Quick answer: A 20 kW whole house generator running at half load burns about 1.7 to 2 gallons of propane per hour, or roughly 150 to 200 cubic feet of natural gas. At typical propane prices that lands in the $5 to $8 per hour range, so a full day of outage power costs somewhere between $120 and $200 in fuel. Natural gas units come in far cheaper per hour, and gasoline portables sit in between but need babysitting. The exact number depends on three things: the fuel, the load, and the size of the machine.
The purchase price of a standby unit gets all the attention, but the cost to run a whole house generator is what you actually feel during a multi-day outage. A three-day January blackout can burn through hundreds of dollars of propane, and that math changes what size generator makes sense in the first place. This page works through the fuel consumption numbers for each fuel type, then puts them together into per-hour and per-day costs you can check against your own fuel bill.
What drives the cost to run a whole house generator
Three variables do almost all the work.
Fuel type. Natural gas is piped, so you pay per therm or per Ccf on your utility bill and never refill anything. Propane is bought by the gallon and delivered to a tank, at a per-gallon price that swings by region and season. Gasoline is bought retail, in cans, at whatever the station charges during the emergency that made you need it.
Load. Generators burn fuel in proportion to the work they do, not in proportion to their rating. A 20 kW unit loafing along at a quarter load sips a fraction of what it drinks at full output. This is why two neighbors with identical generators can spend wildly different amounts during the same outage: the house running two air conditioners pays several times what the house running a fridge and a furnace pays.
Machine size. A bigger engine swallows more fuel at the same load percentage, and most people buy more generator than they need. Running the sizing numbers before you buy is the cheapest fuel-saving move available. The generator size calculator works through your actual circuit list, and the what size generator for a 2800 sq ft house walkthrough shows the arithmetic on a real example.
Natural gas generator cost per hour
On natural gas, a 20 kW class standby draws on the order of 200 to 250 cubic feet per hour at high load, which is roughly 2 to 2.5 therms. Your utility bill states the exact rate; residential gas in most of the United States has traded in the $1 to $2 per therm range in recent years, delivery charges included. Multiply it out and a heavily loaded 20 kW unit costs about $3 to $5 per hour to run. A day of full outage power lands near $75 to $120.
That per-hour number is the whole story, because the pipe keeps supplying the machine. There is no tank to drain, no delivery to schedule mid-storm, and no practical runtime limit. The trade-off is sitting in the natural gas vs propane decision rather than the fuel bill: if your street has gas service, the running cost argument is basically settled in favor of it.
One number worth checking before you commit: the meter. A 20 kW unit at full load wants roughly 250,000 BTU per hour, and older residential meters and half-inch gas lines can struggle to pass that alongside a furnace and a water heater. Your installer should verify meter capacity, not just quote the generator.
How much propane does a generator use
Propane consumption scales with load. For a 20 kW standby, the figures that matter:
| Load | Burn rate (approx.) |
|---|---|
| Quarter load | 1 to 1.2 gallons per hour |
| Half load | 1.7 to 2 gallons per hour |
| Full load | 3.5 to 3.6 gallons per hour |
At half load, a 250-gallon tank holds about 200 usable gallons, which is four to five days of runtime. The propane tank sizing guide covers this arithmetic in detail, including the cold-weather vaporization margin that matters when the tank is sitting in a January wind.
For cost, take the per-gallon price off your last delivery slip. Propane has traded in a wide band, roughly $2 to $3.50 per gallon depending on region and season, so the honest statement is: half-load running on a 20 kW unit costs about $4 to $7 per hour, and a day of it costs $100 to $170. A full 500-gallon tank represents roughly $1,000 to $1,500 of fuel sitting in the yard, which is worth knowing before an outage rather than during one.
Smaller machines drink proportionally less. A 14 kW unit at half load is closer to 1.2 to 1.5 gallons per hour, and the 14 kW Generac price guide goes through that class in detail.
How much gas does a generator use (portables)
A mid-size gasoline portable, the 7,000 to 9,500 watt class that backs up essential circuits, burns roughly 0.6 to 1 gallon per hour at moderate load. At $3-plus per gallon of gasoline, that is $2 to $4 per hour, which looks cheaper than propane until you handle the logistics: a 6 to 8 gallon tank needs refilling every 8 to 12 hours, in the weather that caused the outage, using fuel that goes stale over a season of storage.
For essential-circuit duty rather than true whole-house coverage, the portable whole house generator page maps out which machines can actually feed a panel through a transfer switch. Fuel stabilizer and a rotation habit for your stored cans are part of the real cost; gasoline you cannot start an engine on is the most expensive gasoline there is.
A 24-hour outage, priced three ways
Putting the numbers together for a typical scenario, a 20 kW class machine at half load covering a house for one day:
- Natural gas: roughly 45 to 50 therms, about $50 to $100 depending on your rate
- Propane: about 40 to 48 gallons, roughly $100 to $170 at recent delivery prices
- Gasoline (large portable, essential loads only): about 15 to 20 gallons, $50 to $80 plus the refueling trips
The pattern holds at any duration: natural gas is the cheapest per hour when available, propane is the reliable no-degradation choice where gas service ends, and gasoline wins on equipment cost but loses on effort. Outage frequency matters as much as outage length. The power outages by state data shows how different the exposure is by region, and that should feed into how much running cost you are willing to tolerate.
How to cut the running cost
The biggest lever is load discipline, and it is free. Every watt you do not draw is fuel you do not burn.
- Size the machine to the house, not the ego. An oversized generator running at tiny load still burns its idle fuel. Work the circuit list in the generator size calculator before buying.
- Shed the second refrigerator and the electric dryer during an outage. Heating elements are the fuel hogs; a dryer can add more than a dollar an hour on propane.
- Manage the big cycling loads. If the well pump and the AC compressor do not start at the same moment, the generator runs lower and leaner. Staggering starts is exactly what a load manager does, and the 50 amp load manager explainer covers how they work.
- On propane, fill before storm season. Emergency deliveries carry emergency pricing.
When the fuel math pushes you toward a smaller, smarter setup, the standby vs portable generator comparison and the where to buy a generator guide cover the decision from the spending side.
FAQ
How much does it cost per hour to run a whole house generator on propane? A 20 kW unit at half load burns 1.7 to 2 gallons per hour. At $2 to $3.50 per gallon, that is roughly $4 to $7 per hour. Quarter-load running drops it to $2 to $4 per hour.
Is a natural gas generator cheaper to run than propane? Usually, yes. Natural gas at recent utility rates runs about $3 to $5 per hour for a heavily loaded 20 kW unit, against $5 to $8 for the same load on propane, and there is no tank to refill. The gap narrows wherever gas is expensive and propane is cheap locally, so check both rates on your actual bills.
How long will a 250-gallon propane tank run a whole house generator? At half load on a 20 kW unit, about four to five days. The tank holds around 200 usable gallons after the 80 percent fill rule, and the burn rate is roughly 1.7 to 2 gallons per hour.
Does a generator use fuel when the house draws almost nothing? Yes. Even at no load the engine idles and consumes fuel, typically on the order of half a gallon per hour for a 20 kW class machine. That idle burn is one more argument for right-sizing rather than buying the biggest unit on the lot.
Conclusion
The cost to run a whole house generator comes down to fuel times load: propane at $4 to $7 per hour at half load on a 20 kW machine, natural gas at roughly $3 to $5, and gasoline portables in the $2 to $4 range with refueling labor on top. A day of outage power is a $50 to $170 line item depending on the fuel, and a multi-day event scales linearly from there. Run the sizing math before you buy, watch the heating loads during an outage, and price the fuel options on your own utility bills, because local rates vary more than any of the national averages do.
