Sizing a generator correctly is vitally important to ensure you can power all your equipment when you do not have mains electricity and also keep your running costs to an optimal level.

In order to get an accurate reading of what size generator you need it is always best to have a qualified electrician do a load test at your premises. If this isn't possible or you're just looking for an indication there are 2 additional methods which can help you determine roughly what size you require.

Have an electrician perform a load test and quote on installation.
You can manually add up all the various items which you need to run off the generator.