By Technical Director, Phil Powell

We hear a lot about batteries these days and how they will revolutionise the home energy sector. They are fast becoming a must-have for the early adopters, but do they live up to the hype?

A) Batteries work in 3 ways, or sometimes in a combination of 3 ways


They store surplus generation from the solar PV system for use in the evening.


They are recharged with a cheap off-peak tariff overnight, to be used during the next day when the tariff is more expensive.


They act as an uninterruptible power supply (UPS) and provide power during power cuts to keep electrical devices running for a period, e.g heart rate monitors. 

B) What're they made of?

The current most cost effective option is lithium ion batteries. For longevity they generally should not discharge below a minimum depth of discharge (DOD), and most systems have a device fitted to prevent that happening. The DOD for lithium batteries is 20%, unless the battery is used for emergency supply during power-cuts when this can be overridden.

So, when you are quoted a lithium ion battery capacity of 10kW, you’re actually only getting 80% of that capacity, i.e 8kW.

C) Costs

The current cost of a lithium ion battery is around £900 per rather kWh capacity. A dedicated inverter is also needed to convert the direct current (DC) stored by the batteries, to alternative current (AC) so it can then flow into your main circuit.

So, a 11.6kWh lithium ion battery (a typical size for a household) and inverter, should cost around £10,500 supplied, not including installation labour.

D) Savings

So, going back to our previous three scenarios.


If the battery is used for storing surplus solar energy just once a day, it will store a maximum of 80% of the rated capacity. This can then be used in the evening and night.

So, a 4.8kWh would save 3.84kWh a day (4.8 x 80% being the DOD), multiplied by the 365 days of the year, gives a maximum of 1,400kWh a year. If you’re currently paying 32p/kWh to your supplier, then that represents a saving of £448 a year.

However, this assumes you have surplus solar energy everyday to store in your battery system, and that your solar is generating at 100% even in the winter. In reality, you will have insufficient surplus power and won’t use all the power in the evening, especially in the summer – unless you have a car charger.

You need to be clear about how much you are likely to use the batteries.


If the battery is used to store cheap rate power over night for use during the day, it’s easier to estimate the savings.

First, work out the cost differential between your day and night rates. E.g, 32p/kWh during the day and 15p/kWh at night, which would be 17p/kWh.

Second, to calculate the yearly savings, it would be 80% of the rated battery capacity multiplied by the 365 days of the year, multiplied by the cost differential. Using the same 4.8kWh battery example, the calculation would be:

3.84kWh x 365 days x 17p = saving £238 per year (1,400kWh x 17p)

Here, you will charge up 100% every night but you won’t necessarily use 100% of the stored power everyday.


The battery would be used (hopefully) infrequently and only during a power cut.

The consideration here is the cost of providing emergency blackout power against the cost of disruption to your electrical devices, e.g medical breathing apparatus.


Depending on your needs, your solar generation and your export, it can be difficult to decide if a battery system is right for you.

It also gets more complicated if you have an electric vehicle.

However, our data shows that households will use around 50% of their battery storage per year – and we’re happy to go through these with your personal circumstances.