Tool · Generators
Generator power calculator
Select the devices you want to power at the same time, and the calculator works out the continuous generator power you need, with a margin for the motor starting surge, and suggests models from our range. The device powers in the list are approximate typical values; if you know the exact power from the device plate, enter that.
- Margin
- starting surge of the largest motor + 20%
- Recommendation
- by rated (continuous) power
The calculator adds up the continuous power of all the selected devices. Then, for the device with the highest starting power (continuous power × surge factor), it adds the difference between that power and its continuous power, because motors do not, as a rule, start at the same time. It adds a 20% margin to the resulting sum. It recommends models from our range whose rated (continuous) power is greater than or equal to the result, from the smallest to the larger. Example: a refrigerator 150 W, lighting 100 W, television 100 W, router 20 W, gas boiler 150 W and a well pump 1,000 W give 1,520 W; the pump with a factor of 3 adds 2,000 W, a total of 3,520 W, and with a 20% margin about 4,220 W, so the calculator suggests models with a rated power of 5.0 kW or more, for example the KS 7000E (5.5/5.0 kW). The calculator does not take into account the distribution across phases, running on gas or the condition of the installation.
How to use the calculator
- Select the devices that must run at the same time, not everything you have.
- Where you know the exact power from the plate, change the suggested value.
- See the power you need and the suggested models.
- If you want the generator to start by itself when the power fails, narrow your choice to models with an ATS.
What the calculator does not know
- The exact starting surge of a device. When starting, an electric motor draws several times more current than when running, roughly 2 to 3 times, and some pumps and compressors even more. The calculator uses an approximate factor.
- The distribution across phases. With a three-phase generator, single-phase devices cannot use the full output of the generator.
- Gas. Dual fuel models on LPG deliver about 10% less power than on petrol. If you will run on gas, take a model with that much more power.
- Your installation. Which circuits the generator may supply and how to connect it is determined by an electrician.
Connecting to the house installation
A generator may be connected to the house installation only by an authorised electrician, through a mains/generator changeover switch (manual or ATS). Without a changeover switch, current from the generator can flow into the public mains, which is dangerous for the people working on it. Do not connect a generator to the installation through a socket and do not run it in an enclosed space.
How to estimate the power without the calculator, step by step, is described in the guide How to choose a generator for your home.
Frequently asked questions
Why does the calculator add a margin for the starting surge?
The motors of a refrigerator, pump, compressor and air conditioner briefly draw several times more current when starting than when running. Without that margin the generator can stall, or the voltage can drop, just as the motor starts.
How accurate are the device powers in the list?
They are approximate typical values. The exact power is on the plate or label of the device, so enter it for a reliable result.
What if the calculator suggests a generator that is too large?
Switch off devices with a heating element (water heater, kettle, oven) while the generator is running, or do not start them at the same time. The power you need often drops by several kW as a result.
Can I connect the generator to the house myself based on the result?
No. The calculator helps you choose the power, and the connection to the house installation is made by an authorised electrician, through a mains/generator changeover switch.