What Size Generator Do I Need?
Short answer: size a generator by running watts plus the single largest starting surge. Add the running watts of everything you’ll run at once, then add only the biggest motor’s starting watts on top. Backup essentials need about 1,500 running / 2,500 surge watts (a 2,000–3,000W unit); add a well or sump pump and you need ~5,000–7,500W; a whole house with central air and electric heat needs a 10,000–22,000W standby.
Most generator “size” advice tells you to add up the watts on every appliance and buy a generator at that number. That math is wrong twice over: it ignores that you rarely run everything at once, and it ignores the brief startup surge that actually determines whether a motor will spin up. Here is the honest method — the same one Honda and Champion publish — that our free calculator uses.
Step 1 — Understand running watts vs starting watts
Every motor-driven appliance has two numbers. Running watts (also called rated or continuous watts) is the steady power it draws once it’s up and running. Starting watts (surge or peak watts) is the brief spike a motor demands at the instant it switches on, as it overcomes inertia — often two to three times the running figure, and gone in a fraction of a second.
A refrigerator that runs on about 200 watts can spike to 1,200 watts the moment its compressor kicks in. A 1/2 hp well pump that runs near 1,000 watts can surge to 3,000 watts or more. Resistive loads — lights, a microwave, a space heater, an electric water heater — have no motor, so their starting and running watts are essentially the same. The full breakdown is in our running vs starting watts guide.
Step 2 — List what you’ll actually run at the same time
You almost never run every appliance simultaneously. The honest load is the set of things you genuinely need on at once — typically the fridge, the heating system’s blower, some lights, the internet, and a pump if you have one. The microwave, the washing machine, and the coffee maker get used in short bursts, often when you can pause something else. Here are typical wattages to build your list from:
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 150–800 | ~1,200 |
| Furnace blower (gas, 1/4–1/2 hp) | 600–875 | 1,000–2,350 |
| Sump pump (1/3–1/2 hp) | 800–1,050 | 1,300–2,150 |
| Well pump (1/3–1/2 hp) | 750–1,000 | 1,400–4,000 |
| Window AC (10,000 BTU) | 1,500 | 2,200 |
| Central AC (~3 ton) | 3,500–4,100 | 5,000–6,000 |
| Electric water heater | 4,500 | 4,500 |
| Microwave | 1,000–1,500 | 1,000–1,500 |
| Lights (each) | as marked (e.g. 60W) | same |
| TV / media | 120–300 | 120–300 |
Typical ranges, from Honda Power Equipment and DonRowe appliance charts — always confirm your own appliance’s nameplate, since motor sizes vary.
Step 3 — Do the sizing math
This is the whole method in two lines:
- Add the running watts of everything that will run at once. This is the steady load your generator carries continuously.
- Add the single largest starting surge on top — only one motor starts at a time, so you add only the biggest one, not all of them.
Honda’s own worked example makes this concrete. A fridge, furnace fan, five lights, a TV, and a microwave have a running total around 1,400–2,900 watts depending on what’s on, but because the largest single starting surge is about 1,600 watts, a 3,000-watt generator runs the whole set — you don’t need to add every appliance’s surge together. Your generator fits when its rated watts cover the running total and its surge watts cover that peak. The calculator does this for you →
Step 4 — Choose portable, inverter, or standby
| Type | Typical size | Best for |
|---|---|---|
| Inverter | 1,000–4,000W | Quiet, clean power for electronics, RVs, camping, and light home backup. Fuel-efficient, but pricier per watt. |
| Portable | 3,000–8,500W | Home essentials plus a well or sump pump. Started by hand, refueled by hand, run through a transfer switch or interlock. |
| Standby | 10,000–26,000W+ | Whole-house backup including central AC and electric heat. Permanently installed, auto-start, runs on natural gas or propane. |
Step 5 — Connect it safely: transfer switch or interlock
A generator can power your home two ways. The safe, legal way to feed your home’s circuits is a transfer switch (a separate sub-panel of selected circuits) or an interlock kit (a plate on your main panel that makes it physically impossible to have utility power and generator power on at the same time). Both isolate your house from the grid while the generator runs.
The dangerous shortcut some people take — a “suicide cord” that backfeeds the generator into a wall outlet — is illegal, can electrocute utility crews working on the line, and can start a fire when the power returns. Never backfeed. Any connection to your home’s permanent wiring, and any standby installation, must be done by a licensed electrician to NEC and local code.
The mistakes to avoid
Undersizing for the surge. The most common error is buying a generator at your running-watts total and forgetting the startup spike. The generator boots fine, then the well pump or AC compressor kicks on, the brief surge exceeds the generator’s capacity, and it stalls or trips. A well pump is the classic culprit: a unit that runs on 1,000 watts can demand 3,000+ at startup, so a “3,000-watt” generator that looks adequate on paper can’t actually start it alongside anything else.
Adding every surge together. The opposite error wastes money — summing the starting watts of every appliance and buying a far bigger generator than you need. Only one motor starts at a time, so you add only the single largest surge.
Ignoring fuel and run-time. A bigger generator burns more fuel. If you only need the essentials, an undersized fuel plan on an oversized unit is its own kind of mistake. Size to the real load, and keep a realistic fuel supply.
Frequently asked questions
How many watts do I need to run my house?
It depends what runs at once. Essentials (fridge, furnace blower, lights, TV) total ~1,500 running watts with a ~2,500W surge. Add a pump and the surge passes 5,000W. A whole house with central AC and electric heat needs 10,000–22,000W (standby).
Do I add up running watts or starting watts?
Both. Sum the running watts of everything on at once, then add only the single highest starting surge on top — one motor starts at a time. The generator’s rated watts must cover the running total, its surge watts the peak.
Should I get a portable or standby generator?
Portable (3,000–8,000W) is cheaper and covers essentials plus a pump, but is manual and needs a transfer switch. Standby (10,000W+) is auto-start, runs whole-house including central AC, but costs far more and needs professional install. See whole-house picks.
Can a generator run a well pump?
Yes, but the surge is large — a 1/2 hp well pump runs ~1,000W but spikes to 2,100–4,000W. You usually need a 5,000–7,500W generator to start it alongside the essentials. See running vs starting watts.
Related: Generator Sizing Calculator · What Will a Generator Run? · Running vs Starting Watts · Best for a Whole House · Best for a Power Outage