Propane Generator Run Time Calculator

Five generators, five published propane figures. Each row is what the maker prints on its own product page, plus the efficiency that figure works out to once you put the tank energy on the same basis the calculator uses.

The last two rows print a run time without saying what tank it was measured on, so there is nothing to divide by. The first three do, and the arithmetic is the same every time: a full 20 lb cylinder is 20 / 4.2 = 4.76 gallons, and 4.76 x 91,420 / 3,412 = 127.6 kWh of fuel energy. Champion's 201121 turns that into 3,600 W for 5 hours, which is 18 kWh out of 127.6 kWh, or 14.1%. Run the same division on the other two and you get 13.3% and 7.9%. The calculator defaults to 14%, the average of the two half-load figures, which is why it reproduces Champion's and Generac's published hours instead of beating them.

1. Find your propane weight. A standard BBQ tank holds 20 lbs of propane when full. Larger tanks for whole-house generators hold 100-500 lbs. Weigh the tank and subtract the empty weight (stamped "TW" on the collar) to find remaining fuel.
2. Add up your loads. List every device you will run simultaneously. A refrigerator draws 150-400W (check the compressor rating, not the running average). Space heater: 1,500W. Sump pump: 800-1,500W starting, 400-800W running.
3. Check your generator efficiency. One pound of propane holds about 21,767 BTU: 91,420 BTU per gallon (AFDC, higher heating value) divided by 4.2 lb per gallon (PERC, at 60°F), or roughly 6.38 kWh. How much of that reaches your sockets is the only figure you have to judge. Three makers' published propane run times work out to 7.9% at quarter load and 13.3-14.1% at half load. Standby sets are better: PERC's own fuel table, supplied by Generac, works out to 14-24% at half load and 17-27% at full load for 8 kW to 150 kW machines.
4. Run the calculator. Enter your values above. Then add a 15% safety buffer — generators lose efficiency as altitude increases and temperatures drop.

A propane generator is a heat engine, so most of the fuel leaves as heat and exhaust. The published numbers say how much. PERC's propane generator fuel table, supplied by Generac, prints fuel use in BTU per hour for standby sets from 8 kW to 150 kW at half and full load. Divide the electricity produced by the fuel burned and the answer lands between 14% and 24% at half load, and between 17% and 27% at full load. Portable generators do worse: the three makers' run times in the table above imply 13.3% and 14.1% at half load.

Light load is where the losses bite. Champion publishes 14.5 hours at quarter load for its 100396 inverter on a 20 lb cylinder, and that works out to 7.9%, roughly half the figure the same class of machine reaches at half load. The engine still has to spin, pump and overcome its own friction whether it is delivering 700 W or 2,800 W. This is also why the calculator's answer drops faster than you might expect when you raise the load: it holds efficiency constant, so it cannot show the part-load penalty on its own. PERC's two columns can: doubling the load from half to full raises fuel use by less than double, which makes full-load run times 39% to 48% shorter than half-load ones rather than exactly half.

Altitude costs power on top of that. Engines lose roughly 3 to 3.5% of rated output per 1,000 feet of elevation, so a 5,000 W generator at 5,000 feet delivers about 4,250 W. You are then running a higher share of a smaller machine, which is the light-load penalty working against you from the other direction.