A generator’s fuel cost depends on three factors: the generator size (kW), the load percentage (how much of its capacity you are using), and the fuel type. A 5 kW gasoline generator at 50 percent load costs approximately 1.50 to 2.00 dollars per hour. A 10 kW unit at the same load costs 2.50 to 3.50 per hour. Our Generator Sizing Calculator estimates fuel consumption and hourly cost for any configuration.
Generator Fuel Consumption by Size and Fuel Type
| Generator Size | Gasoline (50% load) | Diesel (50% load) | Propane (50% load) |
|---|---|---|---|
| 3 kW | 0.4 gal/hr ($1.40) | 0.3 gal/hr ($1.20) | 0.5 gal/hr ($1.38) |
| 5 kW | 0.6 gal/hr ($2.10) | 0.4 gal/hr ($1.60) | 0.8 gal/hr ($2.20) |
| 7.5 kW | 0.9 gal/hr ($3.15) | 0.6 gal/hr ($2.40) | 1.2 gal/hr ($3.30) |
| 10 kW | 1.2 gal/hr ($4.20) | 0.8 gal/hr ($3.20) | 1.6 gal/hr ($4.40) |
| 15 kW (standby) | 1.8 gal/hr ($6.30) | 1.1 gal/hr ($4.40) | 2.4 gal/hr ($6.60) |
| 20 kW (standby) | 2.4 gal/hr ($8.40) | 1.5 gal/hr ($6.00) | 3.2 gal/hr ($8.80) |
Prices assume $3.50/gal gasoline, $4.00/gal diesel, $2.75/gal propane (2026 US averages). Actual fuel prices vary by region and season.
The Fuel Cost Formula
Fuel consumption (gal/hr) = Generator rated consumption at 50 percent load × (Actual load / 50 percent). A generator at 25 percent load uses roughly 60 percent of its 50 percent load consumption — not half, because the engine has a baseline fuel requirement even at idle. At 75 percent load, consumption is roughly 85 percent of full-load consumption. This is why right-sizing your generator matters: an oversized generator running at 20 percent load wastes 40 percent of its fuel on engine friction and heat rather than useful electricity.
Daily and Monthly Generator Costs
For a typical power outage where you run the generator 8 hours per day, a 5 kW gasoline generator costs about 17 dollars per day (0.6 gal/hr × 8 hours × $3.50). Over a 3-day outage: $50. Over a week: $120. For frequent or extended outages, diesel generators are 30 to 40 percent more fuel-efficient than gasoline, and the savings compound over days of operation. Propane is less efficient per gallon but stores indefinitely (gasoline degrades in 6 to 12 months without stabilizer), making it the better choice for emergency-only generators that sit idle most of the year.
Generator vs Solar: Long-Term Cost Comparison
A 5 kW generator costs $800 to buy plus $17/day in fuel when running. A 5 kW solar system with battery costs $14,000 to $18,000 installed but has zero fuel cost for 25+ years. If you run the generator 30 days per year (common in storm-prone areas), fuel alone costs $510 annually — plus $200 to $400 per year in maintenance (oil changes, air filters, spark plugs). Over 10 years, the generator costs $7,100 to $9,000 in fuel and maintenance alone, excluding the purchase price. The solar system costs more upfront but produces free electricity every day, not just during outages. Compare both options in our Solar vs Generator Calculator.
Generator Safety: Carbon Monoxide
Generators produce carbon monoxide (CO) — a colorless, odorless killer gas. Never run a generator indoors, in a garage, or in any enclosed space. Place it at least 20 feet from the house with exhaust pointed away from windows. Install battery-powered CO detectors on every floor. More people die from generator CO poisoning during outages than from the outages themselves.
Transfer Switch Requirements
Never backfeed a generator into a wall outlet — this electrocutes utility workers. Use a manual transfer switch ($200-500 installed) or interlock kit ($100-200) to safely connect to your home panel. Standby generators include automatic transfer switches. Our Generator Sizing Calculator sizes both the generator and transfer switch.
Fuel Storage Tips
Gasoline lasts 3-6 months (12 with stabilizer). Diesel lasts 12 months. Propane stores indefinitely. For emergency-only generators that sit idle between outages, propane is the best fuel choice — no degradation, no spill risk, and refillable tanks are available at most hardware stores.
Energy Cost Impact
Monthly cost formula: (Watts × hours per day × 30) ÷ 1,000 × electricity rate. At the US average of $0.16/kWh, a 1,500W appliance running 2 hours daily costs $14.40/month. A 100W device running 8 hours costs $3.84/month. Even small wattage reductions compound over a year — saving 50W over 8 daily hours saves $23 annually. Calculate all your appliances combined in our Electricity Bill Calculator.
Measuring Your Actual Consumption
Published wattage ranges are approximations. Your specific model may draw more or less depending on age, settings, and condition. For exact numbers, use a plug-in watt meter (Kill A Watt P4400 or Poniie PN2000, both under $25) between the appliance and the wall for 24 hours. The meter captures running watts, peak surge, and total kWh consumed — the precise values for sizing solar panels, batteries, or generators. Look up typical values first in our Appliance Wattage Database with 140+ devices.
Sizing Solar Panels and Battery Backup
Use the average wattage for energy calculations (how many panels you need) and the startup surge for inverter and generator sizing (how big the inverter must be to handle the peak). Our Solar System Calculator handles both — enter all your appliances and get the complete system specification. For battery backup runtime, use our Battery Backup Calculator. For generator sizing, use our Generator Sizing Calculator which accounts for motor startup surge automatically.
Reducing Energy Consumption
The cheapest watt is the one you never use. Before sizing a solar system or generator, consider whether you can reduce consumption first. Upgrading to Energy Star rated appliances typically reduces consumption by 20 to 50 percent. Using timers and smart plugs eliminates standby power draw (which accounts for 5 to 10 percent of total home electricity use). Switching from incandescent or halogen to LED lighting reduces lighting consumption by 75 percent. Every watt you eliminate from your load reduces the solar panel count, battery size, and inverter capacity — saving thousands of dollars on the system.
