What Size Generator for Whole House Backup: How to Size It Right

You are asking what size generator for whole house backup you need, and the answers online range from a rule of thumb to a sales pitch. The stake is real. A generator that is too small trips its breaker the moment your air conditioner tries to start, which is exactly when you need it. One that is too large costs more to buy, more to run and more to install, for capacity you never use.

This guide shows how whole house generator sizing is done properly: what is measured, which loads decide the size, how fuel type changes the numbers, and a worked example with clearly labeled example values so you can see the method. The method matters more than any chart.

Whole house generator sizing is measured in kilowatts

Standby generators are rated in kilowatts, written kW. One kilowatt is a thousand watts. A generator's rating tells you how much power it can deliver continuously. Your house's needs are measured the same way, which is why the question "what size whole house generator do I need" is really a question about how many kilowatts your house draws when the things you care about are running together.

Two kinds of numbers matter. Running load is what each appliance draws while it operates. Starting load, sometimes called surge or inrush, is the brief spike some equipment draws as it starts. Motors and compressors draw several times their running current for a moment at startup. A generator has to handle both: the steady total, and the spike when the largest motor kicks on while everything else is already running.

Why guessing by square footage fails

The most common shortcut is sizing by square footage. It fails because two houses of the same size can have very different electrical loads. One has gas heat, a gas range and a gas water heater. The other is all electric with strip heat, an electric range, an electric water heater and an EV charger. Their generator needs are not close.

The second common shortcut is sizing to the panel rating, for example assuming a 200-amp panel needs a generator able to supply 200 amps. That usually oversizes the generator, because no house draws its full panel rating at once. The panel rating is a ceiling, not a measure of use.

The reliable method is a load calculation: listing the loads the generator will carry, their running and starting values, and how they combine. It takes an electrician a short visit and some arithmetic, and it is the only number worth building a generator purchase on.

We do this as a standalone written report, priced in our book as the Load calculation and capacity report task, from $709. It is signed by a Texas licensed master electrician and gives you a size you can take to any generator dealer. Starting price. Your tech confirms the final number on site before any work.

Central air is usually the deciding load

In San Antonio, the air conditioner is almost always the load that sets the generator size. A central AC compressor is a large motor, and its starting current is the biggest single spike most houses produce. If the generator cannot absorb that spike while the refrigerator, lights and other loads are already running, the compressor stalls or the generator trips.

Houses with two AC systems raise the stakes, because the second compressor may start while the first is running. The calculation has to account for that, or the generator has to be told not to let it happen.

Two tools reduce the size an AC load demands. A soft starter is a device installed at the compressor that ramps the motor up gradually and cuts the starting spike. A load management module lets the generator system hold off a large load, such as the second AC or the water heater, until there is capacity to start it. Both are common, both are inexpensive next to stepping up a generator size, and both have to be planned into the install.

Whole home or essential loads

The next decision is what the generator will run. Whole-home backup means everything in the main panel stays on, which requires the largest generator and usually load management. Essential loads backup means a chosen set of circuits, typically refrigeration, lighting, internet, the furnace blower, a bedroom AC or a window unit, and the well pump if there is one, wired through a transfer switch that serves only those circuits.

Essential loads backup lowers the generator size, often substantially, and lowers fuel use. The trade is that some of the house goes dark in an outage. For many owners, keeping one zone cool and the kitchen running is the outcome that matters, and they would rather not pay to back up the dryer.

The transfer equipment changes with the choice. Our post on generator interlock or transfer switch in Texas explains the difference, and our generator installation page covers how we do it.

Natural gas versus propane changes the rating

Most standby generators are sold with two output ratings, one for propane and a lower one for natural gas. Natural gas carries less energy per unit, so the same engine produces less power on it. If your generator will run on a natural gas line, size against the natural gas rating. Buyers who read the larger propane number and then connect to natural gas end up with less generator than they paid for.

Natural gas has the advantage of continuous supply from the utility line, which matters in a long outage. Propane comes from a tank on site, and runtime depends on the tank size and how hard the generator is working. Both are common around San Antonio. In-town houses with gas service often use natural gas. Rural properties and Hill Country homes without a gas line often use propane.

The gas meter and piping also have to deliver enough gas for the generator along with your other gas appliances. That is a plumbing and gas utility question, and it should be checked before the generator is ordered.

A worked sizing example

The numbers below are example values to show the method. They are not a rule and they are not your house. Real running and starting values come from the nameplates on your equipment and the load calculation.

• Example house: gas furnace, gas water heater, electric range, one central AC system, refrigerator, lighting, internet and a garage door opener. Owner wants whole-home backup on natural gas.
• Example running loads: AC compressor and air handler 5 kW, electric range in use 4 kW, refrigerator and freezer 1 kW, lighting and electronics 1.5 kW, furnace blower off in summer, miscellaneous 1 kW. Example running total: 12.5 kW.
• Example AC starting spike: an additional 8 kW for a moment at compressor start, without a soft starter.
• Example worst case: 12.5 kW running, with the compressor starting while the rest is on. Without a soft starter the generator must absorb a brief demand well above its running total.
• Example result: with a soft starter cutting the spike, and a load management module holding off the range when the AC starts, a generator with a natural gas rating comfortably above 12.5 kW covers this example house. Without those measures, the example calls for a noticeably larger unit.

The point of the example is the structure. Running loads set the floor. The largest starting load sets the margin. Load management and soft starters reduce the margin you have to buy. Fuel type decides which rating you compare against. Your own numbers will differ, and they should come from your equipment, not ours.

Once you have a size, the next question is cost. Our guide to whole house generator cost in San Antonio breaks it down, and every flat price we publish is on our pricing page.

Size for the house you will have, not only the house you have

A generator is a purchase you expect to keep for many years, and the house it serves will change in that time. An EV charger, a pool, a converted garage with its own mini-split, or a switch from a gas water heater to a heat pump unit all add load. If any of those are likely, include them in the load calculation now as planned loads, so the size you buy still fits after the change.

The opposite mistake is also common: buying a much larger unit to cover loads that may never arrive. A larger generator burns more fuel at light loads and costs more to install. The better approach is usually a correctly sized unit with load management, which can be configured to hold off the new loads during an outage when they arrive.

Signs that an existing generator was sized wrong are easy to spot. The air conditioner fails to start or the generator shuts down when it does. Lights dim hard every time a compressor kicks on. The generator runs at full output for hours in summer. If you see those, a load calculation tells you whether a soft starter or load management fixes it, or whether the unit is simply too small.

If you already own a generator that struggles, call us to schedule a look, and we will tell you which fix fits before quoting anything.

What the electrical connection involves

Whatever size you settle on, the generator has to be connected to the house correctly. That means a transfer switch or approved transfer equipment so the generator never feeds power back onto the utility line, the right conductors between the generator and the switch, the load management or soft start wiring if you are using it, and a permit and inspection.

In our flat-rate book, the electrical side of a standby generator starts with the Standby generator electrical connection task, from $5,329. The written quote adds the transfer equipment and any load management your sizing calls for, and you approve it before work starts. Starting price. Your tech confirms the final number on site before any work.

The generator pad, gas piping and placement clearances are part of the same project. We coordinate the electrical work with the gas work so the generator is started, tested and inspected as one system, and we pull the electrical permit with the City of San Antonio Development Services Department or the local permit office where the house sits.

Get the size right before you buy

Wattsmith Electric is veteran-owned and licensed by TDLR under TECL 41806. We size generators from a load calculation, put the size and the reasoning in writing, and quote the electrical connection flat before any work starts. If the right answer is a smaller generator with a soft starter, we say so.

Call us and we will tell you over the phone whether your house needs a visit to size. To schedule a load calculation, contact Wattsmith Electric. Owners in the Hill Country can also read our generator readiness guide for Timberwood Park and Bulverde.