The output penalty, with a real number
This is the clearest data point in the whole comparison, and it comes straight from a manufacturer's own catalog.
Generac rates the Guardian 24kW at 24 kW on liquid propane and 21 kW on natural gas — the same engine, the same unit, a 3 kW difference. That is a 12.5% reduction for choosing natural gas.
The reason is energy density. Propane carries more energy per unit volume than natural gas, so an engine drawing the same volume of fuel makes less power on gas.
Practically, this means you size against the fuel you will actually use. A unit marketed as 24kW is a 21kW machine if you connect it to the gas main, and if your calculation needed 22kW you have just under-bought.
The comparison
| Natural gas | Propane |
|---|
| Output from the same unit | Lower — 21 kW on the Guardian 24kW | Higher — 24 kW |
|---|
| Supply | Unlimited, from the main | Finite — whatever is in the tank |
|---|
| On-site storage | None needed | A tank, sited and permitted |
|---|
| Refilling during an outage | Not applicable | A delivery truck that may not come |
|---|
| Fuel goes stale | No | No |
|---|
| Cold-weather behavior | Unaffected | Vaporization slows in severe cold |
|---|
| Available everywhere | No — needs a main at the property | Yes |
|---|
| Vulnerable to | Main interruption after earthquakes or breaks | Running the tank dry |
|---|
The 21/24 kW figures are Generac's published dual ratings for the Guardian 24kW. Everything else is the general behavior of the two fuels rather than one manufacturer's claim. Fuel prices vary far too much by region and season for a running-cost comparison to be useful here.
Natural gas: the case for
It never runs out. That is the whole argument and it is a strong one. A gas-fed standby unit can run for a week without anyone doing anything, which is precisely the scenario people buy standby generators for.
No tank to site, no permit for the tank, no delivery to schedule, nothing to monitor. The weekly exercise cycle draws from the main and appears as a small line on the gas bill rather than as a slowly emptying tank.
And in an extended outage, "unlimited" is worth more than 12.5% of output, because the thing that actually ends most standby runs is fuel rather than capacity.
Natural gas: the case against
- The output penalty is real and you have to size around it. Compare units on their natural-gas rating.
- It requires a main. Rural properties frequently have no gas service, which settles the question.
- Supply pressure and pipe sizing matter. A standby unit is a large gas appliance, and the existing pipe run to the house may need upsizing. This is a real installation cost that often surprises people.
- The main can be interrupted. Rare, but correlated with exactly the disasters you are preparing for.
Propane: the case for
- More output from the same unit — 24 kW rather than 21.
- Available anywhere. No main required, which makes it the default for rural properties.
- You own the fuel. A full tank before the season is genuine independence from any utility.
- It does not degrade. Unlike gasoline, propane stored for years is still propane.
- It may already be there. If you heat, cook or dry with propane, the tank and the supplier relationship exist already.
Propane: the case against
- The tank is finite. This is the whole trade. Consumption at load is substantial and a multi-day outage will make a real dent.
- Refilling during a widespread outage is uncertain. Delivery trucks are also affected by the storm, and everyone calls at once.
- The weekly exercise cycle draws it down steadily, which owners are sometimes surprised by. Factor it into your refill schedule.
- Cold slows vaporization. A larger tank performs better than a small one in severe cold, which matters because winter is when you need it.
- Siting and permits. The tank has setback requirements and takes up yard space.
How to actually choose
For most people this is decided by circumstance rather than preference:
- You have a gas main at the property. Choose natural gas, and size against the natural-gas rating. Unlimited runtime beats 12.5% of output for the outages standby generators exist for.
- You have no main. Propane, and size the tank for the longest outage you consider plausible rather than the average one.
- You already have a propane tank for heating. Propane, almost certainly — the infrastructure and the supplier are already there.
- You are in earthquake country with a main. Worth talking to your installer about whether propane, or a battery fallback, better suits a region where gas supply and power fail together.
Sizing follows the fuel, not the other way round
The sequence that avoids the common mistake:
- Decide the fuel from the list above — it is usually determined by your property.
- Work out what you actually need to run.
- Compare units on their rating for your fuel, not on the headline number.
- Add margin, then talk to an installer about gas pipe sizing or tank siting before committing.
Sizing a standby unit works through the load calculation, and the installation guide covers what the fuel choice does to the job itself.
