Portable Generators for Home Back-up Guide

Our interest in generators started back in 2012 when we were renovating our home and wanted to install a back-up system. What at first seemed a simple idea became rather complicated to understand. We've developed this page to try and provide a definitive easy to follow resource for anyone looking to buy or connect up a portable generator. It does not cover larger commercial or fixed generator installations. 

What type of generator should I chose?

Portable Generator

  • A portable generator uses a combustion engine to directly turn an alternator, which generates AC electricity.
  • Typically the most cost effective type of generator per kWh output, they are great for inductive loads like power tools which need a lot of current to get started.
  • A governor controls the engine’s speed to maintain an output which has a steady voltage and frequency. The process is not entirely precise and can be affected by the load on the generator. Such variation is generally fine when supplying many lights and power tools.
  • However, sensitive electronic items such as computers, home automation systems and microwaves are critically dependant on a smooth pure sine wave output with a very stable voltage and frequency. Using a standard portable generator may damage sensitive electronic equipment.

 Inverter Generator

  • An inverter generator uses a combustion engine to turn an alternator, which produces high frequency AC electricity. Unlike a standard portable generator this is then converted into DC, before an inverter converts it back to AC. Whilst it sounds a little complicated, these additional steps essentially ensure that the output is not so dependent on the running speed of the engine and is unaffected by the load placed on the generator.
  • Inverter generators provide a very stable pure sine wave output, with a consistent voltage and frequency which is as good as that from any mains supply.
  • The increased stability makes them the ideal choice for powering sensitive electronic equipment. For this reason, they are recommended for home back up supplies and motor homes.
  • They are more expensive per kWh output, but are generally more compact and light weight. They are easily identified in pictures by their outer plastic casing.

What size of generator do I need?

In order to establish what generator power output you require, we've developed a simple Excel spreadsheet that does the hard work for you. Download the file here and follow the instructions below. Note that the spreadsheet will not work in 'read only mode' either click on the option to edit or download it.

Step 1

Find the rating of all of the appliances which will be connected to the generator at any one time, by checking the appliance rating plate/label. If that fails, Google the operating manual.

If the rating is only stated in Kilowatts (kW) or Amps (A), the spreadsheet has a handy converter to change them to Watts (W):

Step 2

Enter the name and rating in Watts (W) of each appliance. These must be recorded in either the resistive OR inductive load column. 

Resistive loads are items which do not contain a motor. They require a steady amount of current; the start up and operating current are the same. Examples include: 

  • Lights
  • Computers
  • TV
  • Entertainment systems
  • Kettles

Inductive loads are items which contain a motor. When an inductive load is first powered up, it draws a much larger start up current, typically 2.5-3 times its normal operating current. For example a 500w drill may briefly draw 1250-1500w when it starts up. The appliance will generally only state the power that it normally consumes, not the start up power required on the label. Examples include: 

  • Power tools such as drill, saws, planers and sanders.
  • Refrigerators
  • Microwaves
  • Electric deep fryers
  • Coffee machines
  • Hair dryers
  • Pumps

Step 3

The spreadsheet will automatically calculate the continual power and peak power that you require from a generator. Choose a generator which has a continual AND peak rating which is equal or greater to those calculated

Example Calculation

In this example, the generator is being used to provide emergency back-up power to a home. The total resistive load is 450W, and the total inductive load 500W.

The generator required in the example would therefore need to have a continual rating of at least 950W AND a peak power rating of at least 1950W.

Is run time Important?

Think about the scenarios in which you will use the generator to determine the required approximate continuous run time. This is the maximum length of time that the generator will need to run for without being turned off. Ensure that you choose a generator with a continual run time which meets or exceeds your requirement.

How do I connect up the generator?

The simplest way to use a generator to provide power in a home during a power cut is to use an extension lead to power appliances directly (direct connection). We offer single and 4 gang heavy duty extension leads for this purpose.

Whilst it is possible to connect a generator to the supply side of a home consumer unit to enable all circuits to be powered (subject to the generator rating), this is a complex situation which must be installed by a qualified electrician. A special changeover switch must be fitted to ensure that the home can only ever be powered by either the mains supply or generator at any time, and earthing arrangements need to be carefully considered. Below is a picture of the set up in our own home.

Under no circumstances connect a generator to your home using a lead with a 13A plug on both ends. This is incredibly dangerous; there is a reason that such a lead is often referred to as a ‘widow maker’!

Does the generator need an earth rod and do RCDs work?

Portable generators are normally operated without the use of an earth electrode, as it would be impractical to install one in the ground each time. In this 'floating' system, the earth and neutral are also not connected together. Plug in RCDs and extension leads with RCDs will not operate with a floating system - for this reason, all of our generator extension leads are supplied without an RCD.

When used for a hard wired home back up systems, an electrician will install a suitable earthing arrangement and fit a link between the neutral and earth.  

Further technical guidance on both arrangements can be found in these useful articles from NICEIC and the IET.