Calculate hours of runtime, fuel per hour, and operating costs for gasoline, propane, or diesel generators.
A generator's fuel consumption is not fixed — it varies significantly based on the electrical load you're placing on it. Running a 5,000-watt generator at 50% load uses roughly half the fuel it would at full load, which dramatically extends your runtime and reduces operating costs. Most generator specifications publish a runtime figure at 50% load, which is the standard benchmark. Real-world consumption depends on the actual wattage draw of everything you have plugged in, so building an accurate load list before sizing your fuel reserve is essential.
The fundamental formula is straightforward: fuel per hour equals the generator's rated wattage times its load percentage divided by the fuel type's energy density. Gasoline generators consume roughly 1 gallon per 16,000 watt-hours at 100% load. Diesel engines are more efficient — approximately 1 gallon per 18,000 watt-hours. Propane is less energy-dense by volume, which is why propane generators have shorter runtimes per pound of fuel, though they benefit from indefinite storage life and cleaner combustion.
For home backup power, the most important decision is whether you need to run everything at once or just your critical loads. Running a refrigerator, a few lights, a phone charger, and a well pump simultaneously requires roughly 2,000–3,000 watts. Adding central air conditioning pushes that to 5,000–7,500 watts. Whole-house backup that seamlessly runs everything typically requires 10,000–15,000 watts and a transfer switch installed by a licensed electrician. Matching your generator to actual needs rather than buying the largest available unit saves money on equipment, fuel, and maintenance.
Fuel consumption depends on the generator's wattage and how hard you're running it. A common 5,000-watt gasoline generator at 50% load uses approximately 0.5–0.6 gallons of gasoline per hour. At full load it may use 0.9–1.0 gallons per hour. A 10,000-watt generator under heavy load can burn 1.5–2 gallons per hour. Diesel generators are roughly 15–20% more efficient. Propane generators at the same wattage consume more volume of fuel — about 1.5–2 lbs of propane per hour for a 5,000-watt unit at 50% load. Always check your specific model's spec sheet for actual rated consumption figures, as efficiency varies meaningfully between manufacturers and engine types.
Add up the wattage of everything you need to run simultaneously, then add 25% for startup surge loads (motors, compressors, and pumps draw 2–3 times their running wattage at startup). Critical loads — refrigerator (150–400W running), freezer (100–300W running), well pump (750–1,500W running, 2,000–4,500W starting), lights, phone chargers — typically sum to 3,000–5,000 watts. Running central A/C adds 3,500–5,000 watts. For most households running critical loads only, a 5,000–7,500 watt generator is sufficient. To run a full home without compromise, plan for 10,000–15,000 watts and a properly installed automatic transfer switch.
Untreated gasoline degrades in as little as 30 days as its lighter volatile compounds evaporate and it begins to oxidize. With a quality fuel stabilizer added at the time of purchase — products like PRI-G, Sta-Bil, or Star Tron — gasoline can remain usable for 12–24 months when stored in sealed, opaque, food-grade containers in a cool location. Rotate your stock by using old gasoline in your lawnmower, chainsaw, or vehicle every 6–12 months. Never store gasoline near open flames, in living spaces, or in vehicles parked inside a garage. For long-term fuel storage beyond one year, diesel or propane are far superior options.
Inverter generators produce clean, stable sine-wave power that is safe for sensitive electronics like laptops, phones, medical equipment, and modern appliances with variable-speed motors. They are also significantly quieter (50–65 dB vs 65–75 dB for conventional units) and more fuel-efficient because they throttle the engine down when loads are light. The trade-off is cost — inverter generators are 30–60% more expensive per watt of rated capacity. Conventional generators are better for high-wattage applications like running power tools, well pumps, or air compressors where clean power is less critical and raw capacity matters more. If you need to run sensitive electronics during an outage, an inverter generator is the right choice.
Start by listing every device you want to run simultaneously and finding its wattage, either on the nameplate label or in the product manual. Resistive loads (incandescent lights, space heaters, toasters) run at their rated wattage. Motor loads (well pumps, refrigerators, air conditioners, sump pumps) have a separate starting wattage — typically 2 to 3 times their running wattage — that lasts for 1–2 seconds at startup. Add all running watts together, then identify the single largest motor load and add its starting surge to your running total. That peak surge is your generator's minimum required capacity. Size up at least 10–20% beyond that to avoid running at maximum load continuously, which accelerates engine wear.