Buying guide
Natural gas, propane or diesel?
For a home in Dallas–Fort Worth the answer is usually natural gas, and the reasoning takes about a paragraph. The parts worth reading are the two things almost nobody tells you beforehand: some generators make less power on natural gas, and your gas meter may not be big enough.
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 generally keeps working when the electrical one does not.
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.
| Model | On natural gas | On propane | Difference |
|---|---|---|---|
| Cummins RS13A | 13 kW (54.2 A) | 13 kW (54.2 A) | None |
| Cummins RS17A | 17 kW (70.8 A) | 17 kW (70.8 A) | None |
| Cummins RS20A | 18 kW (75 A) | 20 kW (83.3 A) | 2 kW less on gas |
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.
This is the single biggest reason two apparently similar houses get very different installation quotes. It is also why we will not quote installation without seeing the property — the meter is a physical fact you cannot establish over the phone. See how installation cost actually breaks down.
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.
| 500 gal tank, full load | ~130 hours — about 5 days |
|---|---|
| 500 gal tank, half load | ~221 hours — about 9 days |
| 250 gal tank, half load | ~111 hours — about 5 days |
Real outages almost never hold a generator at full load, so the half-load figures are the more realistic planning numbers. The practical implication for a rural property: a 250 gallon tank is thin for a multi-day winter event, and 500 gallons is the size most people end up wishing they had started with.
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.
Questions about fuel
Which fuel is best for a standby generator in Dallas-Fort Worth?
Natural gas, for almost every home that already has a gas meter. There is no tank to fill, no fuel to go stale, and no runtime limit as long as the utility main holds pressure — which it generally does during the storm and ice events that cause outages here. Propane is the answer when there is no gas main at the address. Diesel is a commercial answer, and it starts at 20kW.
Does a generator make less power on natural gas than on propane?
On some models, yes, and it is rarely disclosed. The Cummins RS20A produces 20kW on LP but 18kW on natural gas — 83.3 amps versus 75 amps at 240 volts. The 13kW and 17kW units are identical on both fuels. The headline number on a spec sheet is normally the propane figure, so if you are on natural gas you should size against the smaller one.
How long will a 500 gallon propane tank run a whole house generator?
A 500 gallon tank holds about 400 usable gallons, because propane tanks are filled to roughly 80%. A 20kW unit at full load burns about 3.08 gallons an hour, giving you around 130 hours — a bit over 5 days. Real outages rarely run a generator at full load, and at half load the same tank stretches to roughly 221 hours, or about 9 days.
Can my existing gas meter handle a generator?
This is the question that catches most people out. A 20kW unit draws around 268 cubic feet of natural gas per hour at full load, which is roughly 268,000 BTU. A standard residential meter and service is often already close to capacity once a furnace, water heater, range and dryer are accounted for. Adding a generator frequently means the gas utility has to upsize the meter, the service, or both — and that is a utility request with its own timeline. It is the single biggest reason two installations at similar houses come back with different prices.
Can I switch fuels later?
The air-cooled Cummins units are field-selectable between natural gas and propane vapor, and they ship set to natural gas. Converting is a configuration change made by a qualified technician, not a parts swap. But the output changes with the fuel, so a unit sized tightly on propane may not cover the same loads on natural gas.
Not sure which applies to you?
The survey answers the meter question, the fuel question and the sizing question in one visit. It is free, and it is the only way to know what your address actually supports.
Rather just talk to someone?
Leave a number and we will call you. No sales script, no obligation.
Got it. We will call you. If you would rather not wait, ring (214) 957-4307 and ask for whoever is on generators.