We are often asked how big a home battery should be to get the best value for money.

The first thing is to understand the three main ways in which a Home Energy Storage system can benefit you:

  1. You can store your excess solar generation electricity and use it at a more convenient time. Since this is electricity you are already exporting and already being paid for, then if you can keep it, it is just free energy. Nice.
  2. You can store additional electricity at the cheap night (Economy 7) rate. This gives you half price electricity. Also nice.
  3. You get an emergency power supply in case of a brown-out or black-out. The larger the battery, the longer this will last.

Your battery storage at home can be configured to provide any or all of these three benefits according to your need. In fact, using our modular battery system means that you can get a smaller installation to cover just one of the benefits, and add individual batteries to it later to increase your scope.



A two battery Installation and inverter with space to increase to six batteries

1) Save Your Solar

If you just want to save your solar, then probably a simple one or two battery system is suitable for you. This is because you want your average daily surplus generation to be able to charge your batteries (not your total generation – it is only the surplus that is free to store in the daytime). The typical numbers for a domestic solar PV array are as follows:


Typical UK Solar PV data for Milton Keynes based on a South Facing 4 kW PV array, courtesy of the European Union Information Service.

The batteries will degrade if allowed to discharge too far and remain discharged for long periods. Remember that in winter you may have little or no surplus generation and so you may need to use grid power to charge the battery on some days to avoid it falling too low. This all happens automatically, but is no longer free. Therefore a frequent compromise might be a single 2.4 kWh battery so that it remains healthy all year round but stores most of the surplus energy.

2) Half Price Electricity

Using Economy 7 to buy cheap electricity at night really maximises the benefit of a home battery storage system. Use Economy 7 to charge your battery and then re-use the battery energy for free during the day. If you don’t already have it, your electricity supplier will usually change to an Economy 7 meter for free.

With Economy 7 the savings increase as your battery capacity grows. Therefore your total capacity should be matched to your average daily usage. When you fill in our enquiry form we provide you with a customised system design based on your real electricity use.

3) Emergency Power Supply

Your PowerBanx system comes with an emergency power socket installed for free as part of our service. In the event of a powercut, this becomes live about 5 seconds after the power fails. You can then plug in key items as required e.g. your router or phone charger, or even a small heater on an extension cable.

For a small additional cost, you can have one or more automatic failover circuits wired in. This means that in the event of a powercut, the chosen circuits will automatically be powered by your battery – even if you are away when a powercut happens. Some examples of what you could have set up:

  • Keeping Cool: Connect your fridge/freezer and make sure your food stays fresh and delicious.
  • Snug and warm: Did you know that even if you have a gas heating system, it needs electricity to be able to start. By adding your boiler circuit to an automatic failover, you can ensure your gas heating keeps you warm on even the coldest day.
  • Keep the lights on: A powercut on a winters evening really is a shock when all the lights go out. Most of us don’t have stocks of candles anymore. By adding a lighting circuit, you ensure you can see what is happening. If you already have LED lights, you also know this will last a long time.