Natural gas, propane or diesel?

The short answer

Natural gas if your house has a gas meter. Nothing to fill, nothing to go stale, and no runtime limit while the main holds pressure. In an ice storm or a summer grid event, the gas system usually keeps working when the electrical one does not, though not always: in the February 2021 freeze Atmos Energy lost service to homes in parts of Grand Prairie for days. Propane stored on site is the hedge against that.

Propane if there is no gas main at the address, which in the Metroplex mostly means rural properties in Parker, Wise, Ellis and outer Denton counties. Propane also stores indefinitely, which is a genuine advantage.

Diesel if the load is large, the runtime requirement is fixed, or code requires fuel on site. It is a commercial answer — the range starts at 20kW and runs to 200kW.

The thing about natural gas nobody mentions

Propane carries more energy per cubic foot than natural gas. On smaller units that does not change the output, but on the larger air-cooled sets it does — and the number printed on most listings is the propane one.

So a 20kW unit on natural gas is really an 18kW unit. That is not a defect — it is physics, and every brand has the same behavior. It only becomes a problem when someone sizes your house against 20kW and then connects it to a gas main. Our sizing calculator asks which fuel you have and sizes against the figure you will actually get.

The gas meter problem

A 20kW generator burns roughly 268 cubic feet of natural gas an hour at full load. That is about 268,000 BTU per hour, from one appliance.

Now add the furnace, the water heater, the range and the dryer, and compare that to what your existing meter and service were sized to deliver. On a lot of houses the honest answer is that they were not sized with a generator in mind, and the gas utility has to upsize the meter, the service line, or both.

Propane: how long does a tank actually last?

Propane tanks are filled to about 80%, so a 500 gallon tank holds roughly 400 usable gallons. A 20kW unit burns about 3.08 gallons an hour at full load and about 1.81 gallons an hour at half load, which puts a 500 gallon tank at roughly 221 hours — about 9 days — on the half-load figure real outages actually produce.

Sizing the tank is a bigger question than this page should answer, because the gallon figure is only half of it: a tank also has to boil liquid into vapor fast enough to keep the engine fed, and cold weather and a low tank work against that at the same time. Propane tank sizing for a standby generator has the runtime table across all three air-cooled models and three tank sizes, the vaporization limit, and the question to put to your propane supplier.

Where diesel makes sense

Diesel gives the highest power density and fuel you control, stored on site and independent of any utility. For a clinic, a restaurant, a data closet or cold storage, that independence is the whole point. It is also frequently what code requires for life-safety loads.

The trade is maintenance. Diesel degrades, absorbs water and grows biological contamination if it sits, so a set that runs rarely needs its fuel managed rather than simply topped up. That is part of what a service visit covers.

See the diesel range, 20kW to 200kW →

One decision that is not about fuel

Whichever fuel you choose, the generator has to be sized against your air conditioning first. In Texas the compressor startup surge sets the requirement far more than square footage does, and no fuel choice changes that.

Consumption and output figures on this page are from the Cummins published specification sheet for the air-cooled Connect range. Download it from Cummins.