Generator Size Calculator: work out the continuous running watts and starting or surge output needed for the appliances you want to keep powered during a power cut. Select your loads, adjust quantities, and replace the typical wattages with your own appliance ratings where available.
UK GENERATOR SIZE CALCULATOR
Generator Size Calculator
Select the appliances you need to run and estimate the generator continuous and starting output required.
Private calculation: Your values are processed in your browser. Backup Power Calculator does not receive or store your calculator inputs.
Generator safety: combustion generators must never be operated inside a home, garage, shed or other enclosed/partly enclosed space. Follow the manufacturer's installation, earthing, connection and carbon-monoxide guidance.
How the Generator Size Calculator works
Using a generator size calculator is not simply a matter of adding the wattage printed on every appliance. A generator has to support the combined running load while also coping with the brief higher demand created when some motors, compressors and pumps start. The Generator Size Calculator accounts for both.
Choose only the equipment you genuinely want to run at the same time. A fridge-freezer, broadband router, a few lights and a television may need far less generator output than trying to support kettles, microwaves, pumps and other high-power appliances simultaneously. The result therefore depends on your chosen backup plan, not just the size of your home.
Running watts vs starting or surge watts
Running watts describe the power an appliance uses while operating. Starting watts, sometimes called surge watts, describe a short higher demand that can occur when equipment starts. Refrigeration compressors, pumps and some motors are common examples.
The calculator adds the selected running loads and then allows for the largest additional startup requirement rather than assuming every appliance reaches its maximum surge at exactly the same instant. It then applies the output headroom you choose and rounds the recommendation upward to a practical generator rating.
Use your own appliance ratings where possible
Typical appliance figures are included so you can get started quickly, but they are planning estimates rather than universal specifications. Use Edit appliance ratings on any selected appliance if you know its actual running and starting wattage from the data label or manufacturer documentation.
Quantity matters as well. If you intend to run several identical loads, increase the quantity so their normal running demand is included. You can also use the extra running-load and extra startup/surge fields for equipment that is not already listed.
What size generator do I need?
The useful answer is the generator size that covers the loads you plan to use, their startup behaviour and sensible headroom. The result therefore shows a recommended minimum continuous output and a recommended starting/surge output. When comparing generators, check both manufacturer ratings rather than looking at a single headline wattage.
If you are planning battery or portable-power backup instead of an engine-driven generator, use the Home Backup Power Calculator to estimate the broader power and energy requirement for your selected household loads.
Generator safety
Combustion generators produce carbon monoxide and must never be operated inside a home, garage, shed or other enclosed or partly enclosed space. Follow the generator manufacturer’s requirements for placement, ventilation, earthing and connections. Do not improvise a connection to household wiring or backfeed a property. Fixed or changeover connections should follow applicable electrical requirements and use a suitably qualified professional where required.
Generator Size Calculator FAQs
How do I calculate what size generator I need?
Add the running watts of the equipment you need at the same time, account for the largest relevant startup demand, and allow suitable output headroom. The calculator performs those steps from the appliances and ratings you enter.
Why is starting wattage higher than running wattage?
Motors and compressors can briefly draw more power while starting than they require once running. A generator may support the normal load but still struggle if its available surge output is too low.
Should I size a generator for every appliance in the house?
Not necessarily. Backup planning is usually more practical when you decide what must remain available during an outage and size for those loads. Trying to power every high-demand appliance at once can increase generator size substantially.
Can I change the appliance wattages in this calculator?
Yes. Each appliance has editable running and starting/surge ratings. Use the actual figures from your equipment where they are available.
Planning note: generator ratings, appliance startup behaviour, altitude, temperature and fuel type can affect real output. Treat the result as a sizing estimate and confirm the exact generator specification before purchase or connection.