Key takeaways
- Generac's published ratings put the Guardian 18kW at 18,000 watts on propane and 17,000 watts on natural gas.
- A Generac 18kW is often the first size that carries one central air system with the essentials, depending on the system and the house.
- If your home is on natural gas, size from the 17,000 watt figure, not the 18,000 on the label.
- Two air conditioning systems, or one large one with electric cooking and a pool, usually point to the 22kW or to load management.
What can a Generac 18kW run comes down to one question in most San Antonio homes: can it carry the air conditioning? Generac's published ratings put the Guardian 18kW at 18,000 watts on propane and 17,000 watts on natural gas. For many houses with a single central air system, that is the size where cooling and the essentials can run together. For others, the system is too large or the list too long. Here is how to tell.
The ratings, engine, breaker and weight are on our Generac 18kW generator page, cited to Generac's product page and spec sheet.
Propane or natural gas changes the number
Unlike the 14kW, the 18kW is rated lower on natural gas than on propane. Generac lists 18,000 watts on propane and 17,000 watts on natural gas, a difference of 1,000 watts. That difference is small on paper, but it matters when you are sizing close to the line.
Many homes inside San Antonio are on natural gas from the street. If yours is, the figure to size against is 17,000 watts. Planning around the propane number on a natural gas house is how a generator ends up undersized on the first hot afternoon of an outage.
Why one central air system is the deciding load
A central air conditioner's compressor is a motor, and a motor draws a surge of current as it starts. In a Texas home, that compressor is usually the largest single load, and its start is the largest moment of demand the generator sees. The running load of the air conditioner may fit easily. The start is what has to fit too, at the same time as everything else that is already running.
The compressor's nameplate and the outdoor unit's data plate list its running amps and its locked rotor amps, which is the starting figure. Those two numbers tell us more than the tonnage alone. A newer, variable speed system often starts more gently than an older single stage one of the same size.
This is exactly what we read at a free Generac sizing visit: the plate on the outdoor unit, the panel, and the list of what you want running.
What else fits beside the air conditioning
With one central air system running, the rest of the 18kW's capacity goes to the essentials. The usual list is the refrigerator and freezer, the lights you use, kitchen and living room outlets, the internet router and the home office, and a well or sump pump where there is one. The nameplate on each appliance gives its running watts, and the total of everything on together has to stay under the rating.
- Usually fine together: one central air system, the refrigerator, lights, outlets, the router and a microwave.
- Often needs managing: an electric range, an electric dryer and an electric water heater, if they run while the air conditioner does.
- Usually pushes past it: a second central air system or a pool pump and spa heater running with everything else.
Soft starters and load management on an 18kW
Two tools let an 18kW do more than its raw rating suggests. A soft starter on the air conditioner's compressor reduces the starting surge, so the start fits more easily beside the other loads. Load management holds back a large appliance, such as the water heater or the dryer, while the air conditioner runs, and lets it run when there is room.
Neither tool adds power. Both make better use of the power the 18kW has. For a home that is just over the line on paper, one of them is often less expensive than moving up a size.
The switch options that include load management are in our guide to generator transfer switch types.
When to look at the 14kW or the 22kW instead
If you do not need cooling during an outage, or your home has a small air conditioner, the 14kW may carry your list, and Generac rates it at 14,000 watts on both fuels. If your home has two air conditioning systems, or one large system with electric cooking and a pool, the 22kW is the next step. Generac rates the 22kW at 22,000 watts on propane and 19,500 on natural gas.
The smaller size is covered in what can a Generac 14kW run, and the larger one in what can a Generac 22kW run.
Mistakes that make an 18kW feel too small
Most complaints about an undersized 18kW trace back to three things. The first is sizing from the propane figure on a natural gas house, which leaves 1,000 watts less than planned. The second is leaving the electric water heater and the dryer free to run while the air conditioner starts, so the start lands on top of two large loads. The third is adding loads after the install, such as a second freezer or a workshop, without checking the total again.
Each of these has a simple fix. Size from the fuel the house actually has. Put the large resistive loads on load management so they wait while the compressor starts. And when you add something big to the house later, ask whether it belongs on the generator at all.
Medical equipment and working from home
Many homes buy a standby generator because someone in the house depends on power: an oxygen concentrator, a CPAP machine, a medication refrigerator, or a home office that cannot go dark. These loads are usually small next to the air conditioner, but they cannot wait their turn. On an 18kW they are placed first on the list, and load management is set so nothing ever holds them back.
If a medical device is the reason for the generator, tell us at the sizing visit. It changes how the list is ordered and which circuits are wired so they never drop, even for a moment, beyond the few seconds it takes the transfer switch to move the house to the generator.
- Medical equipment and medication refrigeration first.
- The air conditioner next, on a soft starter where it helps.
- The kitchen, the lights, the outlets and the router after that.
- Large resistive loads last, managed so they wait.
Write the list down before the visit if you can: which rooms you live in during an outage, which appliances you would miss first, and anything a person in the house depends on. It makes the visit shorter and the recommendation better.
What the install involves
We set the unit where the setbacks from windows, doors and the property line allow, install and wire the transfer switch, pull the electrical permit, and start the unit up and test it under load. The gas line is a licensed plumber's work and is quoted separately. On a natural gas house the plumber also checks whether the meter can supply the generator along with the rest of the house.
The full process is on our Generac generator installation page, and every size we install is on our Generac generators page. We sell and install Generac home standby and commercial generators.
What an 18kW costs
The installed price is the unit, the transfer switch, the wiring, the gas line, the placement and the permit. On an 18kW sized close to the line, a soft starter or a load managed switch can be part of the price too, and it may still cost less than the next size up.
What moves each part is in our Generac generator cost guide. We write your installed price after the sizing visit.