Generator Runtime Calculator
Generator hours, kWh, and refills from fuel and load.
What it does
Generator planning is fuel over burn, then hours times load. Tank size and burn per hour set run time, load turns time into kilowatt hours, and 24 over time sets refills per day. This page keeps those three in one pass.
The default is 7 gal at 0.9 gph with a 1,500 W load. One tank runs 7.8 h, gives 11.7 kWh, and needs 3.1 fills for a full day. Metric defaults convert the same tank to 26.5 L at 3.4 L/h and match the same hours and energy.
Burn belongs to a load. A generator sipping at quarter load can drink double at full load, so brochure figures mislead unless pinned to watts. Meter one real hour at your load when the outage matters.
Safety frames all of it. Run outdoors far from doors and windows, let the unit cool before refueling, and size cords to the load. Numbers never override the manual or local code.
How to use
- Pick imperial or metric first.
- Enter fuel in the tank.
- Enter burn per hour at your load.
- Enter load watts.
- Read hours first, then kWh and fills per day.
Formula
- fuel
- Usable tank fuel in gal or L.
- burn
- Burn at stated load per hour.
- load
- Steady electric load in watts.
Burn must be fuel per hour at the stated load, from the manual or a metered run. Rated tank size can overstate usable fuel, and starting surges draw extra for seconds. Carbon monoxide rules still apply: outdoors only, far from windows.
Worked example
Example: 7 gal portable at half load
7 gal fuel, burn 0.9 gph, load 1,500 W (metric 26.5 L, 3.4 L/h).
- Hours: 7 / 0.9 = 7.8 h.
- Energy: 7.8 x 1,500 / 1,000 = 11.7 kWh.
- Fills: 24 / 7.8 = 3.1 fills/day.
- Metric: 26.5 L at 3.4 L/h gives the same 7.8 h.
- Tool shows 7.8 h, 11.7 kWh, 3.1 fills/day.
Reference table
| Burn | Hours | Fills/day |
|---|---|---|
| 0.5 gph | 14 h | 1.7 |
| 0.75 gph | 9.3 h | 2.6 |
| 0.9 gph | 7.8 h | 3.1 |
| 1.2 gph | 5.8 h | 4.1 |
| 1.5 gph | 4.7 h | 5.1 |
Limits
- Steady load assumed. Cycling loads need averaging.
- Ignores starting surge and altitude loss.
- Usable fuel may sit below rated tank size.
- Safety and code sit outside the math.
Common mistakes
- Using no load burn for a loaded run.
- Reading tank size as usable fuel.
- Forgetting surge on motors and pumps.
- Refueling hot or indoors.
- Undersizing cords for the load.
Try it yourself
Quick checks. Answers sit tucked under each question.
5 gal at 0.8 gph with 2,000 W?
6.3 h, 12.5 kWh, 3.8 fills/day.
5 gal at 0.5 gph, 500 W load?
10 h (5 / 0.5), 5 kWh at 500 W.
Questions
How long will one tank run?
Fuel divided by burn per hour. Here 7 at 0.9 is 7.8 h.
How much energy is that?
Hours times kW load. Here 7.8 h at 1.5 kW is 11.7 kWh.
How many refills per day?
24 divided by hours. Here 3.1 fills per full day.
Does it work in metric?
Yes. Enter liters and L per hour for the same hours and kWh.
Does load change burn?
Yes. Higher load raises burn, so measure burn at your load.
Is this exact?
No. Starting surge, altitude, and old fuel all cut real time. Keep margin.