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Generator Fuel Calculator

Set your generator size and how hard it is working to see the burn rate, how long a tank lasts, how much fuel a day of outage takes, and what a kilowatt-hour costs you.

Where are you?

AWG, NEC, 120/240 V, miles and gallons — we remember this on every calculator.

Generator size

The rated output in kilowatts. A 7,500 W portable is 7.5 kW. If the set is advertised in kVA, multiply by 0.8.

kW
How hard is it working?

The share of rated output you are actually drawing. Most people run a generator at 30–60% during an outage, and part load is where fuel figures diverge most from the headline number.

50 % of rated
Fridge, lights and internet only
10–25%
Essential circuits
30–50%
Most of the house
50–75%
Everything, including AC or heating
75–100%
Fuel

Natural gas is piped, so it never runs out and never needs stabilizer — but it delivers slightly less power and you cannot take it camping.

Fuel tank size

Usable capacity of the tank, not the number stamped on it. Most portables carry 3–9 gallons; a standby diesel set has a belly tank of 25 gallons or more.

gal
Fuel price

Price per gallon of whichever liquid fuel you selected. Propane is usually cheaper per gallon than gasoline but carries less energy, so compare the cost per kWh below rather than the pump price.

per gal
Hours a day you run it

Few people run a generator around the clock. Cycling it — eight hours on, a few off — keeps a freezer cold and roughly halves your fuel bill.

h/day

Your result

Run time on a full tank

8 h 53 min

Burn rate
0.67 gal/h
Output
3.75 kW
Fuel per day
8.10 gal
Cost per hour
$2.16
Cost per day
$25.92
Cost per kWh
$0.58

🪿 The goose says: At 50% load it burns 0.67 gal an hour, so a 6 gal tank lasts 8 h 53 min. Running 12 hours a day costs $25.92 and takes 8.10 gal of fuel.

  • At 50% load this set empties its 6 gal tank in 8 h 53 min. Running 12 hours a day means 1.35 refuels a day and 8.10 gal of fuel.
  • Gasoline goes stale in three to six months, and ethanol-blended fuel absorbs water and gums carburettors faster than that. Treat storage fuel with stabilizer, rotate it through the car twice a year, and never store more than you will use in a season.

Estimate for planning only. Have any electrical work verified by a qualified electrician and installed to code — see the full disclaimer at the bottom of this page.

Fuel burn by load

What a 7.50 kW set on gasoline costs at each point on the dial.

25% load — 1.88 kW
0.45 gal/h 13 h 20 min on a tank · $1.44/h
50% load — 3.75 kW
0.67 gal/h 8 h 53 min on a tank · $2.16/h
75% load — 5.63 kW
0.90 gal/h 6 h 40 min on a tank · $2.88/h
100% load — 7.50 kW
1.13 gal/h 5 h 20 min on a tank · $3.60/h

Consumption is modelled as an idle term plus a marginal term per kilowatt, fitted to published manufacturer consumption tables. It is not a straight proportion of load, which is why quarter load never costs a quarter as much.

What a kilowatt-hour costs on each fuel

Same 7.50 kW set at 50% load, delivering 3.75 kW.

Gasoline
$0.58 / kWh 0.67 gal/h · $2.16/h
Propane
$0.60 / kWh 0.75 gal/h · $2.24/h
Diesel
$0.40 / kWh 0.38 gal/h · $1.50/h
Natural gas
$0.21 / kWh 64.8 ft³/h · $0.80/h

At $3.20/gal gasoline, $3.00/gal propane, $4.00/gal diesel and $1.20/therm gas. For comparison, grid electricity is typically $0.17 per kWh — a generator is emergency power, never cheap power.

How this generator fuel calculator works

Almost every fuel figure you will find quoted is consumption at full load, and almost nobody runs a generator at full load. That single mismatch is why people budget half the fuel they end up needing.

An engine has two separate fuel costs. One is fixed: it burns fuel simply to keep turning at 3,600 rpm, whether or not anything is plugged in. The other is proportional: extra fuel for every kilowatt it actually delivers. Add them and consumption is a straight line, not a proportion — which is why a generator at quarter load uses closer to a third of its full-load fuel rather than a quarter.

Fitting that line to published manufacturer consumption tables gives a per-kilowatt idle term and a per-kilowatt marginal term for each fuel. Those two coefficients reproduce the 25%, 50% and 100% columns in the manuals to within a few percent, across sizes from a 2 kW inverter to a 48 kW standby set.

The fuels themselves then differ in energy density. Diesel carries 138,700 BTU per gallon and burns in a more efficient cycle; gasoline carries 120,000; propane only 91,500, which is why a propane generator empties a tank faster than a gasoline one of the same size even though propane is cheaper per gallon.

  1. Step 1 Fuel burn per hour burn = rated_kW × (idle_term + marginal_term × load_fraction) Gasoline 0.030 + 0.120; propane 0.036 + 0.127; diesel 0.010 + 0.080 gallons per rated kW. Natural gas 2.96 + 11.36 cubic feet.
  2. Step 2 Run time on a tank hours = usable_tank_capacity ÷ burn_per_hour
  3. Step 3 Fuel for a day of outage per_day = burn_per_hour × hours_you_run_it
  4. Step 4 What it costs to run cost_per_hour = burn_per_hour × fuel_price
  5. Step 5 Cost per kilowatt-hour cost_per_kWh = cost_per_hour ÷ (rated_kW × load_fraction) The number worth comparing against your electricity rate — and against a bigger battery.
  6. Step 6 Natural gas pricing therms = cubic_feet ÷ 97; cost = therms × price_per_therm One therm is 100,000 BTU, and natural gas runs about 1,030 BTU per cubic foot.

Example: a 7.5 kW gasoline portable at 50% load burns 7.5 × (0.030 + 0.120 × 0.5) = 0.68 gallons an hour. A 6-gallon tank lasts 8 hours 53 minutes. Running it 12 hours a day takes 8.1 gallons — about $26 a day at $3.20, or $0.58 per kilowatt-hour delivered, roughly three and a half times what the grid charges.

Assumptions & caveats

Everything this calculator quietly assumes on your behalf. If one of these does not match your situation, the answer will be off.

  • Consumption uses a two-term linear model fitted to published manufacturer data. Real engines vary by 15% either way depending on age, tune, altitude and how well the governor holds speed.
  • The load percentage is of rated output, not of the surge rating. If your generator is advertised at 9,000 starting watts and 7,500 running watts, the running figure is the one to enter.
  • Tank capacity means usable capacity. Portables cannot run a tank completely dry without picking up sediment, and bulk propane tanks are filled to 80% by law to leave room for expansion.
  • Propane figures assume the tank can vaporise fast enough to keep up. In freezing weather a small cylinder cannot, and the generator will starve regardless of how much gas is left in it.
  • Natural gas is assumed to be at normal supply pressure with a correctly sized line. An undersized gas line is the most common cause of a standby set failing to reach its rated output.
  • Energy content is taken as 120,000 BTU per gallon for gasoline, 91,500 for propane, 138,700 for diesel and 1,030 BTU per cubic foot for natural gas. These are higher heating values and vary a little by season and region.
  • No allowance is made for the fuel used during weekly exercise cycles, which for a standby set add up to a few gallons a year on top of anything an outage costs.

Frequently asked questions

How much fuel does a generator use per hour?

A 7.5 kW gasoline portable at half load burns about 0.68 gallons an hour, and about 1.13 gallons at full load. Diesel sets of the same size use roughly half that, propane about 30% more by volume. The load matters as much as the size, because an engine burns fuel just to spin.

How long will a generator run on a tank of gas?

A typical 6-gallon portable at half load runs 8 to 9 hours, which is why manufacturers quote "8 hours at 50% load" on the box. At full load the same tank gives about 5 hours. Larger standby sets on natural gas never run out at all, since they are piped.

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

A 20 lb barbecue cylinder holds about 4.6 usable gallons, which runs a 7.5 kW generator for roughly 6 hours at half load or 3 hours 40 minutes at full load. On a small 2 kW inverter it stretches to about 24 hours. In freezing weather expect less, because a small cylinder cannot vaporise gas fast enough to keep up.

How much propane does a whole house generator use?

A 22 kW standby set burns about 2.2 gallons an hour at half load and 3.6 at full load. A 500-gallon tank holds 400 usable gallons, so it will carry that generator for roughly 7 days of continuous running at half load, or two to three weeks if you cycle it 12 hours a day. Sizing the tank is really a question of how long an outage you want to survive without a delivery.

Is propane or gasoline cheaper to run a generator?

Gasoline is usually cheaper per kilowatt-hour despite similar pump prices, because propane carries only about three-quarters of the energy per gallon. Propane wins on everything else: it stores indefinitely, burns cleaner, does not gum carburettors and can be delivered in bulk. For a generator that sits unused most years, propane is the better choice even at a small running-cost penalty.

How much does it cost to run a generator for a day?

A 7.5 kW gasoline portable at half load, run 12 hours a day, uses about 8 gallons — roughly $26 at $3.20 a gallon. A 22 kW standby on natural gas costs more like $20 to $30 for the same period, and never needs refuelling. Either way you are paying three to five times the grid rate per kilowatt-hour, which is the price of having power at all.

Why does my generator use so much fuel at low load?

Because most of the fuel goes into keeping the engine turning rather than into the electricity. A conventional generator runs at a fixed 3,600 rpm whatever the load, so idle consumption is nearly constant. Inverter generators solve exactly this problem by varying engine speed with demand, and they can cut light-load consumption by half.

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Safety disclaimer

Never run a generator indoors, in a garage, in a crawlspace or within 20 feet of a window or door. Carbon monoxide from a portable generator is odorless and kills in minutes; it is the leading cause of death after a storm. Let the engine cool before refuelling — petrol spilled on a hot muffler ignites — and store fuel in approved containers away from the house.