Key takeaways
- Generac generator sizing starts with a list of the loads you want running during an outage, not with the size of the house.
- In a Texas home the central air conditioner or heat pump is usually the load that decides the size, because of the current it draws when the compressor starts.
- A soft starter on the compressor and load management modules on large appliances can let a smaller unit carry the same house.
- Whole house coverage and essential circuit coverage are two different plans. Decide which one you want before comparing any units.
Generac generator sizing is the first question in any home standby project, and it is the one most often answered badly. A unit that is too small trips its own breaker the first hot afternoon the air conditioner starts. A unit that is too large costs more to buy, more to fuel and more to install than the house ever needed. The right answer comes from the loads in your home, measured and listed, and in Texas the air conditioner usually settles it. This guide walks through how a sizing is done, so you know what to expect from the visit and what to ask about.
We sell and install Generac home standby and commercial generators. If you would rather have someone walk the house with you, our free Generac generator sizing visit is the way to start. The rest of this post explains what happens during one.
Start with a load list, not square footage
Two houses of the same size can need very different generators. One has gas heat, a gas water heater and a gas range. The other is all electric, with a heat pump, an electric water heater, an electric range and a pool pump. Square footage tells you nothing about which one you are looking at. A load list does.
The list names every load you want running during an outage and how much power it needs. Some of that comes from nameplates on the equipment, some from the panel schedule, and some from the way the household actually uses the house. A family that works from home and keeps a refrigerator, a freezer and a well pump running has a different list from a couple who only want the lights, the fridge and one air conditioner.
- Heating and cooling: central air conditioner, heat pump, air handler or furnace blower.
- Kitchen: refrigerator, freezer, range, microwave, dishwasher.
- Water: electric water heater, well pump, sewage or sump pump.
- Everyday circuits: lights, outlets, garage door, internet and phones.
- Large extras: pool pump, spa, electric vehicle charger, shop tools.
Why the air conditioner usually decides the size
Most loads draw about the same current when they start as when they run. A compressor motor does not. When a central air conditioner or a heat pump starts, its compressor pulls a large burst of current for a moment before it settles down to its running current. That starting current is printed on the unit's nameplate, often as locked rotor amps, and it is far higher than the running number.
A generator has to be able to supply that burst without its voltage sagging so far that lights dim, electronics reset or the compressor fails to start. In a Texas summer, the outage you are planning for is very often a hot one, and the air conditioner is the load the household cares about most. So the sizing question becomes: which unit can start this compressor, with everything else on the list already running? Two systems in one house make the question harder, because the second compressor may try to start while the first is already running.
We wrote a general guide to sizing for whole house backup in our post on what size generator a whole house needs. This post stays on the Generac side of that question and on the tools that change the answer.
Soft starters and what they change
A soft starter is a device added to an air conditioner or heat pump compressor that ramps the motor up more gently, so the starting burst is smaller. It does not change how much the system draws once it is running. It changes the peak the generator has to supply in the first moment.
That matters for sizing because the starting burst is often what pushes a house into a larger unit. With a soft starter on the compressor, a smaller generator may be able to start the same air conditioner with the same loads running. Whether one fits your equipment, and how much it changes the answer, depends on the compressor and the starter. It is a question to raise during the sizing visit, not something to assume.
Load management: sharing the generator
Load management lets a generator carry more of the house than it could run all at once. Generac systems can use load management modules that control large appliances such as a second air conditioner, an electric water heater, a pool pump or an EV charger. When the generator is near its limit, the module holds that load off. When there is room again, it lets it run.
In practice that means the house works close to normal during an outage, but the biggest loads take turns. The upstairs air conditioner may wait while the water heater recovers, or the pool pump may sit idle until the house cools down. For a lot of Texas homes, load management is the difference between a unit that fits the budget and one that does not.
Whole house or essential circuits
There are two broad ways to set up a home standby system, and the sizing depends on which one you choose.
- Whole house: the transfer switch moves the entire panel to the generator. Every circuit is available, and load management keeps the largest loads from running at the same moment.
- Essential circuits: a smaller set of circuits, such as the refrigerator, lights, outlets, internet and one air conditioner, is moved to a separate panel that the generator supplies. Everything else stays dark during an outage.
Whole house coverage is simpler to live with, because nobody has to remember which outlets work. Essential circuit coverage can allow a smaller unit. Neither one is right for every house. Decide which you want first, then size to it.
The transfer switch is a large part of that decision, and we compare the types in our guide to generator transfer switch types. Our transfer switch installation page covers how that part of the job is done.
Fuel and the size of the unit
A home standby unit runs on natural gas or propane, and the larger the unit, the more fuel it needs at full load. That means the gas meter or the propane tank has to be able to supply the unit you choose. On natural gas, the meter and the line from it have to carry the generator along with the furnace, the water heater and the range. On propane, the tank has to be large enough for the run time you want.
Gas line and gas connection are not included in our electrical work. That is a licensed plumber's work and we quote it separately, and we coordinate with the plumber so the line is sized for the unit. Our post on natural gas vs propane standby generators covers the fuel decision in more detail.
What a sizing visit covers
A sizing visit is where the load list gets built. We look at the electrical panel and its circuits, read the nameplates on the air conditioners, heat pumps and other large equipment, ask how the household uses the house during an outage, and look at where the unit and the transfer switch could go. We also note the gas meter or the propane setup, because the fuel supply has to match the unit.
From that we recommend a unit size, whole house or essential circuit coverage, any soft starter or load management that makes sense, and the transfer switch to go with it. You get a written price before any work is done. In San Antonio the installation needs an electrical permit through the City of San Antonio Development Services Department, which we pull, and the work is inspected under the 2023 National Electrical Code the city adopted effective May 1, 2025.
For how the install itself is done, see our Generac generator installation page and our generator installation service. To have the house sized, book the Generac generator sizing visit.