Storm Risk · Generators
Cost to run a portable generator for 24 hours
When a storm knocks out power, a portable generator keeps essentials running. But fuel costs add up fast. This guide walks you through the math so you know what to budget before you need one.
The fuel cost depends on three things
A generator's 24-hour cost is not a fixed number. It depends on how much fuel it burns, what fuel costs in your area, and how hard the generator is working. A smaller generator running a few light loads costs far less than a large one feeding an air conditioner.
Manufacturers publish fuel consumption in gallons per hour (or liters per hour). That number changes with the load. A generator running at half power burns less fuel than one at full load. To estimate cost, you need to know your generator's size in watts, how much you'll actually run it, and the price per gallon of fuel where you live.
How to calculate fuel use for your setup
Start by identifying what you plan to run. A refrigerator typically needs 600 watts while running. A laptop charger uses about 90 watts. A space heater draws 1500 watts. Add up the wattage of the items you want running at the same time, then check that your generator can handle the total.
Next, look up your generator's fuel consumption chart. Most portable generators have this in the manual or on the manufacturer's website. The chart shows gallons per hour at 25%, 50%, 75%, and 100% load. If you add up 1800 watts of devices and your 5000-watt generator is running at 36% load, find the fuel rate closest to that percentage.
Here's a worked example: suppose your 5000-watt generator burns 0.5 gallons per hour at 50% load, and you run it for 12 hours a day (not continuously). That's 6 gallons per day. At $3 per gallon (an arithmetic illustration), the daily cost is $18. Over 24 hours of continuous run, it would be $36 at that same load. But if you use only 6 hours a day, cost drops to $9 per day.
Real fuel consumption varies by generator type
Smaller portable generators (3000 to 4000 watts) typically burn 0.4 to 0.6 gallons per hour at half load. Mid-size models (5000 to 7500 watts) often consume 0.6 to 1.2 gallons per hour at the same load. Larger units (10,000 watts or more) may burn 1.5 to 2.5 gallons per hour.
Inverter generators, which produce cleaner electrical output, are often more fuel-efficient than conventional models of the same wattage. Some inverter models adjust their engine speed to match the load, burning less fuel during light-use periods.
Fuel type matters too. Propane generators consume more fuel by volume than gasoline units because propane has lower energy density, but propane is often cheaper per unit and stores longer. Natural gas generators are less common for portable use but avoid the need to store fuel.
Why continuous 24-hour running is rarely realistic
Most homes do not run a generator at full capacity all day. Overnight, you may need only a refrigerator, some lights, and a charger. During the day, the load might spike when the water heater runs or clothes dryer starts, then drop again. This variation significantly reduces fuel use compared to worst-case estimates.
Use the generator sizing tool to match your essential loads to a unit that fits your needs. A generator sized to what you actually run costs far less to operate than an oversized one.
Also plan for refueling. Most portable generators hold 5 to 25 gallons. A 7500-watt model at half load might burn 1 gallon per hour, meaning it would empty a 20-gallon tank in 20 hours. Knowing your burn rate helps you plan fuel resupply during outages.
What fuel prices can vary and how to budget
Gasoline and propane prices fluctuate by region and season. Before storm season, check the price at your local gas station and use that for estimates. A $3 per gallon price is a common starting point in many states, but prices vary widely. Use a realistic local price rather than national averages.
If you are planning for a multi-day outage, fuel supply also matters. Gasoline can be hard to find during widespread outages; propane is sometimes easier to locate. Some owners keep a 5 or 10-gallon jerry can of stabilized fuel on hand so they can refuel without leaving home during an outage.
Maintenance costs beyond fuel
Running a generator continuously stresses the engine. Most manufacturers recommend an oil change after the first 5 to 10 hours of operation, then every 50 to 200 hours depending on the model. An extended outage can trigger maintenance sooner than you'd expect. Budget for oil and filters as part of longer-term backup power costs.
If your generator sits unused for months before a storm, old fuel gums up the carburetor. Add fuel stabilizer to stored gasoline, or drain the tank and carburetor before long storage. Propane does not degrade, but equipment still needs annual inspection for leaks and wear.
Putting it all together before the storm
To know your real 24-hour cost: (1) identify your essential loads in watts, (2) look up your generator's fuel consumption at that load level, (3) multiply gallons per hour by the hours you'll actually run it, (4) multiply gallons by your local fuel price.
During an outage, your actual usage often ends up lower than worst-case planning, which is good news for your budget. Run essential items during the day, reduce nighttime loads, and unplug anything not in use.
Start with the 48-hour checklist to map out what you'll need to power. Then use your load calculations to pick a generator size and estimate fuel costs. Having a clear plan before storm season means no surprises when the power goes out.
Before buying fuel, size the machine itself with the honest Generator Sizing tool, and plan the rest of an outage with the Supply Calculator.
Questions people ask
How many gallons of fuel does a portable generator use in 24 hours?
It depends on the generator size and load. A mid-size generator at 50% load might burn 0.8 gallons per hour, or about 19 gallons per 24 hours. A smaller unit at light load might use only 6 to 8 gallons. Check your generator's manual for the fuel consumption chart at the load percentage you expect to run.
Is it cheaper to run a generator on propane or gasoline?
Propane generators typically consume more fuel by volume (because propane has lower energy density), but propane is often cheaper per gallon and stores indefinitely without degrading. Gasoline is denser, so a gasoline generator burns fewer gallons for the same work. Compare local prices for both fuels and your generator's consumption rates to find the better option for your area.
Can I run a portable generator 24 hours straight without damage?
Most portable generators are designed for continuous operation, but running one all day stresses the engine. Check the manufacturer's recommendations for your model. Many suggest an oil change after the first 5 to 10 hours and every 50 to 200 hours thereafter. Keep the fuel tank full and monitor the engine temperature to prevent overheating.
How do I reduce fuel costs when running a generator during an outage?
Run only essential loads, unplug items not in use, reduce nighttime loads, and avoid starting large appliances at the same time. Use the generator sizing tool to match your unit to what you actually need, not a worst-case scenario. Many outages end before continuous 24-hour power is necessary, which keeps fuel costs lower than worst-case planning.
Sources
This article is educational and is not professional contractor, insurance, or engineering advice. Some links in our articles may earn us a commission at no cost to you, and never change what we recommend.
