"How long is my fuel actually going to last?"
I usually get asked this when a storm is brewing. Here's the obvious problem: there is no single magic number.
The easiest way to understand your generator is to think about your own vehicle. If you’re towing a heavy boat uphill with the A/C blasting, you’re going to burn through a tank of gas a whole lot faster than if you’re just cruising down the highway. Your generator is the exact same way. If the grid goes down and you try to run your 5-ton A/C unit at 68 degrees, cook a roast in the oven, and run the dryer all at once, you will burn fuel twice as fast as you would if you just keep the fridge cold, a few lights on, and the ceiling fans spinning.
If You Have Natural Gas (NG)
You’re in great shape. Because your generator is hooked directly into the municipal utility lines, your fuel supply is virtually unlimited. Unless a Category 4 physically rips up the underground gas lines or the city loses pressure, your generator will run indefinitely.
If You Have Liquid Propane (LP)
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This is where we have to do some math. If you are someone who likes exact numbers, here is how you calculate your true runtime.
Step 1: The 80% Rule
Propane tanks are never filled to 100% capacity. Because propane expands—especially in the Florida heat—suppliers only fill them to 80% for safety. So, if you buy a 250-gallon tank, you really only have 200 gallons of usable fuel.
Step 2: Know Your Burn Rate
A standard home standby unit (like a 20kW or 24kW) burns roughly 1.5 to 2 gallons of propane per hour at a 50% electrical load. If you push it to a 100% load, that jumps to about 2.5 to 3.5 gallons an hour.
Step 3: The Calculation
Take your usable gallons and divide them by your burn rate to get your total hours.
(Total Tank Capacity x 0.8) ÷ Hourly Burn Rate = Total Hours of Runtime.
To save you from pulling out your calculator, here is a realistic cheat sheet for a home running a typical emergency load (cycling the A/C reasonably, keeping the fridge cold, and using basic lights at roughly 2 gallons an hour):
- 120-Gallon Tank (holds ~96 gallons): 96 gallons ÷ 2 gal/hr = 48 hours (1.5 to 2 days).
- 250-Gallon Tank (holds ~200 gallons): 200 gallons ÷ 2 gal/hr = 100 hours (4 to 5 days).
- 500-Gallon Tank (holds ~400 gallons): 400 gallons ÷ 2 gal/hr = 200 hours (8 to 10 days).
My bottom-line advice: If we take a direct hit and you know the grid is going to be down for a while, manage your power.
- Bump your thermostat up to 78 degrees—that alone can save you a day's worth of fuel.
- Skip the electric dryer.
- And don't worry about the fuel going bad in the heat; your only real enemy is running out of it before the utility trucks show up.