---
title: "What Size Generator Do I Need? Calculator | Watt Cottage"
description: "Generator size calculator: add your fridge, freezer, sump pump and lights to get running and starting watts, a size band and the order to start them in."
url: https://wattcottage.me/
---

Generator size calculator

# What size generator do I need?

Add up the **running watts** of everything that has to run at the same time, then add the **biggest start-up surge** on top. The running total is the rated (running) output your generator needs; the total with the surge is the starting (peak) output. A fridge, a freezer, a ½ hp sump pump, six LED bulbs, the router and two phone chargers come to about 1,600 running watts and 2,700 watts at the worst moment, so look for at least 1,600 W running and 2,700 to 3,800 W starting, and start the sump pump first.

## How the calculator works

It follows the makers’ own method. Champion Power Equipment’s sizing guide (24 April 2026) says to add up all the running watts, then add the highest starting wattage among your appliances to get your peak need. Honda adds what it calls power management: count only what runs continuously, then add the single biggest occasional need, because not everything has to start or run at once.

So you get three numbers. The **running watts** are everything set to “with the rest”, added up. The **worst moment** adds the extra surge of the motor that surges most, as when the sump pump kicks in while everything else is on. The **makers’ quick rule** adds that load’s whole starting wattage instead, which builds in a margin. Your size band runs from the worst moment to the quick rule, rounded up to the next 100 W. Champion advises a generator that exceeds your calculated wattage by a safe margin, so aim for the upper half of the band.

## Running watts and starting watts

A heater or a light draws the same power from the moment you switch it on. Anything with a motor or compressor, such as a fridge, a sump or well pump or a furnace blower, needs a burst of extra power as it starts, and the size of that burst varies a lot. On Honda’s chart an Energy Star fridge or freezer runs on 132 to 192 W but needs 1,200 W to start; a ½ hp sump pump runs on 1,050 W and needs 2,150 W. A coffee maker needs the same 600 W either way. The American Red Cross puts it as some machines drawing up to three times as much power when starting. So the presets are only a starting point: the real figures are on the appliance’s rating label (the Red Cross says to check the back and sides) or in its manual. If the label only gives amps, multiply them by the volts: 120 V in most US homes, 230 V in the UK. The [watts, amps and volts calculator](https://wattcottage.me/watts-amps-volts-calculator/) does it for you.

Where a maker gives a range, the presets use the top of it (Champion: LED bulbs 8 to 15 W, a router or modem 10 to 20 W). UK figures from Electrical Safety First are in the [extension lead calculator](https://wattcottage.me/extension-lead-overload-calculator/).

The example list. Dark blocks are each motor’s start-up draw; light blocks are what is already running. Starting the biggest surge first keeps the highest moment at 2,442 W instead of 4,700 W.

## A worked example

The list the calculator opens with runs on 192 W (fridge) + 192 W (freezer) + 1,050 W (sump pump) + 90 W (six LED bulbs) + 20 W (router) + 40 W (two phone chargers) = 1,584 W. The sump pump has the biggest extra surge: 2,150 W to start against 1,050 W running, 1,100 W more. Everything running plus that surge is 2,684 W, the worst moment; the quick rule, 1,584 + 2,150, gives 3,734 W. Rounded up: at least 1,600 W running and 2,700 to 3,800 W starting.

## Stagger the big loads

Honda’s example shows why order matters: if you start the fridge and the furnace at separate times, you need 1,600 W for the moment of starting, not 3,200 W. Start the motor with the biggest surge first, then the next biggest, and switch on lights and electronics last; the start order under your result does exactly that. For short jobs, Honda suggests turning off the fridge and furnace for a few minutes while you use the microwave: set those appliances to “on its own” and they count as an occasional need, not part of the running total. The Red Cross also says to stagger the operating times of your equipment if necessary to prevent overloads.

**Carbon monoxide kills, and it gives no warning.** The US Consumer Product Safety Commission says never to run a portable generator inside a home, garage, basement, crawlspace or shed, even with doors or windows open, and to run it outside only, at least 20 feet (about 6 metres) from the house, with the exhaust pointing away. Not on a porch or in a carport: they are too close. The CDC gives the same 20 feet from any window, door or vent. In the UK, the Health and Safety Executive for Northern Ireland says not to use a generator indoors or in partially enclosed spaces, or near open doors and windows.

Fit battery-operated carbon monoxide alarms, or ones with battery backup. If one sounds, get outside: the CPSC says to call 911 from there. In the UK, the NHS says to call 999 if anyone has trouble breathing, chest pain or confusion, and NHS 111 for other symptoms. CPSC’s latest report counts an average of nearly 100 deaths a year in the US from carbon monoxide from portable generators.

Where the generator goes: outdoors, at least 20 feet from the house with the exhaust pointing away (CPSC, CDC), and kept dry. A heavy-duty outdoor cord brings the power in.

## Connecting it safely

- **Never plug a generator into a wall socket** to power the house wiring. CPSC calls this backfeeding: it is extremely dangerous, can electrocute utility workers and neighbours, and bypasses some of your home’s circuit protection. HSENI warns that connecting a generator to your home could cause injury or death.
- **To power the house circuits,** the Red Cross says to have an electrician install a transfer switch to the National Electrical Code and your state and local codes. In the UK, treat it the same way: it is work on your home’s wiring, so ask a registered electrician. We give no wiring instructions on purpose.
- **Otherwise, plug appliances into the generator itself,** or use a heavy-duty, outdoor-rated extension cord whose rating, in watts or amps, is at least the total of what is plugged into it (Red Cross; CPSC says the cord’s wattage rating should exceed that total).
- **Keep it dry**, and let it cool before refuelling.

## Common mistakes

- **Buying on running watts alone.** A 1,600 W generator may stall when the sump pump starts.
- **Forgetting 240 V loads.** Honda marks its 8-inch electric range element (2,100 W) and water heater as needing 240 V. Check the nameplate and the generator’s outlets.
- **Counting the heating system.** A gas or oil furnace still needs electricity for its blower; Honda lists a ½ hp furnace fan at 875 W running and 2,350 W starting.

We don’t estimate fuel use: we found no maker’s general method that works across models, so go by the run time your generator’s maker publishes. Hosting on Thanksgiving, Thursday 26 November 2026? If a storm takes the power out, plan the meal around what doesn’t need the electric oven, and keep the fridge shut: the [power cut checklist](https://wattcottage.me/power-cut-checklist/) has the food timers. To run a fridge from a battery instead, see the [power station runtime calculator](https://wattcottage.me/power-station-runtime-calculator/).

## More tools for a power cut

- [Power station runtime calculator](https://wattcottage.me/power-station-runtime-calculator/): How long a battery will run your fridge, router and lights, and what to switch off to last the night.
- [UPS runtime calculator](https://wattcottage.me/ups-runtime-calculator/): Minutes of backup for a router and modem, from the battery or the maker’s runtime chart, with VA vs watts explained.
- [Watts, amps and volts calculator](https://wattcottage.me/watts-amps-volts-calculator/): Watts to amps at 120, 230 or 240 V, and what a 13 A plug or a 15 A circuit can take.
- [Extension lead overload calculator](https://wattcottage.me/extension-lead-overload-calculator/): Add up what’s on one lead or cord, and why a heater belongs in the wall socket.
- [Power cut checklist](https://wattcottage.me/power-cut-checklist/): Before, during and after, with fridge and freezer timers and a printable fridge-door card.

## Recommended

### CleanTempMail

Want a generator’s manual, a runtime chart or a one-off discount code that sits behind an email form? A temporary inbox gets it without the newsletters that follow: no sign-up, and the emails are deleted automatically after three days. Use your real address for the order itself, the warranty registration and your utility’s outage alerts.

[Open a temporary inbox](https://cleantempmail.com/)

### WebTraceCheck

Found a generator or power station at a price far below other shops, through an ad? Look the shop up with the free DNS and domain lookup: a domain registered last month is one warning sign, never proof either way. Then open the link in a disposable cloud browser that is deleted after the session.

[Try the disposable browser](https://webtracecheck.com/)

## Sources

All checked on 5 November 2026. Wattages are each maker’s typical figures for its own guide, not measurements of your appliances.

- Honda Power Equipment: [Generator wattage estimation guide](https://powerequipment.honda.com/generators/generator-wattage-estimation-guide) and [What size generator do I need?](https://powerequipment.honda.com/generators/generator-how-much-power) (power management).
- Champion Power Equipment: [Generator wattage chart](https://www.championpowerequipment.com/generator-wattage-chart/) and [What size generator do I need?](https://www.championpowerequipment.com/news-updates/what-size-generator-do-i-need/) (24 April 2026).
- Electrical Safety First: [Ratings of electrical appliances](https://www.electricalsafetyfirst.org.uk/safety-advice/home-and-people/house-maintenance/ratings-of-electrical-appliances/).
- American Red Cross: [Safe generator use](https://www.redcross.org/get-help/how-to-prepare-for-emergencies/types-of-emergencies/power-outage/safe-generator-use.html) (prepared with technical advice from the CDC, NFPA and CPSC).
- US Consumer Product Safety Commission: [Winter weather safety tips](https://www.cpsc.gov/Newsroom/News-Releases/2026/CPSC-Issues-Winter-Weather-Safety-Tips-to-Prevent-Fires-and-Carbon-Monoxide-Poisoning) (16 March 2026) and [Portable generator hazards](https://www.cpsc.gov/s3fs-public/Portable_Generator_Safety_Alert_2017_5123.pdf) (safety alert, revised September 2017).
- CDC: [What to do to protect yourself during a power outage](https://www.cdc.gov/natural-disasters/response/what-to-do-protect-yourself-during-a-power-outage.html) (reviewed 26 August 2026).
- Health and Safety Executive for Northern Ireland: [Safety tips: using a portable generator in an emergency](https://www.hseni.gov.uk/news/safety-tips-using-portable-generator-emergency) (26 January 2025).
- US Office of Personnel Management: [Federal holidays](https://www.opm.gov/policy-data-oversight/pay-leave/federal-holidays/) (Thanksgiving Day 2026: Thursday 26 November).

## Questions

**What size generator do I need to run a fridge and a freezer?**

On Honda’s chart an Energy Star fridge or freezer runs on 132 to 192 W and needs 1,200 W to start. Both together run on up to 384 W; the worst moment, one starting while the other runs, is about 1,400 W, and the makers’ quick rule gives about 1,600 W. Add the rest of your list, and check your own appliances’ labels.

**What is the difference between running watts and starting watts?**

Running (rated) watts are what an appliance draws while it runs. Starting (surge or peak) watts are the brief extra draw of a motor or compressor as it starts. A generator lists both: its running output must cover everything on at once, and its starting output must cover the biggest surge on top.

**Is a 3,000 watt generator big enough for a house?**

It depends on what you run, not the size of the house. Honda’s own example of a fridge, a furnace fan, five lights, a TV and a microwave fits a 3,000 W generator when the motors are started one at a time and the microwave is used on its own. A well pump, electric heating or an electric range can each need more.

**Can I plug my generator into a wall socket to power the house?**

No. That is backfeeding, which CPSC calls extremely dangerous: it can electrocute utility workers and neighbours and bypasses your home’s circuit protection. To power house circuits, have an electrician install a transfer switch. Otherwise plug appliances into the generator with heavy-duty outdoor cords.

**How far from the house should a generator be?**

At least 20 feet (about 6 metres), outdoors, with the exhaust pointing away from the house, says the US CPSC; the CDC says at least 20 feet from any window, door or vent. Never in a garage, basement, shed, porch or carport, even with the doors open.