The two numbers that matter

Every estimate here is consumption rate × hours × price. The consumption rates come from Generac's published Guardian spec sheets at half and full load. The prices default to recent EIA national averages — residential propane around $2.67/gallon, natural gas around $14.50 per thousand cubic feet — but fuel prices move by region and season, so the editable price fields are the honest version of this tool: plug in your contract or utility rate.

The punchline the article lands on: natural gas runs at roughly half the hourly cost of propane, and it never needs a delivery truck. A 22 kW unit at half load costs about $3.30/hour on gas versus $6.75/hour on propane. On propane, the question isn't just the bill — it's whether your tank holds enough for the outage. A two-day outage at half load is roughly $325 on propane or $160 on gas.

Honest limits

Generac numbers. These consumption rates are Generac Guardian spec sheets. Kohler, Cummins, and portables will differ — sometimes substantially. Your generator's own spec sheet beats this table; the custom-rate mode exists for exactly that.
Half load is the honest default. Real outages bounce around half load for most homes. Full load is shown as the upper bound, not the plan.
Exercise cycles are noise. The weekly self-test runs at reduced RPM — about 0.5 gal/hr of propane on a 22 kW unit — which works out to a few gallons a year. It doesn't move the math; a dead battery caught in March is worth far more than the fuel.
Prices in this tool are defaults, not quotes. Propane recently ran over $3.50/gal on the East Coast and under $2.20 in the Midwest. If you're budgeting for winter, use your supplier's locked price.

Where every number comes from

No invented numbers. Consumption rates are Generac Guardian manufacturer specs (half and full load), republished in our whole-house generator cost guide; prices are U.S. EIA national averages.

Unit	Propane, 1/2 load	Propane, full load	Natural gas, 1/2 load	Natural gas, full load
10 kW	0.97 gal/hr	1.48 gal/hr	101 cu ft/hr	127 cu ft/hr
13 kW	1.54 gal/hr	2.45 gal/hr	154 cu ft/hr	225 cu ft/hr
16 kW	1.70 gal/hr	2.99 gal/hr	182 cu ft/hr	245 cu ft/hr
20 kW	2.37 gal/hr	3.56 gal/hr	204 cu ft/hr	301 cu ft/hr
22 kW	2.53 gal/hr	3.90 gal/hr	228 cu ft/hr	327 cu ft/hr

Frequently asked questions

Propane or natural gas — which is cheaper to run?

Natural gas, by roughly half, at current EIA prices. It also means no tank, no deliveries, and no refueling math during a long outage. Propane's advantages are availability — no gas main on your road, propane is your piped-fuel equivalent — and indefinite shelf life in the tank.

How long will my propane tank last in an outage?

A 500-gallon tank holds about 400 usable gallons (filled to 80%). A 22 kW unit at half load burns 2.53 gal/hr, so that's roughly 150 hours — a little over six days. Size the tank to your realistic outage length, not your optimistic one.

Does the weekly exercise cycle burn much fuel?

Almost nothing. It runs at reduced RPM — about 0.5 gal/hr of propane on a 22 kW unit — which totals a few gallons a year. The value is the self-test catching a dead battery or fault code, not the fuel it burns.

Can I use this for a diesel generator or a portable?

Yes — pick "Custom rate," enter the consumption from your generator's spec sheet (published at half and full load), and your local fuel price. The math is identical; only the preset rates are Generac-specific.

Why does my installer's fuel estimate differ?

Load assumptions and prices. An installer sizes fuel from your expected load at your local price; this tool starts from spec-sheet rates and national averages. Same arithmetic, different inputs — ask them which two they used.

Want the full picture? Read what a whole-house generator costs or size your generator first.