What do you need to power?
Check an appliance and set how many. Watt values are typical averages — confirm against your appliance nameplates before you buy. Missing something? Enter its nameplate values under “Your appliance.”
Recommended size
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Your load breakdown
| Item | Watts |
|---|---|
| Total running watts (everything at once) | — |
| Surge allowance (largest motor's extra starting draw) | — |
| Peak watts needed (running + surge) | — |
| 25% safety margin | — |
| Total required | — |
This plan powers:
How generator sizing works
Running watts vs. starting watts
Every appliance draws running watts — the steady power it needs to keep going. Appliances with electric motors draw something extra: a starting surge when the motor first spins up. A cold compressor or pump motor briefly looks like a near-short-circuit to the generator (technicians call it locked-rotor current), so the surge often runs two to three times the running watts for a few seconds. Take a refrigerator: 700 running watts, but 2,200 watts at the instant the compressor kicks. A generator has to satisfy both demands — enough steady capacity for everything running at once, plus enough headroom for the biggest motor's startup kick. Resistive loads (heaters, toasters, lights) have no surge: starting watts equal running watts.
Exactly what this calculator does — a worked example
The calculator totals the running watts of everything you selected, then adds only the single largest starting surge — because motors rarely all start in the same second. Then it adds a 25% safety margin. Say you pick a refrigerator, a microwave, LED lights, and a sump pump:
- Running watts: 700 + 1,000 + 150 + 1,050 = 2,900 W
- Largest surge: the fridge's 2,200 − 700 = 1,500 W extra at startup
- Total: (2,900 + 1,500) × 1.25 = 5,500 W → a 7.5 kW portable generator
This mirrors the method in published sizing guides (sum the running watts, add the highest starting watts, add a margin) — done the same arithmetic, with the surge expressed as the extra draw above running load so the breakdown is transparent.
Why the 25% margin
- Nameplates are optimistic. Appliance labels show ideal-condition draw; real loads vary.
- Motors get thirstier with age. Wear and dust raise both running and starting draw over time.
- Generators derate in heat and altitude. A generator at 5,000 feet or in 100°F air makes less than its rated watts.
- Running pinned at 100% is abusive. Sustained full load shortens engine life and invites nuisance overload trips. Headroom is cheap insurance.
Portable vs. inverter vs. standby: which one?
| Type | Typical range | Strengths | Trade-offs |
|---|---|---|---|
| Inverter | up to ~4 kW | Quiet, fuel-sipping, clean power for electronics | Most watts-per-dollar; not for central AC |
| Portable (conventional) | ~7.5–12 kW | Most watts per dollar; 240V models run well pumps and dryers | Louder, thirstier, manual start |
| Standby | ~20 kW and up | Starts itself, runs the whole house, no refueling | Costly, professionally installed |
Four questions settle it:
- Do you need 240 volts? Well pumps, electric dryers, and central AC usually need 240V — check that any portable you buy has a 240V outlet.
- Must it start itself? If outages strike while you're away (sump pump, frozen pipes), only an automatic standby qualifies.
- How long are your outages? Hours: a portable with a few gallons of stabilized fuel. Days: standby on natural gas or propane, or a portable plus serious fuel storage.
- Do electronics matter? Laptops, modems, and medical devices prefer an inverter's clean sine wave; a conventional portable can make some UPS units and chargers misbehave.
Frequently asked questions
What is the difference between running watts and starting watts?
Running watts are the steady power an appliance needs to keep operating. Starting watts are the short burst a motor-driven appliance draws for a few seconds when it starts — often two to three times its running watts. A generator must handle the running total of everything at once plus the largest single starting surge, which is why both numbers matter.
What will a 7500 watt generator run?
A 7,500 running-watt portable generator comfortably covers an essentials list: a refrigerator (700W), freezer (500W), sump pump (1,050W), microwave (1,000W), a bundle of LED lights (150W), and a TV with internet gear (300W) — 3,700 running watts total, about 6,500 watts of required capacity once you add the largest starting surge and a 25% safety margin. That leaves room for a furnace fan or well pump to cycle, but it will not carry a central air conditioner plus an electric dryer at the same time.
Should I buy an inverter generator or a conventional generator?
Inverter generators are quieter, more fuel-efficient at partial load, and produce cleaner power that is safer for electronics — best for loads under about 4,000 watts, camping, and tailgating. Conventional portable generators deliver more watts per dollar and suit heavier essentials like well pumps and furnace fans. Whole-home backup is a different category: a permanently installed standby generator.
How long will a generator run on a tank of fuel?
Runtime depends on tank size and how much load the generator is carrying, not just its wattage rating. A typical 7,500-watt portable with a 6-gallon tank runs around 8–11 hours at half load, and longer under lighter loads because most generators sip fuel when idling against a small load. Check the manufacturer's stated runtime at 50% load and plan your fuel storage around it.
Do I need an electrician to install a generator?
Yes if the generator connects to your home's wiring. A licensed electrician must install a transfer switch or interlock so generator power can never back-feed onto the utility lines — back-feeding can electrocute line workers. For a portable generator running extension cords only, no electrician is needed, but never plug a generator into a wall outlet.
What size generator do I need for an RV?
Most RVs plug into a 30-amp outlet, which caps usable power at about 3,600 watts. A rooftop RV air conditioner typically draws around 1,300 running watts with a 1,650-watt starting surge, so a 3,000–4,000 watt inverter generator runs the AC plus lights, a fridge, and phone charging. Use the custom-appliance row above to enter your AC's exact nameplate values for a precise answer.
How should I store gasoline for a generator?
Store gasoline in approved containers in a cool, well-ventilated spot away from the house — never in the room where the generator runs. Add fuel stabilizer the day you buy the gas; stabilized fuel stays fresh up to 12 months, while unstabilized gas starts degrading in about 30 days. Rotate your stock seasonally, and never refuel a hot generator: shut it off and let it cool first.