# Generator Fuel Calculator: Gas or Propane · Larder & Well > Source: https://larderandwell.cc/power/generator-fuel-calculator-how-much-gasoline-or-propane-to-store Off-grid power ## Generator Fuel Calculator Enter the generator, what it carries, the fuel, the hours a day and the days you want covered. You get the gallons or pounds, the cans or cylinders, and how to keep them fresh. By Ruth Calloway, November 14, 2026 A mid-size portable generator of about 5,500 running watts, carrying a refrigerator, a freezer, a furnace fan and some lights, burns about half a gallon of gasoline an hour. Run it eight hours a day in shifts and two weeks takes about 56 gallons, which is twelve 5-gallon cans. On propane the same two weeks takes about 300 pounds, fifteen grill-size cylinders or three 100-pound ones. Put your own generator and load in below to get the figure for your house. Your generator and your outage Generator running watts The running (rated) figure for the fuel you will burn, 500 to 20,000 Average load in watts What it carries most of the time, from the sizing guide Fuel Gasoline Propane Diesel Hours a day it runs 0.5 to 24\. Shifts of a few hours are common Days to cover 1 to 180 Work out the fuel to store #### Gasoline for 14 days * **0.50 gal**Per hour * **4.0 gal**Per day * **56 gal**In total * **12**5-gallon cans A 5,500 W generator carrying 2,000 W for 8 hours a day. One 5-gallon can runs this load for about 10 hours. 56 gallons is more than the 25 gallons of gasoline that International Fire Code guidance allows at a house, so check with your fire marshal, or move part of the plan to propane. Add stabilizer the day you fill each can, mark the date on it and pour the oldest can into a car each month so the stock turns over within a year. If you do not yet know your average load, work it out first with our guide to [sizing a generator for the things a house must run](/power/what-size-generator-do-i-need-for-my-house). It lists what a refrigerator, a freezer, a well pump and a furnace fan draw, which is the number this calculator needs. ### How the calculator estimates fuel A generator engine burns fuel in two parts. Some goes into just keeping the engine turning at speed, and that share depends on the size of the machine, whether anything is plugged in or not. The rest rises with the load. That is why a big generator idling along on a single refrigerator wastes so much gas, and why half load is the figure every maker prints. The calculator uses one formula for that shape. For gasoline it allows a fixed 0.041 gallons per hour for each kilowatt of running rating. On top of that it adds 0.138 gallons per hour for each rated kilowatt, multiplied by the share of the rating the load uses. A machine at 50 percent therefore burns 0.11 gallons hourly per rated kilowatt. The split between the fixed part and the load part comes from the burn Generac publishes for a 7.5 kW propane unit, as reported by [SlashGear](https://www.slashgear.com/1989834/how-long-propane-generator-run-on-20-pound-tank): 0.87 gallons hourly at half load and 1.42 at full. The level comes from the run times that makers print for their portable gasoline models. A Champion with 5,500 running watts is listed at 10 hours on its 5.9-gallon tank, about 0.59 gallons each hour, and the formula gives 0.61\. A 3,500-watt class portable listed near 0.4 comes out at 0.39. Propane and diesel are worked out from gasoline by their energy. The federal Energy Information Administration's conversion table says [a gallon of motor gasoline holds 120,214 Btu, propane 91,452 and diesel 137,381](https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php). So the same work takes about 1.31 gallons of propane, or 0.87 gallons of diesel, for each gallon of gasoline. Propane is then turned into pounds at about 4.24 pounds a gallon, the figure that puts 20 pounds in a grill cylinder of about 4.7 gallons. This is an estimate, and it leans to the safe side. Inverter generators throttle down at light load and often beat it by a sixth or more. Diesel engines usually burn less than their energy share, so the diesel answer is generous. Cold weather, a dirty air filter and long runs near full rating all push real burn above it. The number printed on your own generator's spec sheet or manual beats any formula, and if it says your machine runs 9 hours at half load on a full tank, use that. ### Worked example: gas, then propane Take a portable rated at 5,500 running watts on gasoline. In an outage it carries a refrigerator, a chest freezer, a gas furnace's blower, a few LED lights and phone chargers. With the compressors cycling, that averages around 2,000 watts. 1. Share of rating in use: 2,000 ÷ 5,500 = 0.36. 2. Burn per hour: 5.5 × (0.041 + 0.138 × 0.36) = 0.50 gallons. 3. Burn per day at eight hours: 0.50 × 8 = 4.0 gallons. 4. Fourteen days: 4.0 × 14 = 56 gallons, which is 11.2 cans, so twelve 5-gallon cans. Now switch the same generator to propane. Dual-fuel machines make about 10 percent less power on propane, so its propane rating is about 4,950 watts and the same 2,000-watt load is 40 percent of it. The formula gives 0.48 gallons of gasoline's worth an hour, times 1.31, or 0.63 gallons of propane. That is 2.7 pounds an hour, 21 pounds a day and about 300 pounds for two weeks. Fifteen 20-pound cylinders do it, or three 100-pound cylinders. Two weeks of gasoline on that schedule already passes what most fire codes allow at a house. That one result decides a lot of plans, and it is the reason long-outage households drift toward propane. ### How long a generator runs on 5 gallons These are the calculator's results for one 5-gallon can of gasoline at two common loads. On a mid-size machine, half load is roughly the cold storage plus a furnace fan, and quarter load is one fridge and the lights. | Running watts | Half load | Quarter load | | ------------- | --------- | ------------ | | 2,000 W | 23 h | 33 h | | 3,500 W | 13 h | 19 h | | 5,500 W | 8 h | 12 h | | 7,500 W | 6 h | 9 h | | 9,500 W | 5 h | 7 h | Look at what the bigger machine costs you. A 9,500-watt generator running the same refrigerator burns roughly twice the fuel of a 3,500-watt one, because so much of its gas goes into spinning a large engine. For an outage measured in weeks, the right-size generator saves more fuel than any stabilizer or storage trick. ### Run it in shifts Few households need a generator running around the clock. [Ready.gov](https://www.ready.gov/power-outages) says a closed refrigerator keeps food cold for about four hours and a full freezer holds its temperature for about 48 hours. So a pattern of three or four hours in the morning and the same in the evening keeps both cold, charges phones and batteries, runs the well pump to fill the pressure tank and the water jugs, and uses about half the fuel of running all day. Plan the heavy jobs, a washing machine or a well pump refilling a cistern, inside those windows so the load runs while the engine is already burning fuel. Night running is where fuel quietly disappears. A generator left on overnight for one lamp and a phone charger burns its fixed share of fuel for very little work. A small battery pack or power station charged during the day covers those hours for nothing. Running in shifts also means fewer refueling stops, and every refuel has to happen with the engine off and cool, outdoors. ### Storing gasoline for a generator Gasoline is the fuel most generators burn and the hardest to keep. Honda warns that its properties [can quickly lead to stale fuel](https://direct.powerequipment.honda.com/generators/eu-generator-fuel-recommendations), and treats anything stored past four weeks as storage that needs a plan. Its advice for the fuel can is short: * Add stabilizer to the can every time you fill it with fresh fuel, following the stabilizer maker's dose. * Use a self-sealing can, which keeps vapor in and moisture out. * Keep the cans and the generator out of the sun in a cool, dry place, because heat and air speed up the breakdown. * Avoid fuel with more than 10 percent ethanol. Ethanol pulls water from the air into the fuel. Stabilizer buys time but has limits. Gold Eagle says its [STA-BIL fuel stabilizer keeps fuel fresh for up to 24 months](https://www.goldeagle.com/brands/sta-bil/). Honda's own guidance is more cautious: stabilizers have a shelf life of their own, they will not bring stale fuel back, and once stabilized fuel has sat in a can for 12 months it should go into a car's tank while that tank is fairly full. I would plan around Honda's 12 months and treat anything past it as a bonus. The legal ceiling is lower than most people expect. The [Lehi Fire Department's guidance](https://www.lehi-ut.gov/departments/fire-department/fire-prevention/home-fuel-storage-limits/), based on the International Fire Code, caps gasoline at a house at 25 gallons, ideally in a detached garage or shed, no more than 10 gallons in an attached garage and none at all in a basement. Empty cans count as full. Above 5 gallons it asks for a 2A10BC fire extinguisher kept 10 to 50 feet from the cans. Gas in a vehicle's tank does not count against the total, which is one honest way to carry more: keep every car topped up when a storm is coming. Your own town's code may differ, so ask the fire marshal before you buy the twelfth can. ### A rotation plan for gasoline Rotation is what makes a gasoline stock work. Number each can and write the fill date on a strip of tape. Each month, pour the oldest can into the car, refill it at the station, add stabilizer and put it at the back of the line. With twelve cans that turns the whole stock over in a year, exactly inside Honda's window. With five cans, empty one every couple of months and no can is ever older than ten months. Do the same with the generator itself. Honda says a generator run less than once a month should have its carburetor drained, because the small amount of fuel in the float bowl goes stale first. A monthly run of 20 or 30 minutes under some load, outdoors and well away from the house, keeps fresh fuel in the system and tells you it starts before you need it. ### Propane for a long outage Propane's big advantage is that it does not go stale in a sealed cylinder, so a stock bought this year will still run the generator years from now with no stabilizer and no rotation. The cost is bulk and power: it carries about three quarters of the energy of gasoline per gallon, and a dual-fuel generator makes roughly 10 percent less on it. Storage rules are also kinder. The same Lehi guidance allows portable cylinders up to 25 gallons of total capacity, which it describes as five 20-pound grill cylinders or one 100-pound cylinder, kept outside the house in a detached garage or shed. It allows only two 1-pound disposable cylinders inside a home or attached garage, and it asks for at least 10 feet between stored propane and stored gasoline. A household that wants more can install a permanent tank, and the International Fire Code allows up to 2,000 gallons of water capacity in populated areas with a permit and a licensed installer. That is where a long-outage plan usually ends up. Ninety days of the worked example above, at 21 pounds a day, is around 1,900 pounds, or about 450 gallons. A 500-gallon tank, filled to the usual 80 percent, holds about 400 gallons, which covers most of a quarter with careful shifts. Our [90-day outage plan](/power/90-day-power-outage) puts power last among the pillars for that reason: by then a generator is a fuel problem first. ### Diesel Diesel generators are mostly the larger standby and farm machines, and the calculator's diesel answer is the gasoline figure scaled by energy, so a real diesel engine will often beat it. Diesel is a combustible liquid, not a flammable one, and the Lehi guidance allows up to 60 gallons outside the house, with a lower cap in the garage and none below ground, with two 20BC extinguishers once you pass 25 gallons. Keep it in approved containers marked for diesel, apart from the gasoline, and rotate it through a tractor or truck on the same principle as gasoline. ### Where the fuel and the generator go Every gallon this page tells you to store has to be burned outdoors. Ready.gov puts generators at least 20 feet from windows, doors and attached garages, and wants a battery-powered carbon monoxide alarm on every level of the house. Refuel with the engine off and cooled, never in a garage, and keep the stored cans away from the running machine. The full rules, with transfer switches and backfeeding, are in our guide to [generator safety and carbon monoxide](/power/generator-carbon-monoxide-safety), which also lists current generator recalls. ### My take I would size the fuel stock from the shifts, not from the generator's full-load run time on a tank. Eight running hours is a realistic daily target for a house protecting its cold food, and most families find they can do it on less once the freezer is full and the doors stay shut. For a two-week plan, five stabilized gasoline cans on a rotation plus five 20-pound propane cylinders, burned in a dual-fuel generator, stay inside typical fire-code limits. At the worked example's 2,000 watts that stock lasts about eleven days on eight-hour shifts and a little over two weeks on six-hour ones. For anything longer, I would put money into a fixed propane tank before a second generator or a bigger one. Propane does not need rotating, and the largest fuel saving in this whole calculation is running a generator sized to the load. This calculator gives an estimate from published fuel energy figures and manufacturer run times. Your own generator's manual is the better source where it gives a burn rate, and your town's fire code sets how much fuel you may keep. ### Sources * [U.S. Energy Information Administration: British thermal units and fuel conversion factors](https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php) * [Honda Generators: Avoiding fuel-related problems](https://direct.powerequipment.honda.com/generators/eu-generator-fuel-recommendations) * [Lehi Fire Department: Home fuel storage limits](https://www.lehi-ut.gov/departments/fire-department/fire-prevention/home-fuel-storage-limits/) * [Ready.gov: Power outages](https://www.ready.gov/power-outages) * [SlashGear: Propane generator runtime on a 20-pound tank](https://www.slashgear.com/1989834/how-long-propane-generator-run-on-20-pound-tank) * [Gold Eagle: STA-BIL fuel stabilizer](https://www.goldeagle.com/brands/sta-bil/) How we write: we work from government, fire-service and manufacturer guidance and say what we have not tested. See our [editorial policy](/editorial-policy). ### Generator fuel questions How long will a generator run on 5 gallons of gas? At half its rated load, a 3,500-watt portable gets about 13 hours from 5 gallons, a 5,500-watt about 8 and a 7,500-watt about 6\. Lighter loads stretch that, and an inverter model at a quarter load can run well over a day. How much gasoline does a generator burn in a day? A mid-size portable carrying a refrigerator, a freezer, lights and a furnace fan burns about half a gallon an hour. Run it eight hours in shifts and that is roughly 4 gallons a day, or a 5-gallon can and a bit. How many 20-pound propane tanks do I need for a week? For a 5,500-watt dual-fuel generator carrying about 2,000 watts for eight hours a day, plan on a little over one 20-pound cylinder a day, so eight cylinders for a week. A 100-pound cylinder replaces five of them. How long does stored gasoline last? Untreated gasoline can start to go stale within about a month in a small engine. With stabilizer added the day you buy it, in a sealed can kept cool, it is good for a year by Honda's guidance, and that is the point to pour it into a car and refill. Is propane better than gasoline for a long outage? Propane does not go stale, so a stock bought this year still runs the generator in five years, and a household can usually store more of it legally. The cost is about 10 percent less power from the same generator and more bulk to move. Written by Ruth Calloway Pantry, water and homestead skills Ruth Calloway writes Larder & Well's guides to food storage, water, off-grid power and the old household skills. She works from extension-service bulletins, USDA and Ready.gov guidance, manufacturer manuals and years of homesteader discussion threads, and she says plainly where the sources disagree. She does not claim to have tested what she describes; when a method is untested or risky, her articles say so. [rcalloway@larderandwell.cc](mailto:rcalloway@larderandwell.cc) Last checked 2026-11-14 Every guide on generators, fuel and staying warm without the grid is in the [off-grid power section](/power). --- [Larder & Well](/) Keep the household fed, watered and warm when the stores close. Food storage, water, off-grid power, homestead skills and the old ways, written for American households that want to stay fed, watered and warm when the grid, the economy or the supply chain fails. [Send a correction or a question](/contact) © 2026 Larder & Well. Independent, written from sources you can open. A page that ever carries a paid link says so above it. Nobody pays for a place in our guides.