pumping, well-water needs, and overnight risk can make the schedule matter.

Why manufacturer runtime matters

Honda Canada lists the EU2200i at approximately 3.2 hours per tankful at rated load and 8.1 hours at 25% load. Westinghouse currently lists the WGen10500TFc with a 36 L gasoline tank, 19 hours at 25% load and 9.5 hours at 50% load. Those two examples are enough to show why one generic “generator litres per hour” value is nonsense.

When you have two manufacturer points that bracket your target load, ReadyHome uses simple piecewise linear interpolation. We chose that because it stays inside known data instead of fitting a fancy curve to a tiny number of points. If your target load is above the highest point you provide, the calculator stops rather than extrapolating.

Confidence levels

• High confidence: your target load matches a manufacturer runtime/consumption point for the fuel you selected.
• Moderate confidence: your target load falls between two manufacturer points and the tool interpolates between them.
• Planning estimate: your target is below the lowest published point, so the tool holds consumption at that lowest published rate instead of projecting toward zero. This is deliberately conservative.
• Needs more data: your target is above the highest published point. Add another manufacturer point instead of trusting an extrapolation.

Even “high confidence” means high confidence in the calculation from manufacturer data, not guaranteed field runtime. Temperature, altitude, maintenance, fuel condition, and changing household loads still matter.

Load management can stretch fuel much farther than buying a bigger tank

Fuel planning should use average sustained load, not every appliance's starting surge added together. A sump pump or refrigerator may pull a large current for a second while starting, but treating that transient as an all-day load would wildly overstate fuel demand. Use the generator size calculator for starting watts and voltage compatibility; use this tool for multi-hour average load and fuel.

Fuel storage and refuelling safety in Canada

A calculated fuel requirement is not permission to store that amount at home. Storage requirements vary by province, municipality, building type, fuel, and container. For example, Ontario's current Fire Code sets specific limits for Class I liquids in dwelling units and attached garages/sheds; those Ontario numbers should not be copied across Canada as though they were a national residential allowance.

Canadian Propane Association guidance says portable cylinders should be stored upright in a secure, well-ventilated outdoor area away from heat and ignition sources. Health Canada generator guidance says shut the generator down and let it cool before refuelling. Read the full generator safety guide before an outage, not while standing beside a hot engine with a fuel can.

Methodology

The core formulas are deliberately boring:

• load % = average load watts ÷ fuel-specific running watts × 100
• published fuel rate = fuel quantity ÷ runtime
• runtime = available fuel ÷ estimated fuel/hour
• outage fuel = estimated fuel/hour × run hours/day × outage days
• cost = calculated fuel × user-entered local unit price

Exact physical unit conversions are used for US gallons/litres, pounds/kilograms, and cubic feet/cubic metres. Propane liquid-density conversions are intentionally unnecessary in the main calculator because cylinder fuel is more cleanly handled by mass.

Frequently asked questions

How long will a generator run on 20 litres of gas?

It depends on the generator and load. Find a manufacturer runtime or fuel-consumption point at a known load, enter 20 L as available gasoline, and the calculator will estimate runtime from that model-specific rate. If your target load is above the manufacturer's only published point, add another point rather than scaling fuel use linearly.

How long will a 20 lb propane tank run a generator?

At a published manufacturer load point, divide 20 lb by the model's pounds-per-hour consumption — or enter the manufacturer's stated runtime on a 20 lb cylinder directly. The resulting energy/runtime calculation still does not guarantee that a small cylinder can vaporize propane fast enough in cold weather.

Does a generator use half as much fuel at 50% load?

No universal rule says it will. Real manufacturer fuel curves include baseline engine consumption and can behave differently between inverter and conventional designs. Use actual manufacturer points.

Can I use starting watts to calculate generator fuel use?

No. Fuel planning should use average sustained running load. Starting watts are brief motor/compressor surges and belong in generator-capacity sizing.

How much natural gas does a generator use?

Manufacturers commonly publish natural-gas flow in CFH, m³/h, or BTU/h at specified load points. Enter those model-specific values here. Do not infer gas-line suitability from the consumption result.

Can I store whatever amount the outage calculator says I need?

No. The result is theoretical consumption, not a legal or safe storage recommendation. Check your provincial/local fire and fuel-storage requirements and use approved containers and professional fuel systems where required.