Free Generator Wattage Sizing calculator
Find the right generator wattage for your home in one click. Pick the appliances you'll run during an outage and this calculator adds their running watts to the single largest startup surge, then sizes the generator — updated live, as you type.
On this page15 sections
Hardest to start: Furnace / gas-furnace blower — its surge sets the peak. Start it before the others.
Planning estimate only, based on typical appliance ratings. Confirm each appliance's data plate before you buy.
Results are estimates. Consult a professional.
How the generator wattage calculator works
A generator has to do two jobs at once. It has to supply the steady running watts of everything switched on, and it has to survive the brief spike when a motor kicks on. This calculator adds up the running watts of every appliance you select, finds the single hardest-starting appliance, and stacks that one startup surge on top. The total, plus a safety margin, is the generator size you need.
The reason only one surge counts is timing. Motor-driven appliances surge for a fraction of a second when they start, then settle to their running draw. Two of them almost never hit that peak in the same instant, and you can deliberately stagger the ones that might. So a generator that can absorb the worst single surge, on top of everything already running, will start anything in your set.
Running watts vs starting watts
Every appliance has two wattage numbers, and getting them straight is the whole game. Mix them up and you either buy twice the generator you need or one that stalls the moment the fridge cycles on.
Running watts — the steady draw
Running watts, also called rated or continuous watts, are what an appliance pulls while it works normally. A refrigerator settling into its cycle draws about 700 running watts. An LED bulb draws 10. This is the number a generator must supply for as long as the appliance is on, and it is the figure on most data plates.
Starting watts — the startup surge
Starting watts, also called surge or peak watts, are the extra burst a motor or compressor needs to break from a standstill. That same 700-watt fridge can spike to 2,100 watts for a moment as its compressor kicks on — three times its running draw. Pumps, AC compressors and furnace blowers all surge this way. Resistive loads — lights, kettles, space heaters — have no motor, so their starting watts equal their running watts.
What goes into your generator size estimate
Three pieces build the estimate. Get each one right and the generator starts everything and runs cool; miss one and you are either overpaying or stalling mid-storm.
Total running watts — the baseline
Add the running watts of every appliance you plan to power at once. This is the load the generator carries continuously. A few LED lights add almost nothing; a space heater or window AC adds a lot. Drop anything you can live without during an outage and this number falls fast.
The single largest startup surge
Find the appliance with the biggest gap between its starting and running watts, and add that one gap on top of the running total. Only the largest counts. A furnace blower that jumps from 875 to 2,350 watts adds a 1,475-watt surge; if that is the biggest jump in your set, that is the only surge the generator has to absorb.
Headroom — the safety margin
Generators run best, and last longest, at 70–80% of their rated capacity, never at 100%. Headroom adds a cushion so the unit never sits at its limit. The calculator defaults to 25% and lets you pick 10%, 25% or 50%. Larger headroom also leaves room to plug in one more thing without re-sizing.
A worked example using the generator calculator
A winter storm knocks out the power. Maria wants to keep the essentials going: her refrigerator (700 W run / 2,100 W start), the gas-furnace blower (875 / 2,350), the sump pump (1,050 / 2,150) and six LED bulbs (10 W each, no surge). She uses the default 25% headroom.
Step 1 — Add the running watts
700 + 875 + 1,050 + (6 × 10) = 2,685 running watts. That is the steady load the generator must carry.
Step 2 — Find the single largest surge
Startup gaps: fridge 1,400 W, furnace blower 1,475 W, sump pump 1,100 W, lights 0. The biggest is the furnace blower at 1,475 W — the only surge that gets added.
Step 3 — Get the peak load
2,685 running + 1,475 surge = 4,160 W peak load. This is what the generator faces at the worst moment — everything running while the furnace blower starts.
Step 4 — Add headroom
4,160 × 1.25 = 5,200 W. Maria should buy a generator rated at about 5,200 starting watts, or roughly 5.2 kW.
Appliance wattage chart: running and starting watts
These are typical figures for common household appliances. Use them for planning, then confirm the exact numbers on each appliance's data plate or manual before you buy a generator — ratings vary by model, size and age.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 700 | 2,100 |
| Sump pump (1/3 hp) | 1,050 | 2,150 |
| Furnace / gas-furnace blower | 875 | 2,350 |
| Well pump (1/2 hp) | 1,000 | 2,100 |
| Window AC (10,000 BTU) | 1,200 | 1,800 |
| Microwave (1,000 W) | 1,000 | 1,000 |
| Space heater | 1,500 | 1,500 |
| Coffee maker | 1,000 | 1,000 |
| TV / Wi-Fi / chargers | 200 | 200 |
| LED light bulb | 10 | 10 |
Typical values compiled from Honda, Generac and Lowe's appliance wattage charts. Resistive loads (heater, kettle, lights) have no startup surge, so starting watts equal running watts.
What size generator do I need?
If you want a ballpark before adding up individual appliances, match your goal to a generator class. The right size depends entirely on how much you want to run at once.
| What you want to run | Generator size | Type |
|---|---|---|
| Fridge + a few lights + phone chargers | 2,000–3,500 W | Inverter, portable |
| Outage essentials (fridge, furnace fan, sump pump, lights) | 4,000–6,500 W | Portable |
| Most of a small home, including a well pump | 7,500–10,000 W | Large portable |
| Whole home with central AC | 12,000–20,000+ W | Standby / whole-house |
Sizes assume the running-plus-largest-surge method with headroom. Whole-home backup with central air conditioning usually needs a permanently installed standby generator.
An inverter generator is worth the premium for electronics: it produces cleaner power and throttles its engine to the load, so it sips fuel when you are only running a fridge and a router. For the essentials tier, that fuel saving over a long outage is real.
Can you overload a generator?
Yes, and it happens two ways. Knowing both is the difference between a generator that runs your house for days and one that trips its breaker every time the compressor cycles.
Running overload — too much steady load
Plug in more running watts than the generator is rated for and it overloads: the breaker trips, or on cheaper units the engine bogs down and overheats. This is why headroom matters — sitting at 100% capacity for hours shortens the engine's life even when nothing trips.
Surge overload — not enough starting watts
The sneakier failure is surge overload. The generator runs your fridge fine for an hour, then the compressor cycles, demands 2,100 watts for a second, and the generator stalls because it had no surge headroom. The appliance never starts, or it trips the generator mid-cycle. Sizing for the largest startup surge prevents this.
How to run more on a smaller generator
The cheapest way to need less generator is to start fewer surges at once. Because only the single largest surge sets your size, controlling when motors start lets a smaller unit do a bigger job.
- Stagger the big starts. Start the fridge, then wait a minute and start the sump pump, then the furnace. Never let two compressors surge in the same instant.
- Power motors before electronics. Let the high-surge appliances settle into their running draw first, then plug in the TV, router and chargers.
- Shed loads you don't need. A space heater or coffee maker can wait. Dropping one 1,500-watt resistive load frees real capacity instantly.
- Choose an inverter for sensitive gear. Cleaner power protects electronics, and load-following throttling cuts fuel use on light loads.
- Confirm data plates, not estimates. A 1/2 hp well pump surges far harder than a 1/3 hp one. Read the actual nameplate before you commit to a size.
Backup power is one piece of outage readiness. If you are also planning water storage off your roof, the rainwater collection calculator estimates how many gallons a storm will fill.
Generator wattage definitions
How accurate is this generator calculator?
The arithmetic is exact. Total running watts plus the single largest surge, times the headroom factor, is the precise figure the running-plus-largest-surge method produces. If your appliance wattages are right, the recommended size is right.
The appliance presets are typical values, and on purpose. A refrigerator's real running and starting watts vary by model, size, age and how cold it is asked to get; a well pump's surge depends heavily on its horsepower. The defaults here come from published Honda, Generac and Consumer Reports charts, but the only numbers that size your generator correctly are the ones on your appliances' data plates. Read them, enter your own figures, and size up rather than down — a generator that stalls mid-outage is far more costly than a little spare capacity.
Frequently asked questions about the free Generator Wattage Sizing calculator
About this Generator Wattage Sizing calculator
This calculator runs entirely in your browser — nothing you enter is sent anywhere. It sizes a generator using the method Honda and Generac publish: total running watts across the appliances you select, plus the single largest startup surge, plus a safety margin so the unit runs at 70–80% of its rating rather than its limit.
It's part of our home & garden calculators — and you can browse every tool from the full calculator directory. The appliance presets are typical published values; always confirm the running and starting watts on your own appliances' data plates before you buy.