What Size Generator Do I Need?

The size of generator you need is decided by two numbers you build yourself: the total running watts of everything you want on at once, and the starting watts of the single worst moment, when your biggest motor fires while everything else is already running. You get both from a load list, and you build a load list out of your panel directory and the nameplate stickers on your appliances.

You do not get it from square footage. That is the honest headline, and it is worth sitting with for a second, because square footage is how nearly every generator gets sold.

Two 1,800 square foot houses on the same street can need machines that differ by half. One has gas heat, gas water heat, and city water. The other has a heat pump, an electric water heater, and a submersible pump in a well out back. Same footprint, completely different electrical appetite. The square footage never knew the difference, and neither does anyone quoting from it.

This guide is the arithmetic. If you want the other half of the purchase, the fuel decision, air-cooled versus liquid-cooled, and what actually goes wrong with these installs, that is choosing a whole-house generator. If you want national published cost ranges, those are in whole house generator cost. This page is about getting to a number.

Running watts and starting watts are two different specifications

Look at any generator and you will find two wattage figures on the label, and understanding why there are two is most of the battle.

Running watts, sometimes called rated or continuous watts, is what the machine can produce indefinitely. This is the real capacity, and it is the number the generator lives at.

Starting watts, sometimes labeled surge or peak, is a larger figure the machine can produce for a few seconds. It exists for one reason, and that reason is motors.

Loads come in two flavors, and they behave completely differently at the moment you switch them on.

Resistive loads turn electricity into heat. An electric water heater element, a range burner, a baseboard heater, an old incandescent bulb. These are the polite ones. A 4,500 watt water heater element draws 4,500 watts the instant it energizes and 4,500 watts an hour later. Running and starting are the same number, and you can add them up on a napkin.

Inductive loads turn electricity into motion. Compressors, pumps, blowers, the motor in a well casing. These are the impolite ones. At the instant a motor starts, the rotor is not turning yet, and the motor briefly behaves almost like a short circuit. It pulls several times its running current, commonly four to eight times, until the rotor comes up to speed. That spike lasts under a second. It is also the thing that stalls generators.

When an undersized machine meets a starting surge it cannot cover, it does not fail gracefully. The engine bogs, voltage sags, lights across the house dim, anything with a clock reboots, and usually the generator's own breaker trips. The engine keeps running the whole time, which is the confusing part: nothing looks broken, and yet nothing works.

So the sizing question has two halves. Running watts have to cover the steady-state sum. Starting watts have to cover the worst instantaneous moment. A machine can pass one test and fail the other.

Where the real numbers live, and it is not a website

Generator wattage charts are all over the internet, and they are a fine starting point for a first draft. They are also generic, frequently copied from each other, and often based on appliances that are older and hungrier than yours. Treat them as a sketch.

The authority is the nameplate on your own equipment, and the panel door in your own house.

Start at the panel. Open the door and read the directory. Every circuit in the house is on that list, which makes it the only complete inventory you have of what your house can even ask for. Go down it and mark each circuit as must-run, nice-to-have, or off. That single pass does more to size a generator honestly than any online calculator, because it forces the decision the calculator quietly makes for you.

Then read the labels. Nearly every appliance carries a plate or sticker with its electrical draw, usually on the back, inside the door frame, or on the side of the outdoor unit. Some list watts directly. Most list amps and volts, and for a load list, volts times amps is close enough. On motors that math slightly overstates the true wattage, which is the safe direction to be wrong in.

A few label conventions worth knowing, because they trip people up:

• LRA on a motor or compressor nameplate stands for locked rotor amps. That is the starting surge, stated by the manufacturer, in the one place it is actually accurate for your machine. RLA is rated load amps, the running figure.
• MCA and MOCP on an HVAC nameplate are minimum circuit ampacity and maximum overcurrent protection. Those are wire and breaker sizing numbers. They are not what the unit draws, and using MCA as a load figure will oversize your generator.
• A microwave labeled 1,000 watts is describing its cooking output, not its draw. It pulls roughly half again that much from the outlet. Small appliances lie in this direction routinely.

The two loads that decide the whole answer

Once the list exists, you will find something slightly deflating: most of it does not matter. Lights, phone chargers, televisions, routers, and a modern refrigerator together add up to less than people imagine, especially in an LED house. You could run all of that on a small machine.

The size of your generator is decided by two loads, and usually only two.

The well pump

If your water comes from a well, everything about your outage changes. No electricity means no water, which means nothing to drink, no way to flush a toilet, and no way to wash your hands. For a lot of houses on the rural roads out past Coosa, around Plainville, and up in Chattooga County near Summerville, that single load is the actual reason a generator gets bought, and the food in the freezer is the second reason.

A submersible well pump is also one of the hardest things in a residential system to start. It is a motor at the bottom of a deep hole, lifting a column of water, and it draws a hard surge every time the pressure switch calls. Commonly published figures put a half horsepower pump around 800 to 1,000 running watts, with a starting surge in the range of several thousand. A one horsepower pump roughly doubles both. Your pump's own nameplate, or the paperwork from whoever set it, beats every one of those figures.

The practical consequence is that a well pump can force a generator up a size band all by itself, and it is the reason the small inverter generator that handles your refrigerator beautifully will trip on the first call for water.

The air conditioner

The other decider, and in a Georgia August not a trivial one.

A central air conditioner is the largest motor most houses own, and it starts the way a well pump does, only bigger. A three ton system might draw somewhere around 3,500 to 5,000 watts while it runs, counting the outdoor compressor and the indoor blower. The instant the compressor starts, that same system can momentarily demand several times as much. That momentary demand is what sizes the machine, and it is why generators that comfortably carry a whole house of ordinary loads still cannot start the AC.

There is a real lever here that does not get mentioned enough. A soft starter, sometimes sold as a hard start kit, is a device installed at the condenser that ramps the compressor up instead of slamming it on. Published figures for the reduction in starting current vary by unit, but the effect is substantial, often cutting the surge by half or better. On a generator project that can be the difference between two size bands of machine, and it is a far smaller purchase than the size band. It is worth raising with whoever handles your HVAC before you finalize a generator size.

Winter runs on the same logic with the numbers rearranged. If the heat itself comes from gas, the electrical side of it is small, because you are only powering a blower and a control board. If the heat comes from electricity, the appliance is the load, and electric resistance strip heat in particular is among the largest things any house owns. Check which one you have before you assume winter is the easy season.

Load management, and why "whole house" usually means something narrower

Here is where the arithmetic stops being straight addition and starts working in your favor.

Straight addition assumes the worst case is permanent. It is not. The worst case is a moment, and modern standby systems are built to make sure that moment never contains two big motors at the same time.

The tool for that is load management, and it works by scheduling rather than by capacity. The controller watches total demand and postpones things. It parks the water heater for the seconds the compressor needs. It holds a second air conditioner until the first one has settled. It drops the dryer if the number climbs toward the limit. Nothing is denied, only delayed, and the delays are measured in seconds.

That changes the arithmetic in your favor, because the question stops being "what is the sum of everything" and becomes "what is the largest group that can honestly be running together." The second number is smaller, sometimes dramatically. It is the reason a real load calculation on a real panel so often lands under the figure a brochure suggests, and it is not something you will find emphasized in a sales pitch built on square footage.

Which brings up the phrase itself. "Whole house" on a spec sheet almost never means every circuit at full draw at the same moment. It means the house feels normal, with the controller doing the staging quietly enough that you do not notice. That is the design working, not a compromise in it. Worth knowing in advance, though, so that a brief pause somewhere in the house during an outage reads as the system doing its job rather than as something wrong.

It also means the essential-circuits option deserves a fair hearing. A manual transfer switch carrying six to ten chosen circuits, or a portable feeding the panel through an interlock, covers the refrigerator, the freezer, the furnace blower, the well pump, and the lights for a fraction of the standby project. That is the comparison in transfer switch vs. interlock kit, and for a lot of households it is the right answer.

If you only want to keep the food cold

This is a common enough goal that it deserves its own paragraph, and the answer is genuinely reassuring.

A refrigerator is one of the easiest loads to back up. A modern one draws a couple hundred watts while the compressor runs, less when it is coasting, with a brief spike at start that a small machine covers without effort. A chest freezer is similar and often lighter. Add some LED lights, phone and laptop charging, and a fan, and you are still working with a small total.

A generator in the 2,000 watt class, particularly an inverter model, handles that comfortably. What it will not do is start a well pump or touch a central air conditioner, so be clear with yourself about which problem you are solving. If the goal is strictly food and lights, you can also skip the electrician entirely: outdoor-rated extension cords run from a properly placed portable generator to the fridge and freezer are legal, permit-free, and effective. We wrote up the food side of this, including the federal guidance on how long a closed refrigerator and freezer actually hold, in how long food lasts in a power outage.

Whatever size you land on, the placement rule does not scale down with it. A small generator makes carbon monoxide just as readily as a large one, so it goes outdoors only, well clear of the house, exhaust aimed away from any opening, and never inside a garage, carport or basement, open door or not. Working CO alarms indoors belong to this setup rather than sitting alongside it.

Rough bands, offered as orientation only

With every caution above still standing, here is what the size bands tend to cover in practice. Use them to know roughly which conversation you are in, not to place an order.

• Around 2,000 watts. Refrigerator, freezer, lights, charging, a fan. The food-and-comfort tier.
• Roughly 3,500 to 5,000 watts. The above plus a furnace blower, a sump pump, and a small well pump, with the big items taken one at a time.
• Roughly 7,500 to 9,500 watts. Essentials plus a well pump without much staging, and possibly a window unit. Central air is generally still out of reach.
• Standby machines in the high teens to mid twenties of kilowatts. True whole-house behavior for most single-family homes here, including starting a central air conditioner, with load management doing the staging.

Notice that the jump from the third band to the fourth is almost entirely about one appliance. That is not an accident, and it is the clearest illustration of why the air conditioner and the well pump are the two loads worth being careful about.

What we actually do, and what your panel has to say about it

When we size a generator, the work happens at your panel and your equipment, not at a desk with your address in a calculator. We read the directory, walk the loads with you, take the nameplate figures off the real appliances, sort the list into what must run and what can wait, and then work the worst-case starting moment rather than just the total. That is a load calculation, and it is the only method that survives contact with an actual outage.

Two results come up often. The first is that the honest number is smaller than the one the homeowner arrived expecting, which is a good outcome for everybody except a salesperson working from an address. The second is that the panel has to be settled before a generator can land on it at all: an older service with nowhere to put the transfer gear, no spare spaces, or a brand of panel with a reputation. That is a separate project with its own timeline, and finding it during the estimate beats finding it on install day. It sits on our panel upgrades and breaker box replacement page.

If you want a number for your own house, the shortest path is to list what has to keep running when the power drops, then photograph two things: the inside of the panel door with the directory legible, and the data plate on the outdoor air conditioning unit. Those two pictures answer most of the sizing question. Everything else about the install is on our generator installation page.

Do it on a calm week. A load list is a twenty minute job with the lights on and an impossible one by flashlight with the freezer warming up.