How it works

The basic equation

runtime = fuel available ÷ fuel use per hour. Everything else is about estimating fuel use per hour as honestly as possible.

Two modes: Quick Estimate vs Manufacturer Data

Quick Estimate is for when you don't know your generator's published runtime. It uses a generic fuel-consumption pattern for typical portable generators. A generic model cannot perfectly predict any specific generator — two units with the same wattage can burn noticeably different amounts of fuel — so results are labeled a Planning Estimate and shown as a range.

Manufacturer Data is for when you have your generator's published runtime specification. A manufacturer's published figure generally provides a better baseline for that specific generator than any generic model, so this mode starts from the numbers you enter. Results are only described as based on manufacturer specifications when this mode is in use.

GeneratorRuntime.com has not tested generators itself, and no manufacturer specifications are built into the calculator. Both modes produce estimates; real-world conditions always affect runtime.

Load

Load is the average electrical draw as a share of the generator's running (rated) watts. When you pick appliances, average load = Σ (running watts × quantity × "on %"). The "on %" accounts for appliances that cycle, such as refrigerators (default 50%). Loads below 10% are treated as 10% because an engine never burns less than its idle consumption.

Quick Estimate model

Fuel use per hour is modeled as a straight line with a fixed idle component:

gallons/hour = rated kW × (a + b × load)

• Gasoline, conventional: a = 0.05, b = 0.10
• Gasoline, inverter: a = 0.025, b = 0.12
• Diesel, conventional: a = 0.035, b = 0.075
• Diesel, inverter: a = 0.02, b = 0.085

These coefficients are generic planning values chosen to sit near typical published runtimes of portable and standby generators (for example, about 0.75 gal/h for a 7,500 W conventional unit at 50% load). They are not measurements of any particular model.

Propane and natural gas use the gasoline curve converted by energy content: propane gallons = gasoline gallons × 114,000 ÷ 91,500 (≈ 1.25), and natural gas cubic feet = gasoline gallons × 114,000 ÷ 1,030 (≈ 111). Enter the generator's rating on that fuel, since dual-fuel units produce less power on gaseous fuels.

Because the model is generic, results are shown as a ±25% range on fuel use.

Manufacturer Data model

Your spec gives a baseline rate: rate at spec = tank capacity ÷ rated runtime. We scale it to your load using the shape of the same load curve:

rate at your load = rate at spec × (a + b × your load) ÷ (a + b × spec load)

When your load matches the spec load, the ratio is exactly 1, so the middle of the result equals the manufacturer's runtime — the generic coefficients do not alter your baseline. At other loads, the calculator is estimating how runtime changes as load changes. Results are shown with a narrower ±10% range. For natural gas, manufacturers publish consumption (cubic feet per hour) rather than runtime, so that figure is used as the baseline instead. We never supply or assume manufacturer data.

Fuel units

• Propane: 1 gallon ≈ 4.2 lb. A 20 lb cylinder ≈ 4.7 gal; 30 lb ≈ 7.1 gal; 40 lb ≈ 9.5 gal; 100 lb ≈ 23.8 gal.
• Bulk propane tanks are sized by water capacity and filled to about 80%; the gauge reads percent of capacity.
• Natural gas: about 1,030 BTU per cubic foot; 1 therm = 100,000 BTU ≈ 97 cubic feet.

Starting (surge) load

We assume one motor starts at a time while everything else runs: max starting load = Σ running watts + largest (starting − running) extra. If that exceeds your generator's surge rating (or its running rating when no surge rating is entered), you'll see a warning.

Known limitations

• The Quick Estimate coefficients are generic and can be off by more than 25% for some models.
• The load curve is linear; real engines are not perfectly linear.
• Temperature, altitude, fuel quality, and maintenance are not modeled.
• Small propane cylinders can struggle to vaporize fast enough in cold weather at high loads.
• Appliance wattages are typical values; your appliances may differ significantly.