ENEEnergy
What Size Generator Do I Need for My House?
Published 4 min read
The right generator size is the total running wattage of everything you want powered at the same time, plus the extra starting wattage of the single most demanding motor, plus a safety margin of roughly a fifth on top. That number comes from your own appliances, so the answer differs between a flat that only needs the fridge and router kept alive and a family home that wants heating, a well pump and the kitchen running.
Step 1: decide what actually needs power
Start with a list, not a catalogue. During a power cut, most households care about a handful of things:
- the fridge and freezer;
- heating controls, a furnace fan or a boiler pump;
- a sump pump or well pump, where one is fitted;
- lights in a few rooms;
- the internet router and phone chargers;
- medical equipment, if anyone at home relies on it;
- perhaps a microwave, kettle or window air conditioner.
Split the list into "must run" and "nice to have". The first group sets the minimum size; the second tells you how much extra comfort a bigger unit would buy.
Step 2: find the wattage of each item
Every appliance has a rating plate or label, usually on the back, the base or inside the door, and the manual repeats it. Look for watts (W). If only volts (V) and amps (A) are shown, multiply them: volts × amps = watts.
Write down two figures where they apply:
| Figure | What it means | Applies to |
|---|---|---|
| Running (rated) watts | The steady power the item draws while operating | Everything |
| Starting (surge) watts | A brief spike when a motor or compressor kicks in, often several times the running figure | Fridges, freezers, pumps, air conditioners, power tools |
If the label does not give a starting figure, the manufacturer's support page or a quick call usually can. Avoid guessing from generic online tables, because two fridges of the same size can differ widely.
Step 3: do the sum
- Add up the running watts of everything on your "must run" list that could be on at the same time.
- Find the single item with the largest gap between starting and running watts and add that difference to your total. You only add one surge, because motors rarely all start in the same second.
- Add a margin of about 20 to 25 percent. Generators run more smoothly and last longer when they are not at full load, and you will almost certainly plug in something you forgot.
Compare the result with the generator's rated or continuous output, not its peak figure. Peak output is what the unit can deliver for a few seconds; continuous output is what it can sustain.
Step 4: match the type to the job
| Type | Typical use | Points to weigh |
|---|---|---|
| Portable generator | Running a few essentials through extension leads or a transfer switch | Has to be wheeled out, fuelled and started by hand; noisy |
| Inverter generator | Electronics, small loads, quieter operation | Cleaner power output; usually smaller capacity per pound spent |
| Standby generator | Whole-house or large partial-house backup | Permanently installed, starts automatically; needs professional installation and regular servicing |
Fuel matters as well. Petrol is easy to find but does not store well; propane and natural gas keep better; diesel suits larger standby units and is common in commercial settings. If you are weighing a diesel unit, our guide to prolonging the life of a diesel generator explains the servicing it will need.
What is a transfer switch, and do I need one?
A transfer switch is a device wired into your home's electrical panel that lets you move selected circuits from the grid to the generator, and back again, in a controlled way. It keeps the two sources from ever being connected at the same time.
- A manual transfer switch is flipped by hand once the generator is running, and is the usual partner for a portable unit.
- An automatic transfer switch detects an outage, starts a standby generator and switches over without anyone touching it.
Never plug a generator into a wall socket to "backfeed" the house. It can send power back down the line, endangering utility workers and neighbours, and can damage your wiring. Transfer switches and inlets should be installed by a qualified electrician, and many areas require a permit.
Safety basics that come with any size
- Run generators outdoors only, well away from doors, windows and vents. Exhaust contains carbon monoxide, which you cannot see or smell.
- Put a working carbon monoxide alarm on each floor where people sleep.
- Let the engine cool before refuelling, and store fuel in approved containers.
- Use heavy-duty outdoor extension leads rated for the load.
Thinking about efficiency, too
Shrinking the list of must-run items is the cheapest way to shrink the generator. Efficient appliances, LED lighting and a well-insulated home all help. Knowing your normal consumption makes this easier, and our explainer on what a smart meter is and how it works covers how interval readings show which appliances draw the most.
Can a generator run the whole house?
It can if it is large enough, which usually means a permanently installed standby unit sized by an installer who checks your panel and loads. Most portable units are sized for essentials only.
Is bigger always safer?
Not necessarily. An oversized generator costs more, uses more fuel and may run inefficiently at very light loads. Aim for your calculated figure plus a sensible margin.
Questions? Write to us.
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