The idea of storing the electricity produced by your solar PV (photovolatic) array or wind turbine has become more popular in recent years. That’s driven mainly by the falling FiT (Feed-in Tariff) rate, which is now less than 4p/kWh, and the rising price of grid electricity.

Selling surplus electricity to the grid will bring in around 5p/kWh but the electricity we buy from the grid will cost around 15p/kWh. So it would seem to make sense to store that electricity on site for use, and buy less from the grid.

Here we consider the battery storage option.

Types of Battery

Lead-Acid Batteries

There are still a few lead-acid batteries on the market, but most systems use lithium-ion batteries.

Lithium-ion Batteries

The reason for choosing lithium is its longevity. A battery’s life is measured by the number of cycles – charges and discharges – it can endure before its capacity drops. Lead-acid batteries are in the 2,000 to 3,000 range and lithium-ion are in the 6,000 to 10,000 range.

AC or DC?

Beyond the lead-acid or lithium-ion batteries options, there are two different types of battery systems:

  • DC (direct current) systems These systems connect to the PV array, which means an additional inverter is not needed.
  • AC (alternating current) systems.  AC systems connect to the PV array via the electricity meter. In this case, an additional inverter is needed to convert DC electricity produced by the PV array into AC electricity which is usable in the house. Unless there is a particular need for a DC system, AC would be the default option.

How does a Battery System Work?

A battery storage system stores electricity produced by the PV array during the day for use in the evening. If you work at home or spend most of the day at home, then probably most of what is produced can be used during the day and a battery system would be largely redundant.

If not, then you need a system that will use as much electricity as possible in the house, then charge the batteries, and finally, export any excess to the grid.

This grid connection option also offers the option of charging the battery on cheap, overnight grid electricity if the battery has been run down through the evening. In this sense, how the battery works is to do with its control system.

A note of caution: Increasingly the permission of the grid operator is needed to enable battery systems to be connected to the grid and some suppliers are reporting around a 50% rejection rate. A 4kW PV array can be connected to the grid without the consent of the grid operator.

If a 4kW battery system is installed there is a theoretical potential to export 8kW, which needs permission. Check this with your supplier/installer, who may need to complete a G100 application form.

Does Storage Capacity Matter?

Floorstanding battery systems for domestic applications are similar in size to a washing machine. Wall-mounted ones, like the Tesla Powerwall, are slimline but are similar in width and height. The Moixa Smart Battery, meanwhile, measures just 500m x 300mm x 200mm. That’s around the size of a small suitcase.

More important is the electrical size. This is measured in kWh storage capacity and will range from 4kWh for the Powervault G200 to 14kWh for the Tesla Powerwall. However, a battery must not be fully discharged so the total capacity might be 14kW but the usable capacity is 13.5kW. What you need will depend on how much electricity you need to store and how much you want to spend.

As a guide, the ‘average’ UK household uses around 16kWh per day,
most of which will be during the evening.

Another measure is the discharge capacity, which is the maximum the battery can put out at any one time. The Powervault, for instance, can discharge a maximum 1.6kW, which is not enough to run a kettle. The Tesla Powerwall will put out 5kW — enough to keep a small family ticking over. Again, it is a matter of how much you want to spend — the Powervault is half the price of the Powerwall.

Does a Battery Storage System Make Economic Sense?

The real-term value of a battery system is difficult to calculate as there are so many variables, including the way grid electricity prices will change over the next 10 years.

But if we consider:

  • the storage capacity
  • the number of cycles available in the warranty period
  • the installed cost of the system

then we arrive at the unit costs shown in the table below. Bear in mind that the price does not include installation costs or VAT, which is 5% if the battery is installed at the same time as the PV array, but 20% as a retrofit.

However, a good battery system will give independence for around eight months of the year, and the price is fixed for the life of the battery system, while the price of grid electricity is only likely to rise.

Remember, also, that the PV array is likely to last 20 years or more, and the battery system is only warranted for 10 years, although it is likely to last longer than that.

Battery Systems Compared

Model Useable
capacity per kWh
Cycles kWH under warranty* Price Unit cost per kWh Warranty Battery

GoodWe 3.6 Hybrid/Hoppecke

4

2,500 10,000 £4K 40p not known Lead acid
Powervault 4KWh 4 4,000 16,000 £4K 25p 10 years Lithium
Solar Edge LG Chem RESU10 9.3 Unlimited 27,400 £6K 22p 10 years Lithium
Tesla Powerwall 2 13.5 Unlimited 37,800 £6K 16p 10 years Lithium
Solax Hybrid with Pylon 7,2KwH 5.76 6,000 34,560 £4.3K 12p 5 years Lithium
Sonnen 8.8 8 10,000 80,000 £8.7K 11p 10 years Lithium
Moixa 3Kw 2.4 10,000 24,000 £3.45k 14p 10 years** Lithium

* The warranted KwH is a good guide to cost effectiveness but only around 25,000KwH will be put through the batter from a domestic scale PV array. If the battery is charged with cheap overnight electricity it is only likely to increase to around 35,00 KwH.

** Anyone choosing to sign up to Moixa’s Gridshare scheme has an extendable warranty for as long as they remain a member of the scheme.

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