Generator Fuel Consumption Chart (1,000–12,000 W)

How much fuel does a generator use per hour?

Unlike a car, a generator does not have one mileage figure. It has a rate — fuel burned for every unit of work it delivers — and the work depends entirely on what you have plugged in. That rate is called the specific fuel consumption (SFC), and for the small air-cooled gasoline engines in portable generators it sits close to 0.16 gallons per kilowatt-hour across the whole 1,000–12,000 W class. Diesel is far more efficient at roughly 0.07; propane sits the other side of gasoline at about 0.19; piped natural gas needs about 11 cubic feet per kilowatt-hour.

One multiplication turns that into a per-hour figure for any machine:

gal/h = rated kW × load factor × 0.16 (gasoline)

Swap 0.16 for 0.07 (diesel), 0.19 (propane) or 11 (natural gas, in cubic feet) and the same line works for the other fuels. Load factor is the share of nameplate output you are actually drawing: 0.25 for a light fridge-and-lights setup, 0.75 for a typical well-loaded run, 1.00 when the unit is flat out.

Worked example, the figure everybody searches for: a 5,000-watt generator is a 5 kW machine, and at 75% load it is producing 3.75 kW. Multiply through — 5 × 0.75 × 0.16 = 0.60 — and the answer is 0.60 gallons per hour, which is 2.3 litres per hour at 3.785 L per gallon. At $3.50 a gallon that is $2.10 of fuel every hour, and a 5-gallon tank lasts about 8 hours 20 minutes. The same unit at full load jumps to 0.80 gal/h (6¼ hours per tank), at half load drops to 0.40 gal/h, and at 25% load to 0.20 gal/h. Every number in the tables below comes from that one line, using the same constants, so any cell is checkable in a few seconds.

Two caveats before you budget to the gallon. First, the SFC constants are averaged, catalogue-grade figures: your engine's real number drifts with age, carburettor jetting, oil viscosity, air temperature and maintenance. Second, this is an energy-proportional model — it assumes fuel scales with load, which holds well for inverter generators and only roughly for fixed-speed conventional units. The fourth section covers that correction in full, because it is where most published charts mislead people.

Which size uses how much?

The chart below covers the sizes sold as "portable" and "inverter" units, from a 1,000-watt camping box to a 12,000-watt jobsite rig. Read down for your nameplate rating, across for the load you expect to run. The last two columns convert the 75% figure into litres per hour and invert it into hours-per-gallon, which is the number that decides refuelling trips. The sizes cluster by use: up to about 2,200 W is an inverter unit for a van or a couple of lamps, 3,000–4,000 W covers a fridge plus lighting and a furnace fan, and 7,000 W upward is jobsite territory where burn stops being a rounding error. At 12,000 W and 75% load you are spending $5.04 an hour on gasoline alone (1.44 gal/h × $3.50) — which is why, at the top of this table, fuel cost rather than purchase price becomes the dominant number.

The point that surprises most buyers: nameplate watts, not the fuel dial, decide the burn. A 7,000-watt generator idling a single lamp is still a 7,000-watt engine, and on a conventional fixed-speed unit it will burn closer to its full-load rate than the 0.28 gal/h shown in the 25% column. Buying a larger generator to "save fuel at low load" does the opposite — the bigger engine carries more friction and windage losses per kilowatt-hour delivered.

Sizing works better in your favour. A unit running at 70–80% load is the sweet spot: near the top of its efficiency curve, warm enough to burn cleanly, and with headroom for one motor start. Under 30% is where both fuel waste and, on gasoline engines, carbon build-up and wet-fouling start to bite. Run three sizes up and down this table against your actual running watts before you order anything — the wattage calculator adds your appliance list and includes the starting surges.

How long will it run on a full tank?

Consumption per hour tells you the running cost; tank capacity tells you whether you sleep through the outage. Runtime is simply tank gallons divided by the gal/h figure from the first table, so the two tables below are the same arithmetic read out for the tank sizes portables actually ship with — from a 1.2-gallon inverter box to an 8.5-gallon jobsite frame. Hours are given to one decimal; multiply the decimal by 60 for minutes (0.3 h ≈ 18 min).

Halving the load doubles every figure in that table, because the model is linear. Here is the same grid at 50% load — the "fridge, furnace fan and a few lights" regime — which is where the long overnight runtimes live. Read the top-left corner as a warning rather than a spec: a 12,000-watt unit on a 1.2-gallon tank is 0.8 hours at 75% load, which is not a machine anybody runs unattended. The opposite corner, 106 hours for a 1,000-watt box on 8.5 gallons at half load, is arithmetically true and practically meaningless — nobody runs one unit that long in one stretch. What both ends actually teach is the same lesson: runtime is bought with tank volume and modest nameplate watts, not with a bigger engine.

Treat every cell above 10 hours as a plan, not a promise. Real tanks are never filled to the brim — the usable volume is usually 5–10% below the printed capacity, since the fill neck and a float at the bottom leave fuel you cannot draw — and on a conventional generator running at 25–50% load the fixed-RPM effect described below eats part of the theoretical gain. The full-tank runtime guide adds the 2-hour rule for unattended overnight use.

Why the 25% column is optimistic

This is the part almost every published generator fuel chart skips, and it is the difference between a 9 a.m. refill and running dry at 3 a.m. The four load columns above are an energy-proportional model: they assume that drawing a quarter of the power burns a quarter of the fuel. For sizing and comparing machines that is the right model, and it is stated plainly here so you can check it. For predicting a specific tank, it flatters light loads badly on one common class of generator.

The reason is mechanical. A conventional fixed-speed generator holds 3,600 rpm no matter what you draw, because the frequency of the AC output is tied directly to engine speed. Dropping from 4 kW to 1 kW of electrical load does not slow the engine down; it only reduces the torque the engine must produce. Meanwhile the losses that dominate a small air-cooled engine — piston and bearing friction, the windage of that big flywheel spinning at 3,600 rpm, pumping losses against the nearly-open throttle, the fan blowing cooling air — stay roughly constant. Fuel keeps flowing to hold the speed. The practical rule of thumb: at 25% load, a conventional generator often still burns 50–60% of its full-load rate, not the 25% the table implies. A 5,000-watt unit that should sip 0.20 gal/h may drink 0.40–0.48 gal/h, halving your projected runtime.

An inverter generator behaves much closer to the table, precisely because it is not obliged to hold a fixed speed: output frequency is made electronically, so the engine can be throttled to whatever load is present. Drop to a 25% load and an eco-throttle inverter genuinely slows down, and its burn falls toward the proportional figure — though usually not all the way, since idle fueling and a base friction floor remain. The upshot for buying: if you know you will mostly run lightly loaded, the inverter is not just quieter, it is materially cheaper to fuel. If you mostly run hard and steady, the conventional unit's simplicity wins on purchase price and the load columns barely matter.

Two habits fix the uncertainty. First, look up your own machine's rated fuel consumption at rated load in the owner's manual — it is printed there, and it anchors this chart to your engine rather than to an average. Second, measure the tank once: fill it, run your normal load for two hours, refill and read the litres. Divide by two and you have your real gal/h, which you can then feed into the fuel calculator instead of the generic constant.

Gasoline vs diesel vs propane: what actually costs less per hour

Fuel volume and fuel cost are different questions. Diesel wins the volume argument by a mile — 0.07 gal/kW·h against 0.16 for gasoline — and propane loses it, needing 0.19 gallons for the same kilowatt-hour. But cost per hour multiplies those rates by whatever you pay at the pump, so the ranking can shuffle. Here is a 5,000-watt unit at 75% load (3.75 kW drawn), priced at $3.50/gal gasoline, $3.80/gal diesel, $2.75/gal propane and $1.50 per 100 cubic feet of mains gas:

Diesel burns nearly three times the gallons for the same work as gasoline and still costs half as much per hour, because it needs so little of it. Two break-even ratios tell you when that stops being true, and they are the same two used across this site:

• Diesel vs gasoline: 0.16 ÷ 0.07 = 2.29. Diesel stays the cheaper fuel to run until its price exceeds roughly 2.3× the gasoline price. At $3.50 gasoline that means diesel would have to top about $8.05 a gallon. It never does, so diesel wins on running cost and loses on purchase price, noise and fuel-ageing maintenance.
• Propane vs gasoline: 0.16 ÷ 0.19 = 0.84. Propane is cheaper per hour only when it sells below about 84% of the gasoline price — near $2.95/gal while gasoline is $3.50. Worth checking your supplier, but rarely true. Propane also gives back about 10% of the machine's rated output and earns a 3% efficiency penalty per 1,000 ft of altitude on top.

Natural gas is normally cheapest per hour of all when a mains line exists, at $0.62 here, but it cannot be carried — which is exactly why it belongs to standby units rather than the portables in the charts above. The full per-gallon, per-litre and per-kilowatt-hour comparison, including what changes when prices move, is in the diesel vs gasoline vs propane cost page.

How to read a manufacturer's fuel-consumption figure

Spec sheets publish numbers, not justifications, and three conventions distort what you are reading.

The "at 25% load" convention. Many portable generator box claims — "up to 12 hours on a tank" — are quoted at 25% load, because that is where the runtime number looks longest and the curve is flattest. It is a real figure, but it describes one LED lamp and a phone charger, not a house during an outage. When a brochure gives one runtime without a load qualifier, assume 25% and re-derive it yourself at the load you actually intend to run: divide tank gallons by the gal/h from the first table at that load. If the manufacturer's own tank figure and this chart disagree wildly, check which load each one assumed before deciding who is wrong.

Ethanol blend and shelf life. Most US gasoline is E10 (up to 10% ethanol), and E15 is worse. Ethanol is hygroscopic: it pulls moisture from the air, then separates as water in the tank and gums up carburettors. Practical life is about 30–90 days unstabilised, six months to two years with a proper stabiliser, less in humid coastal air. For a standby generator this matters more than any efficiency figure on this page, because the failure mode is a machine that will not start when you need it. Burn through and refresh stored fuel, or run ethanol-free pump gas if your engine is small and older. Diesel is more stable in the can (roughly a year) but grows microbes in water that settles in its tank, so it needs draining and polishing rather than simply topping up.

Altitude derating. Air is thinner at elevation and an engine ingests less oxygen per revolution, so it makes less power: a workable rule is roughly 3–5% of rated output lost per 1,000 ft above sea level, with diesel engines nearer 2–4% and naturally aspirated gasoline units nearer 3–5%. A 5,000-watt machine at 5,000 ft may only deliver about 4,000 W. What people miss is the second-order effect on this chart: if the same lights and furnace fan draw a fixed 3,750 W, then at altitude that is a higher percentage of the remaining capability, so the unit runs further up the load columns — and the fuel per kilowatt-hour delivered rises as well. Altitude also leans the carburettor mixture, which raises fuel use and can overheat the engine if the jetting is never corrected. If you live above roughly 3,000 ft, size the generator for the derated output, not the brochure output, and expect fuel figures toward the higher end of anything published at sea level.

Keep reading

Chasing a single size? Gas use for a 5,000-watt generator gives its full load-by-load breakdown, and runtime on a full tank turns these burn rates into hours per fill plus the refuel-stop arithmetic. On fuel choice, the diesel vs gasoline vs propane cost comparison shows exactly where the break-even prices move. Not sure the size above is yours? Work the appliance list in the generator wattage calculator first. All the browser-only tools on this site are collected under energy tools.