Generator Size Calculator

Buying the wrong size generator is one of those mistakes you only make once. Too small, and your breaker trips the moment your AC compressor kicks in. Too large, and you've spent hundreds of extra dollars on fuel and a machine that's working harder than it needs to at low loads.

This generator size calculator helps you avoid both problems. Add each appliance you plan to run, enter the running watts and starting watts, and the calculator does the rest — totaling your load, converting to kVA, and recommending the right generator size with a built-in safety margin.

Whether you're prepping for hurricane season, outfitting an RV, or pricing out a whole-house standby unit, the math here is what matters. Get this right, and everything else — brand, fuel type, features — falls into place.

At its core, generator sizing means matching a generator's output to the total electrical load you need to run. Simple enough in theory. In practice, there's a wrinkle that catches a lot of people off guard.

Your appliances don't all behave the same way electrically. A lamp draws a steady 60 watts from the moment you flip the switch. A refrigerator draws 150 watts while running, but the instant that compressor kicks on, it pulls 400-600 watts for a few seconds. An air conditioner is even worse — a 3-ton central unit might run at 3,500 watts but surge to 7,000+ watts on startup.

Size a generator based only on running watts and that first startup surge will overload it. Size purely on surge watts and you'll overspend on a generator you don't need. The right approach splits the difference, and this calculator handles that math for you.

If you only learn one thing before buying a generator, make it this.

Running watts (sometimes called rated watts) are the steady, continuous power an appliance draws while operating normally. This is the number on most appliance labels — 150 watts for a refrigerator, 1,200 watts for a window AC, 1,400 watts for a circular saw.

Surge watts (also called starting watts) are the short burst of extra power that motors demand during their first few seconds of operation. Electric motors need a jolt of energy to get the rotor spinning, and that jolt can be substantial. A 1/2 HP sump pump running at 800 watts might surge to 1,500 or even 2,000 watts at startup.

Here's the pattern worth remembering: anything with a motor — refrigerators, AC units, sump pumps, well pumps, power tools — will have surge watts significantly higher than running watts, usually 1.5x to 3x. Resistive loads like toasters, space heaters, light bulbs, and phone chargers? Their running and surge watts are essentially the same. No motor, no surge.

This calculator asks for both numbers per appliance because it's the only way to get a recommendation you can actually trust.

1. Enter your first appliance's running watts. Look for this on the nameplate sticker (usually on the back or bottom of the appliance), in the owner's manual, or search "[your appliance] wattage" online. Enter it in the "Running watts" field.
2. Enter the surge watts. Check the same sources for "starting watts," "surge watts," or "LRA" (locked rotor amps — multiply by voltage to get watts). If you can't find it and the appliance has a motor, estimate 2-3x the running watts. No motor? Use the same number as running watts.
3. Click "Add Another" for each appliance you plan to run. Don't just list everything in your house. Focus on what you'd actually need running at the same time during an outage or at your worksite.
4. Check your results. The calculator shows total rated watts, total surge watts, apparent power in kVA, and — most importantly — the recommended generator size in kVA with a safety margin already factored in.
5. Shop by the recommended kVA number. Look for generators rated at or above that figure. You're set.

If you don't have your appliance nameplate handy, these typical ranges will get you in the right ballpark:

These are estimates. Your actual appliance may differ, so always check the nameplate when the stakes are high — especially for big-ticket items like AC units and well pumps.

Abstract numbers only help so much. Here's what generator sizing actually looks like for the most common scenarios.

The power's out, a storm just rolled through, and you just need the basics: food stays cold, the basement stays dry, you have light and can charge your phone.

What you need: A 3,000–3,500 watt portable generator. These run $400–$800, fit in a garage, and are easy to find at any hardware store. This is the most popular setup for a reason — it covers what actually matters at a price that doesn't sting.

You don't want to just survive the outage — you want AC, hot water, and laundry.

What you need: A 15–20 kW standby generator. These are permanently installed next to your house and kick on automatically within seconds of losing power. Expect to invest $5,000–$15,000 installed, but if you live somewhere with frequent outages, the peace of mind is hard to put a price on.

You want air conditioning at the campsite without waking up every neighbor in a half-mile radius.

What you need: A 3,500–4,000 watt inverter generator. Inverter models produce cleaner power (safer for electronics) and run significantly quieter — around 55–65 dB, which is conversational volume. Your campground neighbors will thank you.

Power tools all day, plus a compressor for nail guns.

What you need: A 7,500–10,000 watt conventional portable generator. Jobsite generators take a beating, so look for steel-roll-cage frames, GFCI-protected outlets, and models rated for construction use. You won't run every tool at once, but that compressor surge is no joke — give yourself plenty of headroom.

When you shop for generators, you'll notice some are rated in watts and others in kVA. They're related, but they're not the same thing, and confusing them can lead to buying a generator that delivers less power than you expected.

Watts = the actual usable power (what your appliances consume). kVA (kilovolt-amperes) = the total electrical output, including power "lost" to inefficiencies in the circuit.

The formula connecting them:

Watts = kVA x Power Factor x 1,000

Most household loads have a power factor around 0.8. So a generator rated at 10 kVA actually delivers about 8,000 usable watts — not 10,000. That 20% gap matters.

Bottom line: when comparing generators, check whether the rating is kW or kVA. If it's kVA, multiply by 0.8 to get a realistic wattage estimate. This calculator shows both numbers so you can shop confidently no matter how the generator is labeled.

It's tempting to match your generator size exactly to your total load. Don't.

Here's what happens in the real world: your refrigerator compressor cycles on at the same moment your sump pump kicks in after heavy rain. Individually, your generator handles either surge just fine. Both at once? That's an overload, even if your running watts are well within range.

Beyond overlapping surges, there are practical reasons to build in headroom:

• Altitude and heat reduce output. Generators lose roughly 3.5% capacity per 1,000 feet of elevation, and hot days shave off even more. A generator rated at 7,500 watts at sea level might only produce 6,500 watts in Denver on an August afternoon.
• Sustained full-load operation burns more fuel and shortens engine life. Running at 75% capacity gives your generator breathing room and can significantly extend the time between maintenance.
• You might add something later. An extra freezer, a medical device, or a space heater on a cold night. A little spare capacity means you won't have to recalculate from scratch.

The standard recommendation is 20–25% above your total surge load. The good news: this calculator already bakes that safety margin into its "Required generator size" output, so you can trust the number it gives you.

Quick guidance: if you're powering sensitive electronics (computers, medical equipment, TVs), go with an inverter generator or a standby unit. Conventional portables produce rougher power that can potentially damage sensitive devices over time. If noise matters — and if you have neighbors, it does — inverter generators are worth the premium.

• Size for what you'll actually run, not your entire electrical panel. Most people don't run the dryer, oven, AC, and water heater simultaneously even when the power is on. Be realistic about your outage priorities.
• Start with your biggest surge load. Central AC and well pumps have the largest startup surges. If you can cover those, everything else usually fits comfortably.
• Gasoline goes stale. If you're buying a portable generator for occasional emergency use, treat your stored fuel with stabilizer or plan to rotate it every 3–6 months. Nothing's worse than a generator that won't start when you actually need it.
• Check your transfer switch requirements. If you're connecting a generator to your home's electrical panel (which you should, rather than running extension cords everywhere), you'll need a transfer switch. Manual transfer switches are affordable; automatic ones pair with standby generators.
• Test it before you need it. Run your generator under load for 30 minutes every few months. Confirm it starts, check oil levels, and make sure your setup actually works. Outage prep done during good weather is infinitely less stressful.