Sizing answers “how many kilowatts?” Runtime answers “how long until the tank is empty?” Fuel burn changes with load — half load is not simply twice the hours of full load — so outage planning needs gal/kWh at the load you will actually run.
Fuel Burn and Runtime Basics
Runtime is tank energy divided by burn rate. For liquid fuels:
Burn rate (gal/hr) ≈ Load (kW) × Specific consumption (gal/kWh)
Runtime (hr) ≈ Usable tank gallons ÷ Burn rate (gal/hr)
Specific consumption is not constant. Engines usually burn more fuel per kilowatt-hour near full load than at half load. That is why a generator at 50% load often runs more than twice as long as the same unit at 100% on the same tank — you are producing fewer kW and doing it at a better gal/kWh.
Use the generator sizing calculator to pick capacity. Use fuel/runtime math to size tanks, propane cylinders, and refill intervals for multi-day outages. Transfer equipment is a separate safety requirement — see the generator transfer switch guide.
Default gal/kWh by Fuel Type
Manufacturer fuel curves beat rules of thumb. When you do not have a curve, these planning defaults match common portable and standby estimates (same baselines as the calculator):
| Fuel | ~50% load | ~100% load | Notes |
|---|---|---|---|
| Gasoline | 0.10 gal/kWh | 0.15 gal/kWh | Most portable open-frame units |
| Propane (LP) | 0.12 gal/kWh | 0.18 gal/kWh | Gallons liquid equivalent; or convert from lb |
| Diesel | 0.08 gal/kWh | 0.12 gal/kWh | Typically better specific consumption |
| Natural gas | 0.09 therms/kWh | 0.14 therms/kWh | Pipeline = burn rate only (unlimited runtime) |
Inverter generators are often more efficient than open-frame units at light load — override defaults when the manual publishes a fuel curve. Altitude, temperature, and dirty air filters all move real burn upward.
Load = 3.5 kW. At 0.10 gal/kWh → 0.35 gal/hr. Runtime ≈ 5 ÷ 0.35 ≈ 14 hours. At 100% (7 kW × 0.15 = 1.05 gal/hr) the same tank lasts only ~4.8 hours — not half of 14.
Load Percent vs Actual kW
Load percent is actual kW ÷ rated kW. A 10 kW unit running 4 kW of house load is at 40%. Essential-circuit outages often sit at 30–60% if you sized the generator with headroom for surge. Running near 100% for long periods shortens engine life and empties tanks fast — and leaves no margin for a well pump or AC compressor kick.
- Measure when you can — a kill-a-watt on 120 V loads or clamp metering on feeders improves load % accuracy.
- Convert amps when needed — use the kW / kVA / amps calculator so panel readings become load kW.
- Surge ≠ continuous — motor starts spike watts briefly; fuel burn tracks average continuous load more than momentary surge.
Gasoline goes stale; propane and diesel store better. Stage refill fuel safely outdoors, away from the running set. Do not refuel a hot generator — shut down, cool, then fill.
Sizing Tanks for Outages
Pick a target runtime (12, 24, 48 hours), estimate average load kW, pick gal/kWh for that load band, then solve for gallons:
Gallons needed ≈ Load kW × gal/kWh × Hours
Add margin for cold weather (higher burn), longer outages than the forecast, and the fact that “usable” tank volume is less than nameplate if you will not run dry. Propane users often size by cylinder count (20 lb / 30 lb / 100 lb) using liquid-gallon equivalents (~0.24 gal/lb as a planning conversion in the calculator).
Fuel Type Tradeoffs
- Gasoline: easy to find, poor long storage
- Diesel: better gal/kWh, heavier sets
- Runtime limited by onboard + jerry cans
- Propane stores for outages; slightly higher gal/kWh
- Natural gas: long runtime if utility gas stays up
- Utility gas can fail in disasters — have a backup plan
Estimate burn rate and tank runtime Enter fuel type, tank size, rated kW, and load percent (or actual kW). Compare 50% vs 100% and daily fuel use.
Frequently Asked Questions
Why does 50% load last longer than half of 100% runtime?
Specific fuel consumption (fuel per kWh) is usually better at partial load than at full load. Half the power at a better gal/kWh yields more than half the hours.
Should I run at 50% on purpose?
Light-to-moderate load is easier on the set and stretches fuel. Extremely light loads on diesels can cause wet stacking over long periods — follow the manufacturer’s guidance for minimum load.
How accurate are these estimates?
Planning accuracy. Real burn varies with engine design, altitude, temperature, and maintenance. Prefer the OEM fuel curve when you have it; treat calculator defaults as order-of-magnitude planning.
Does a transfer switch change fuel burn?
No — it changes which loads you can run safely. Fewer circuits usually means lower average kW and longer runtime. Size the switch and the fuel plan together.
Can I use kVA instead of kW?
Fuel tracks real power (kW). If you only have kVA and power factor, convert first with the kW/kVA/amps tools before applying gal/kWh.
Plan your outage fuel Tank size, load, and fuel type — get hours and burn rate.
VoltKit — Electrical Calculators for the Job
Generator sizing, fuel runtime, voltage drop, load calc, and more. Free on Android. Works offline.