Every fall I get the same phone call. Somebody just bought a standby generator, the installer is coming Tuesday, and now there’s a question nobody warned them about: what size propane tank goes with it? The generator gets all the attention. The tank is an afterthought until it runs dry at 3 a.m. during an ice storm.
I spent twelve years wiring and servicing backup power for homes, and I can tell you the tank is half the system. Pick it right and you barely think about fuel for a week. Pick it wrong and you’re paying for an emergency delivery at holiday rates, or watching the lights blink out because the regulator can’t keep up in the cold.
So let’s work out the right propane tank size for generator backup the way I’d do it standing in your driveway, with a tape measure and a calculator.
Key takeaways
- A generator burns roughly 2 to 3 gallons of propane per hour at half load, so runtime, not just wattage, drives tank size.
- For whole-home standby, a 500-gallon tank is the sweet spot for most families; 250 gallons works if outages are short.
- Never fill a tank past 80 percent, and size for winter, when demand and vaporization limits both work against you.
- Buying the tank almost always beats leasing once you do the math over five years.
Start with burn rate, not tank size
The number that matters is how much propane your generator eats per hour. Manufacturers list this on the spec sheet, usually at 50 percent and 100 percent load.
A typical 22 kW air-cooled standby unit burns about 2.1 gallons per hour at half load and closer to 3.4 gallons at full tilt. A smaller 14 kW unit sips maybe 1.4 gallons at half load. Portable units running on propane land lower, around 0.7 to 1.2 gallons per hour depending on the draw.
Here’s the part people skip: your generator rarely runs at full load. Most homes hum along at 40 to 60 percent once the AC or well pump isn’t kicking on. So I plan around the half-load figure and treat full load as my worst-case cushion.
Multiply your half-load gallons-per-hour by 24 to get daily burn. A 22 kW unit at 2.1 GPH eats about 50 gallons a day. That one number tells you more than any tank chart.
Turn burn rate into the runtime you actually want
Once you know daily burn, decide how long you want to ride out an outage without a refill. This is a personal call, and it depends heavily on where you live.
On a job last winter outside Ithaca, my customer averaged three-day outages every couple of years. We sized for seven days of autonomy so she’d never sweat a delivery truck getting stuck. Somebody in a suburb with rare, short blips might be fine planning for two or three days.
Run the math against a tank’s usable capacity, not its stamped size. Propane tanks only fill to 80 percent to leave room for expansion, so a 500-gallon tank holds about 400 usable gallons.
| Tank size (water gal) | Usable propane (80%) | Runtime at 50 gal/day | Best fit |
|---|---|---|---|
| 120 gal (vertical) | ~96 gal | ~1.9 days | Portable or small partial-home |
| 250 gal | ~200 gal | ~4 days | Short outages, mild climate |
| 500 gal | ~400 gal | ~8 days | Most whole-home standby setups |
| 1,000 gal | ~800 gal | ~16 days | Rural, long outages, high demand |
For most families running a whole-home standby unit, the 500-gallon tank is where I land. It gives real breathing room and doesn’t cost dramatically more to set than a 250.
Two small tanks or one big one?
Some folks already have a pair of 100-pound cylinders for the grill and a fireplace. You can manifold cylinders together for a portable, but honestly, for standby power it’s a headache. A single buried or set 500-gallon tank means fewer connections, fewer regulators, and one delivery to schedule.
Cold weather changes the math
This is the part that bites people, and it has nothing to do with how many gallons are in the tank. Propane has to vaporize to feed the generator, and a tank’s ability to do that drops as temperature falls and as the liquid level drops.
A tank that’s only a quarter full on a 5°F morning has far less surface area boiling off gas. If your generator can’t pull enough vapor, it stumbles or won’t carry the load. This is exactly why a bigger tank helps even if you’d never burn all the fuel: more liquid surface means more reliable vaporization in the cold.
Propane and its tanks are governed by real fire codes for a reason. Setback distances from the house, windows, and ignition sources are not suggestions. Have a licensed installer set the tank and pressure-test the line, and keep the area around it clear. The NFPA 58 standard covers this in detail.
In genuinely cold country I lean one size larger than the runtime math alone suggests. A 250-gallon tank might carry the gallons you need on paper, but a 500 keeps the vapor flowing when it’s brutally cold and the tank is running low.
Buy or lease the tank?
Your propane supplier will happily lease you a tank for a small yearly fee, sometimes free, in exchange for buying fuel only from them. That lock-in is the catch.
Leased tank customers I’ve talked to routinely pay 30 to 60 cents more per gallon because they can’t shop around. Over a 500-gallon tank filled twice a year, that’s real money.
- Owned tank lets you buy from whoever’s cheapest that week
- New 500-gallon aboveground tank runs roughly $1,200 to $1,800 installed
- Buried tanks cost more but disappear into the yard
- Leasing makes sense only if you’ll move soon or use very little fuel
My rule: if you plan to stay in the house more than five years, buy the tank. The fuel savings usually cover the purchase in that window, and you’re never held hostage on price.
A quick sizing walkthrough
Let’s put it together with a real example. Say you’ve got a 22 kW standby unit and you want five days of autonomy.
Half-load burn is about 2.1 GPH, so daily burn is roughly 50 gallons. Five days means 250 gallons of usable fuel. Divide by 0.8 for the fill limit and you need about 312 gallons of tank capacity.
That points straight at a 500-gallon tank, since a 250 only gives you 200 usable gallons, four days at best. Round up, especially if you’re in cold country. Sizing down to save a few hundred dollars is the one regret I hear most.
If you’re still deciding between fuel types before you get this far, my breakdown of how the common generator fuels stack up is worth a read, and if you’re weighing propane against a utility hookup, this look at whether a natural gas generator keeps running in an outage answers the question most people forget to ask.
What to do next
Grab your generator’s spec sheet and find the half-load fuel figure. Multiply by 24, then by the number of days you want to survive without a refill, then divide by 0.8. That’s your minimum tank capacity, and nine times out of ten it rounds up to a 500.
Before you sign anything, get the tank purchase quoted right alongside the generator so leasing doesn’t sneak in by default. And once it’s all set, keep the whole system honest with a proper service routine; my complete generator maintenance checklist covers what to check and when. If a fixed standby unit still feels like too much, a smaller propane-capable portable setup is a fine starting point, and my beginner’s guide to portable generators for home backup walks through it. For a sanity check on operating cost against other fuels, the diesel versus gas running-cost comparison is a useful gut check, and if you’re curious about flexibility, tri-fuel conversions let one engine run on more than propane alone.
For the code and safety side, the NFPA 58 Liquefied Petroleum Gas Code is the authority installers work from, and the U.S. Department of Energy’s propane basics from energy.gov is a solid, jargon-free primer if you’re new to the fuel.
