Natural gas, propane or diesel?

The short answer

Natural gas if your house has a gas meter. Nothing to fill and no runtime limit while the main holds pressure. Gas service can fail too: in the February 2021 freeze Atmos Energy shut off gas to homes near Joe Pool Lake in Grand Prairie for about three days. Propane stored on site is the hedge against that.

Propane if there is no gas main at the address, which in the Metroplex usually means a rural property. Propane does not degrade in storage.

Diesel if the load is large, the runtime requirement is fixed, or code requires fuel on site. It is mainly a commercial choice; the Cummins range runs from 20kW to 200kW.

Output on natural gas versus propane

Propane carries more energy per cubic foot than natural gas. On the Cummins air-cooled range that changes the output of the 20kW units only, and their model name is the propane figure.

So a Cummins 20kW air-cooled unit on natural gas is an 18kW unit. That matters only if the house is sized against 20kW and then connected to a gas main. The liquid-cooled range, 25kW and up, is rated the same on both fuels. Our sizing calculator asks which fuel you have and sizes against the figure you will actually get.

The gas meter problem

A Cummins 20kW air-cooled generator burns 268 cubic feet of natural gas an hour at full load, published as 256,000 BTU per hour, from one appliance.

Add the furnace, the water heater, the range and the dryer, and compare that to what your existing meter and service line were sized to deliver. If they were not sized with a generator in mind, 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, at half load.

Gallons are only half of tank sizing: a tank also has to boil liquid into vapor fast enough to keep the engine fed, and cold weather and a low tank both work against that. 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, which is why clinics, restaurants and cold storage choose it. Some life-safety applications require fuel stored on site.

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, size the generator against your air conditioning first. The compressor startup surge sets the requirement more than square footage does.

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