A Free Calculator · Your Appliances · Updated 2026
What size backup generator do you actually need?
The right generator size isn't about your whole house — it's about the specific
appliances you'll run during an outage, and the brief startup surge the biggest motor
demands. Check the items below and the calculator adds the running load, the largest
motor's surge, and a safety margin, then recommends a standard generator size. Every
formula is shown, and no number is presented as gospel.
Running watts + startup surge·Recommended generator size·Override any appliance from its nameplate
Read this first
The wattages built into this tool are typical values and vary widely — the
nameplate or data label on your own appliance is the authoritative number, so override the
quantities and the custom row with your real figures when you have them. You usually back up
essentials, not the whole house at once. And the safety rules are non-negotiable:
never run a generator indoors, in a garage, or near windows — carbon monoxide is
invisible, odorless, and kills. Have a licensed electrician install a transfer
switch for anything wired into your home; never backfeed a generator into your wiring.
Size your generator from the appliances you'll run
Set a quantity for each appliance you need to back up (the typical running and starting watts are shown next to each). Add your own item in the custom row at the bottom. The recommendation updates as you go.
Choose what you'll run during an outage
W
W
You need about a generator
Total running watts
Peak (starting) watts required
Minimum needed (with margin)
Recommended generator
What you selected
The formulas, in full
Nothing here is a black box. These are the exact calculations the tool runs — the same
arithmetic you could do on paper. The only judgment calls are which appliances you select
and the watt figures you supply.
How the recommended size is derived
1 — Total running watts (everything on at once)
total_running (W) = sum over selected rows of (qty × running_watts)
2 — Largest startup surge (only one motor surges at a time)
max_surge_delta (W) = max over selected rows of (starting_watts − running_watts)
// 0 if every selected load is resistive (no motor surge)
3 — Required peak watts (running load + the single largest extra surge)
5 — Recommended size (smallest standard generator that covers it)
STANDARD_SIZES (W) = 2000, 2200, 3000, 3500, 4000, 4500, 5000, 5500,
6500, 7000, 7500, 8000, 8500, 9000, 9500,
10000, 11000, 12000, 12500, 13000, 14000, 15000
recommended_size = smallest STANDARD_SIZES value ≥ recommended_watts
// if recommended_watts exceeds 15,000 W → consider a standby/whole-house unit
Typical appliance running and starting watts
Motors surge at startup; resistive and electronic loads do not. The table below lists
typical figures for common backup loads — they are approximate and vary by make, model,
age, and size. The nameplate on your own appliance is the authoritative number; treat
these as a starting point and override them in the calculator.
Appliance
Running watts
Starting watts
Notes (motor vs resistive)
Refrigerator / freezer
~150 W
~600 W
Motor (compressor) — large surge when the compressor kicks in.
Chest freezer
~350 W
~1,200 W
Motor (compressor) — surge several times the running draw.
Sump pump (1/3 HP)
~800 W
~1,300 W
Motor — surge matters most; cycles on demand.
Well pump (1/2 HP)
~1,000 W
~2,100 W
Motor — often the biggest surge in the house.
Furnace blower fan (gas furnace)
~600 W
~1,500 W
Motor — only the blower; the heat itself is gas.
Window AC (10,000 BTU)
~1,200 W
~3,600 W
Motor (compressor) — large startup surge.
Central AC (3-ton)
~3,500 W
~10,500 W
Motor (compressor) — very large surge; often standby-only.
Space heater (1,500 W)
~1,500 W
~1,500 W
Resistive — no surge; starting equals running.
Microwave
~1,000 W
~1,000 W
Electronic/resistive — effectively no startup surge.
Lights (about 10 LED bulbs)
~200 W
~200 W
Resistive/electronic — no meaningful surge.
Wattages are typical and approximate, drawn from common appliance categories — your
appliance's actual figures appear on its nameplate or data label and may differ
substantially by size, age, efficiency, and brand. The calculator above includes these
plus several more (water heater, dryer, dishwasher, air compressor, CPAP, and others) and
lets you add a custom item.
How generator sizing actually works
Three ideas explain almost everything about picking a generator size. Get these right and
the recommendation above will make sense rather than feeling like a magic number.
Running watts vs starting watts — why the surge matters
Every appliance has a steady running draw, but anything with a motor — a fridge compressor, a well or sump pump, an air conditioner, a furnace blower — also demands a brief spike of power at the instant it starts. That surge can be two to three times the running figure. A generator must supply both the continuous running load and that momentary surge, or the motor stalls and the generator can trip or stall with it. Resistive loads (heaters, lights, microwaves) have no surge, so their starting watts equal their running watts.
You don't start everything at once
Surges are momentary, and in practice your motors don't all kick on in the same instant. The realistic worst case is that everything is running while the single largest motor surges. So the calculator adds only the largest motor's extra surge on top of the total running load — not the sum of every appliance's surge. Adding every surge together would massively overstate the generator you need and have you buying far more capacity than you'll ever use.
Size up for headroom and future loads, but don't massively oversize
You shouldn't run a generator at 100% of its rating continuously — it runs hotter, louder, and less reliably — so a 20% margin gives breathing room and a little slack for an appliance you forgot or add later. But going far beyond that wastes money up front, burns more fuel at light loads, and can even cause problems for sensitive electronics. The recommendation above rounds up to the next standard size after the margin, which is the sensible middle ground.
How to size your generator step by step
The calculator does the arithmetic, but a trustworthy result starts with a realistic list
of loads and accurate nameplate numbers.
List your must-run appliances
Decide what genuinely has to keep working during an outage — typically a refrigerator or freezer, a sump or well pump, the furnace blower in winter, some lights, and phone charging. Leave off anything you can live without; every load you add raises the size and cost.
Read each appliance's nameplate
Find the data label (often on the back, base, or inside a door) and note the running watts, or the amps and volts to compute watts (watts = amps × volts). Use those real numbers in place of the typical values in the table; pumps and motors usually list locked-rotor or starting amps too.
Identify the biggest motor
Find the single appliance with the largest startup surge — usually a well pump, a sump pump, a central air conditioner, or the furnace blower. That one motor's surge, added on top of everything running, sets your peak demand. It's the number that most often forces a larger generator.
Add a safety margin
Don't size to the exact peak. The calculator applies a 20% margin so the generator isn't pinned at full output, which extends its life and leaves slack. Then it rounds up to the next standard generator size, so you're buying a unit that actually exists on the shelf.
Choose a transfer-switch-compatible unit and get a licensed electrician
To power anything wired into your home — furnace, well pump, hardwired circuits — you need a transfer switch installed by a licensed electrician, and for permanent standby units you'll need a permit. Never backfeed a generator into your wiring. Pick a generator rated above the recommended size with the outlets or inlet your setup needs.
Where to buy
Got your numbers? Here's where to pick up what you need:
The units and terms that show up on a generator spec sheet, an appliance nameplate, or a
buyer's guide — in plain English.
Running (rated) watts
The steady power an appliance draws while it operates normally. The sum of the running watts of everything you'll run at once is the continuous load your generator must sustain — its "rated" or "running" output rating.
Starting (surge / peak) watts
The brief spike of power a device needs at the instant it switches on, mostly from electric motors overcoming inertia. It can be two to three times the running watts but lasts only a fraction of a second. A generator's "starting" or "peak" rating must cover your largest surge on top of the running load.
Watt vs kilowatt
A watt (W) is the basic unit of power; a kilowatt (kW) is 1,000 watts. Generators are sold by both — a "5,000-watt" unit is the same as a "5 kW" unit. Divide watts by 1,000 to get kilowatts (5,000 W ÷ 1,000 = 5 kW).
Transfer switch
A device, installed by a licensed electrician, that safely switches selected home circuits between the utility grid and your generator. It disconnects the home from the grid before the generator connects, protecting equipment and preventing dangerous backfeeding. Required to power anything hardwired into your home.
Inverter generator
A generator that produces clean, stable AC power by converting the engine's output to DC and back to AC electronically. It's quieter, more fuel-efficient at light loads, and safer for sensitive electronics (laptops, phones, TVs) than a conventional generator — typically at a higher price per watt.
Resistive vs inductive (motor) load
A resistive load (space heater, incandescent bulb, microwave element) turns electricity straight into heat or light and has no startup surge — its starting watts equal its running watts. An inductive (motor) load drives a motor and surges at startup. The distinction is why two appliances with the same running watts can need very different generators.
Backfeeding (and why it's dangerous)
Connecting a generator to your home's wiring without a transfer switch — for example, through a dryer outlet with a "suicide cord." It sends electricity backward onto the utility grid, where it can electrocute line workers repairing the outage, and can also damage your equipment when grid power returns. It is illegal in most jurisdictions and lethally dangerous; always use a properly installed transfer switch.
Frequently asked
There's no single number — it depends on what you actually need running at the same time. Most people don't back up the whole house; they back up essentials: a fridge or freezer, a sump or well pump, the furnace blower, some lights, and device charging. That combination often lands in the 3,000–7,500 watt range, which a mid-size portable generator covers. Running the entire house at once — central AC, an electric water heater, an electric range — can demand 15,000 watts or more, which is standby (whole-house) territory. The calculator above lets you check exactly the appliances you intend to run, adds the largest motor's startup surge on top of the running load, applies a 20% safety margin, and recommends a standard size.
Running (rated) watts is the steady power an appliance draws while it operates. Starting (surge/peak) watts is the brief spike a device needs the instant it switches on. Anything with a motor — a fridge compressor, a well or sump pump, a furnace blower, an air conditioner — pulls a surge two to three times its running watts for a fraction of a second as the motor overcomes inertia. Purely resistive or electronic loads (space heaters, lights, microwaves, TVs) have no meaningful surge, so their starting watts equal their running watts. A generator must supply both the total running load and the largest single surge, or the motor won't start.
Almost certainly, with room to spare. A typical refrigerator runs at around 150 watts (with a roughly 600-watt startup surge) and a gas furnace blower runs at around 600 watts (with a roughly 1,500-watt surge). Running both together is about 750 watts continuous; the largest surge on top adds roughly 900 extra watts, so the peak demand is near 1,650 watts. A 5,000-watt generator covers that comfortably and leaves headroom for lights, phone charging, and a sump pump. Use the calculator above to add the specific items you want and confirm — and always check your own appliance nameplates, since the values here are typical, not yours.
For powering anything wired into your home's circuits — a furnace, a well pump, hardwired lighting — yes, you need a transfer switch installed by a licensed electrician. It safely disconnects your home from the utility grid before connecting the generator, which protects your equipment and, critically, prevents backfeeding electricity onto the grid where it can electrocute line workers. Never connect a generator to your wiring with a "suicide cord" or by backfeeding a dryer outlet — it's illegal in most places and lethally dangerous. If you only run appliances via heavy-duty extension cords plugged directly into the generator (a fridge, a space heater, lamps), you don't need a transfer switch, but you're limited to what you can reach with a cord.
No. Never run a generator indoors, in a garage, in a carport, in a basement, or near open windows or doors — even with the garage door open. Generators produce carbon monoxide (CO), a colorless, odorless gas that can build to lethal levels within minutes in an enclosed or partially enclosed space and kills people every year during outages. Always operate a generator outdoors, well away from the house (at least 20 feet is a common guideline), with the exhaust pointed away from windows, doors, and vents. Install battery-powered CO alarms inside your home. This is the single most important rule of generator use — no amount of convenience is worth it.
Both are motor-driven, so the surge matters more than the running watts. A 1/3-HP sump pump runs at roughly 800 watts but can surge to around 1,300 watts at startup; a 1/2-HP well pump runs at roughly 1,000 watts and can surge to around 2,100 watts. Because the generator must handle the largest startup surge on top of everything else running, a well pump in particular drives up the peak demand. To run a well pump plus a fridge and a few lights, you typically want a generator in the 3,500–5,000 watt range once a safety margin is added. Deep or high-pressure well systems can draw more — check the pump's nameplate or the data plate on the control box for the exact running and starting figures, and enter those in the calculator above.
Common mistakes with this calculator
Generator sizing errors tend to leave you either with a too-small unit that trips on startup or a too-large one you'll never fully load.
Sizing to running watts only — ignoring startup surge
Every motor (refrigerator compressor, well pump, AC, sump pump) draws 2–3× its running wattage for the first second or two of startup. A generator must handle the peak surge, not just the steady running load. If you add up only running watts and buy to that number, the generator will trip its breaker or stall every time a motor starts. Add the largest single startup surge on top of the combined running load of everything else.
Adding every motor's full surge together
You do not sum every appliance's startup watts simultaneously. Only one motor starts at a time. The correct method: take the sum of all running watts, then add the starting watts of the single largest motor you'll ever start while the others are already running. Adding all surge figures together results in a wildly oversized generator requirement.
Targeting 100% of rated output with no headroom
Running a generator continuously at or near its rated maximum shortens engine life and risks overload trips when loads fluctuate. A practical rule of thumb is to size so your expected load sits at 70–80% of the generator's rated wattage, leaving headroom for unexpected surges and for the engine to run cooler and last longer.
Forgetting that a transfer switch is required for whole-home backup
Connecting a generator directly to your home's wiring without an approved transfer switch is illegal under NEC Article 702 and dangerous — it can back-feed power onto utility lines and injure lineworkers. If you need to power hard-wired circuits (well pump, furnace blower, whole-home AC), a licensed electrician must install a transfer switch or interlock kit. Factor that cost into your total budget. Source: NFPA 70 / NEC Article 702.