Generator Fuel Consumption Chart: Propane, Natural Gas & Diesel by kW
The question that decides most generator purchases isn't wattage — it's "what will this thing cost me per hour, and how long will my fuel last?" This chart gives typical fuel consumption for air-cooled and liquid-cooled standby generators by size class, at half and full load, across propane (LP), natural gas (NG), and diesel. Figures are typical field values for planning; your unit's spec sheet is the final word, and load percentage moves the numbers a lot.
Generator fuel consumption chart (typical, by size)
| Generator size | Propane, 50% load | Propane, 100% load | Natural gas, 100% load | Diesel, 75% load |
|---|---|---|---|---|
| 7.5–10 kW air-cooled | ~1.0 gal/hr | ~1.6 gal/hr | ~120–140 ft³/hr | — |
| 14 kW air-cooled | ~1.4 gal/hr | ~2.2 gal/hr | ~180–200 ft³/hr | — |
| 18 kW air-cooled | ~1.6 gal/hr | ~2.8 gal/hr | ~230–250 ft³/hr | — |
| 22 kW air-cooled | ~1.7 gal/hr | ~3.2 gal/hr | ~280–300 ft³/hr | — |
| 26 kW air-cooled | ~2.1 gal/hr | ~3.9 gal/hr | ~330–400 ft³/hr | — |
| 30 kW liquid-cooled | ~2.4 gal/hr | ~4.3 gal/hr | ~380–420 ft³/hr | ~1.8 gal/hr |
| 48 kW liquid-cooled | — | — | ~570–620 ft³/hr | ~2.7 gal/hr |
| 60 kW liquid-cooled | — | — | ~700–770 ft³/hr | ~3.3 gal/hr |
| 80 kW liquid-cooled | — | — | ~900–1000 ft³/hr | ~4.4 gal/hr |
| 100 kW liquid-cooled | — | — | ~1,100–1,250 ft³/hr | ~5.5 gal/hr |
| 125–150 kW liquid-cooled | — | — | ~1,400–1,800 ft³/hr | ~6.9–8.3 gal/hr |
Rules of thumb behind the chart: air-cooled units burn roughly 0.12–0.15 gallons of propane per kW-hour of rated output at full load, natural gas runs about 10–13 ft³/hr per rated kW, and diesel sips about 0.055–0.07 gallons per rated kW at 75% load — diesel is the efficiency king, which is why it dominates commercial standby.
The 26kW question everyone asks
"How much propane does a 26kW Generac use per hour?" is one of the most-asked questions in this category — here's the straight answer. A 26kW air-cooled standby at half load burns about 2.0–2.2 gallons of propane per hour; at full load, roughly 3.8–4.0 gallons per hour. On natural gas, expect around 330–400 cubic feet per hour at full load. Real outages rarely run at full load — most homes average 30–50% of capacity — so the half-load number is your planning figure.
Tank runtime math: how long will your propane last?
A 500-gallon propane tank is filled to 80% (400 usable gallons) for thermal expansion. Runtime = usable gallons ÷ burn rate:
| Scenario | Burn rate | 400 gal runtime |
|---|---|---|
| 26kW at 50% load | ~2.1 gal/hr | ~190 hours (~8 days) |
| 26kW at 100% load | ~3.9 gal/hr | ~103 hours (~4.3 days) |
| 22kW at 50% load | ~1.7 gal/hr | ~235 hours (~9.8 days) |
| 18kW at 50% load | ~1.6 gal/hr | ~250 hours (~10.4 days) |
Worked example: sizing the tank for a week of autonomy
You want seven full days of runtime on a 22kW unit, assuming half-load average and 12 hours of runtime per day (you shut it down overnight): 12 hr/day × 7 days × 1.7 gal/hr = 143 gallons — a 250-gallon tank (200 usable) covers it with margin. Running 24/7 instead: 24 × 7 × 1.7 = 286 gallons → you need the 500-gallon tank. The math scales linearly, so keep the chart above handy.
Natural gas vs propane vs diesel: the trade-offs
- Natural gas: unlimited runtime while the utility is up, no tank, no refills — but roughly 8–10% output derating versus LP on the same unit, and useless if the gas main goes down in a disaster.
- Propane: full rated output, stores for decades, works off-grid — but you're bounded by tank size and delivery logistics.
- Diesel: best fuel economy, longest engine life, standard above 30kW commercial — but fuel has a shelf life (~12 months untreated), gels in extreme cold, and on-site storage raises permitting questions.
Altitude derates everything: expect roughly 3–4% less output per 1,000 feet above ~500 feet on naturally aspirated air-cooled engines.
What does a 3-day outage actually cost in fuel?
Fuel math gets real when you price an actual outage. Using mid-2020s US residential averages (~$2.50–$3.50/gal propane, ~$1.20–$1.80/therm natural gas, ~$3.50–$4.50/gal diesel) and a 22kW air-cooled unit at half load:
- Propane: 1.7 gal/hr × 72 hrs = ~122 gallons → $300–$430 for three days of continuous runtime. Intermittent operation (12 hr/day) halves that.
- Natural gas: ~140 ft³/hr at half load × 72 hrs ≈ 100 therms → $120–$180 for the same outage — the fuel-cost winner by a wide margin, which is why NG units dominate suburban installs.
- Diesel (60kW commercial at 75%): 3.3 gal/hr × 72 hrs ≈ 238 gallons → $830–$1,070 — but that unit is backing up an entire small facility, not a house.
Against those numbers, the "expensive" standby generator is usually the cheapest line item in the outage: a single refrigerated truckload of spoiled food, a flooded basement from a dead sump pump, or three nights of hotel rooms for a family of four all cost more than the fuel. The real comparison isn't generator versus nothing — it's generator versus what you lose without one. If your outages are short and your loads are modest, a battery backup system covers the first hours silently and the generator only runs for extended events — our battery runtime calculator sizes that side of the pair.
Frequently asked questions
How much propane does a 22kW generator use per hour?
About 1.6–1.8 gallons per hour at half load, and roughly 3.2 gallons per hour at full load.
Is it cheaper to run a generator on natural gas or propane?
Per hour of runtime, natural gas is typically 30–50% cheaper than propane at recent US residential prices — but propane avoids the output derating and works when the gas utility is down.
How long will a 500-gallon propane tank run a whole-house generator?
For a 22–26kW air-cooled unit at typical half-load operation, 8–10 days of continuous runtime. Intermittent operation stretches that to two weeks or more.
Do generators burn more fuel at higher load?
Yes, but not linearly — most engines are most efficient per kWh between 50–75% load. Running a 26kW unit to power one refrigerator is the least efficient way to make electricity; that's what load management is for.
How much diesel does a 100kW generator use per hour?
Roughly 5.5 gallons per hour at 75% load, around 7 gallons at full load. Diesel's efficiency advantage grows with size.
Does a generator use less fuel at lower load?
Yes — but the relationship isn't linear. Dropping from 100% to 50% load cuts fuel use by roughly 40–45%, not half, because fixed engine losses (friction, cooling, excitation) don't scale down. Below ~30% load, efficiency falls off a cliff and diesel engines risk wet-stacking from incomplete combustion.
Can I extend propane runtime with a second tank?
Yes — dual-tank setups with an automatic changeover regulator are standard practice. Two 500-gallon tanks give a 26kW unit roughly 16 days of half-load runtime, and the changeover lets you refill one bank while running on the other.
Pick your unit — we stock the fuel-flexible ones
Portlandia Electric Supply carries NG/LP standby units from 14kW to 150kW, like the Generac Guardian 18kW home standby, the Briggs & Stratton 18kW PowerProtect, and liquid-cooled workhorses like the Briggs & Stratton 30kW Fortress LPV/NG. Browse Standby Home Generators and the full Standby Generators collection, and don't forget the transfer switch. Still sizing? Read our whole-home generator sizing guide first, then call the counter with your square footage and fuel type.