Adding up appliance wattages is a useful starting point, but it is not the whole sizing job. A refrigerator, freezer, boiler pump or power tool may draw much more for a short period when its motor starts than it uses while running. A generator that appears large enough on paper can therefore overload when several loads begin together.
If you are asking, “what size generator do I need for home backup?”, the answer depends on three things: the equipment you want to run at the same time, its normal running load and its starting surge. You also need sensible headroom for variation in real-world use.
What size generator do I need for home backup?
Start by deciding which appliances and circuits genuinely need power during an outage. Do not size the generator around every appliance in the house unless you intend to run them concurrently. A smaller, carefully chosen load can be more practical than trying to supply the whole property.
Make a list containing:
- Appliances that must remain available, such as a fridge or freezer.
- Useful household loads, such as lighting, internet equipment, a television or a circulation pump.
- Occasional loads, such as a kettle, microwave, washing machine or power tool.
- Loads with motors, compressors or heating elements, because these may affect the generator rating differently.
Check the data plate, user manual or manufacturer specification for each item. Look for watts (W), amps (A), volt-amperes (VA) or kilowatts (kW). In the UK, household equipment is generally designed around a 230V, 50Hz supply, but the generator and appliance ratings still need to be compatible. If a rating is given only in amps, a rough power calculation is volts multiplied by amps; the actual requirement can differ where power factor is involved.
Running load: the power appliances use normally
Running load is the power an appliance draws after it has started and settled into normal operation. Add the running wattage of the items you expect to operate at the same time:
Total running load = appliance 1 running watts + appliance 2 running watts + appliance 3 running watts
For example, you might plan to run a fridge, a few LED lights, broadband equipment and a television together. Use the figures on the appliances or in their manuals rather than relying on a generic online wattage chart. Two products serving the same purpose can have noticeably different demands.
Some loads are intermittent. A boiler pump may not run continuously, and a fridge compressor cycles on and off. That does not mean you can ignore its rating: the generator must still cope when the load is active, and it may need to handle the compressor’s start-up demand at the same time as other equipment.
Starting surge is the part that catches people out
Motors and compressors can draw a short burst of current when they start. This is commonly called starting watts, surge watts or inrush current. Refrigerators, freezers, pumps, fans and some power tools can all produce this effect. The surge may last only briefly, but it can cause a generator to trip, stall or reduce its output if the generator cannot supply it.
Heating loads such as kettles, heaters and conventional incandescent lamps generally do not have the same motor-starting surge, although their running demand can be high. Electronic equipment may have its own start-up behaviour, so the manufacturer’s information remains the best guide.
There is no single surge multiplier that applies to every appliance. A practical calculation is:
Required generator capacity = simultaneous running load + the additional starting demand of the largest relevant start-up event
That wording matters. You do not necessarily add the full starting rating of every motor together, because appliances may not start at exactly the same moment. However, if a fridge, pump and freezer could start while other equipment is already operating, the generator needs enough capacity for that likely combination.
Where the manufacturer gives separate running and starting figures, use them. If it gives only an amp rating or an unclear label, ask the manufacturer or a qualified electrician for clarification rather than treating a guessed figure as dependable.
How much headroom should you allow?
Do not choose a generator whose continuous rating exactly matches your calculated running load. A margin gives you room for small measurement errors, changing appliance behaviour and an extra light or charger. As a rough planning approach, some people allow around 20–25% above the expected continuous load, but that is a starting point rather than a universal rule. Motor surge, generator design, ambient conditions and altitude can all change the result.
Compare like with like when reading specifications. A generator may show a continuous or rated output and a higher maximum or surge output. The continuous figure is the one to use for the ongoing load; the maximum figure must cover the temporary starting demand without being treated as a sustainable rating.
Also check whether the manufacturer quotes watts, VA or kVA. For simple resistive loads these can be close, but motors and electronic power supplies may have a power factor that makes the relationship less straightforward. If your planned load includes several motors or sensitive electronic equipment, professional advice is worthwhile.
A worked sizing example
Imagine a deliberately simplified plan with these illustrative figures:
- Fridge: 150W running, with a higher start-up demand stated by its manufacturer.
- Freezer: 120W running, also with a higher start-up demand.
- Lighting and broadband equipment: 100W combined.
- Television: 100W.
The running total is 470W. That does not mean a 500W generator is automatically suitable. The fridge and freezer may start while the other 200W is already being supplied, so their starting requirements must be checked and added to the relevant running load. If you also want to use a 3,000W kettle, that changes the calculation substantially even though it has little motor surge.
This example uses rounded figures to show the method, not to specify typical appliance consumption. Check your own equipment. A manufacturer’s rating may describe a maximum, an average or a particular operating mode, and actual demand can vary.
Choosing between a small generator and a larger one
A smaller generator can make sense if you are happy to run a defined group of low-power appliances and avoid simultaneous high-demand loads. It may also be easier to store and move, depending on the design. The limitation is that a kettle, fan heater, microwave or washing machine can use much of the available capacity on its own.
A larger generator gives you more flexibility, but it is not automatically the better choice. It may consume more fuel at a light load, be heavier or produce more noise. Generator efficiency and fuel use vary by model and load, so check the manufacturer’s specifications rather than assuming that a larger unit will run for a particular number of hours.
Think in terms of a load plan. You might keep refrigeration, lighting and communications on the generator continuously, then switch on a kitchen appliance briefly while reducing other loads. A written plan helps prevent accidental overloading and makes it clear which appliances are priorities.
Generator sizing is not the same as runtime
Generator size is about power: whether the unit can supply the required watts or VA at a given moment. Runtime is about energy: how much fuel is available and how efficiently the generator uses it over time.
A generator with enough output can still run for less time than expected if the load is high. Fuel consumption depends on the model, fuel type, load, operating conditions and maintenance. Temperature, altitude and extension-cable losses can also affect performance. Use the manufacturer’s fuel-consumption information at comparable loads, and treat any runtime estimate as conditional rather than guaranteed.
Battery systems require the same distinction. A battery’s nominal capacity in watt-hours is not necessarily the energy available to your appliances. Usable capacity, inverter efficiency, discharge limits and battery age all matter. If you are comparing a battery power station with a combustion generator, compare both its continuous output and surge capability as well as its usable energy.
Safety and connection choices
A combustion generator must be operated outdoors, in a well-ventilated position and away from doors, windows and ventilation openings. Never use one indoors, in a garage, basement, shed or another enclosed or partly enclosed space: carbon monoxide can build up without an obvious warning. Follow the manufacturer’s placement and operating guidance, and read the relevant generator safety advice from Electrical Safety First.
Portable appliances may be connected only in ways permitted by the generator and appliance manufacturers. Supplying fixed household circuits is a different matter from plugging in a single appliance. Any permanent connection or changeover arrangement should be assessed and installed by an appropriately qualified electrician, in line with current UK requirements and the property’s electrical system. This article does not provide instructions for altering fixed wiring.
Keep fuel storage, refuelling and exhaust arrangements within the manufacturer’s instructions. Do not refuel a hot generator, and keep children and combustible materials away from the equipment. If backup power is intended for a medical device or other clinically critical equipment, follow the device manufacturer’s guidance and speak to the relevant clinical and electrical professionals rather than relying on a general generator calculation.
Use a load list before buying
The most reliable next step is to write down each appliance, its running watts, any stated starting watts and whether it will run continuously or intermittently. Mark the combinations you expect to use together. Then enter those figures into the UK Backup Power Calculator to compare running load, surge requirements and a sensible planning margin.
Before committing to a model, check its continuous output, maximum output, voltage and frequency, fuel information, noise specification and manufacturer limits. If any appliance’s starting demand is unknown, treat that uncertainty as a reason to obtain better information, not as permission to guess.
Frequently Asked Questions
Can I size a generator by adding all the wattages in my home?
Only if you genuinely plan to run all those appliances at the same time. A better method is to list the appliances needed during an outage, total their simultaneous running load and then account for the highest relevant starting surge. High-power appliances such as kettles and heaters can change the result quickly.
How do I find an appliance’s starting watts?
Check the appliance data plate, manual or manufacturer specification for separate running and starting figures. If the information is missing, contact the manufacturer or ask a qualified electrician. Generic online estimates may be useful for an initial list, but they should not be treated as a precise rating for a safety-critical decision.
Is a generator’s maximum wattage the same as its usable rating?
No. The continuous or rated output is the figure used for the ongoing load. Maximum or surge output is intended to cover a temporary demand, such as a motor starting. Both ratings matter, but the maximum figure should not be treated as the amount the generator can supply continuously.
What size generator will run a whole house?
There is no single whole-house size. It depends on the property’s connected loads, the circuits you want backed up, heating and cooking equipment, motor loads and whether appliances can operate together. A qualified electrician can help define a safe backup arrangement, while a detailed appliance load list provides the starting point for sizing.
Gather the data-plate figures for your priority appliances, decide which ones must run together and use the calculator to test the resulting load before comparing generator specifications.