From the time electricity was first discovered, we have tried to find ways to store it. By 1800, Alessandro Volta (for whom the volt is named) had already developed the first usable battery and we are still working on that same basic principle.

But batteries are not the only option and we now have no less than four potential, practical ways of storing electricity — covered below in more detail.

The problem has existed for years and, until a decade or so ago, we were fairly happy to live with it. The vast majority of self builders and renovators didn’t even attempt such a thing as generating their own electricity.

That all changed with the take-up of photovoltaic (PV) solar panels which now sit on over a million homes in the UK. That they have enjoyed such success is due to a slightly unusual solution to energy management: a Government incentive (Feed-in Tariff) to produce electricity regardless of what you do with it. It was a generation game rather than a question of effectively using what’s generated.

The Four Ways to Store Electricity

We’ve been ‘managing’ electricity for years. Here’s how:

Lead-Acid Batteries
Well-known, well-developed technology that has been used for many years. Batteries from milk floats and even submarines have been used by intrepid renewable energy users. The problems are cost, relatively short lifespan (they have a limited number of charge/discharge cycles) and disposal.

Solid State Batteries
The batteries used in laptops, etc., are solid state batteries. These are usually lithium-ion (Li-ion) or nickel-cadmium (Ni-Cd) batteries and can be three to four times more efficient than lead-acid batteries. They have low power density (poor for things that want power all at once) but can store lots of energy in a small space.

Thermal
There are a number of products available that will enable electricity from renewable energy sources (PV, wind turbines, etc.) to run an immersion heater in a hot water cylinder. Storing electricity as heat is not a new idea and it means that the electricity (usually — at a domestic level) cannot be converted back to electricity.

The Grid
The default option for most renewable energy system owners is to ‘store’ surplus electricity production on the Grid. That is, to sell it (at around 4.7p/kWh) to an energy supplier and then buy it back (at around 13p/kWh) when they need it. Obviously this isn’t ideal considering that around 50 per cent of PV energy is exported back.

Increasingly, however, canny homeowners have begun to find ways to use this ‘free’ electricity. Storing it for future use, in the home, is the next – and highly sensible – step forward.

Other electricity innovations

ImmerSUN
Diverts electricity produced by PV panels into an immersion or storage heater. Claims to save £250 per year

immerSUN-20II-White

Sonnenbatterie
Stores up to 16kWh (useable capacity) of electricity from solar in wall-mounted batteries, with basic models starting at 4kWh

Energy-accumulator-Sonnenbatterie-eco

Solar storage from Save Energy
A UK solar PV battery with sizes upwards from 5kW allowing you to use stored electricity when you need it

Tesla Powerwall
The ‘game changer’ development in solar storage, the Tesla Powerwall 2, which comes onto the UK market in February 2017 and replaces its Powerwall 1.

The battery charges during low rate periods when demand for electricity is lower, and discharges during more expensive rate periods when electricity demand is higher. It can store surplus solar energy not used at the time it is generated and can use that energy later when the sun is not shining. Also, one nice benefit is that it could potentially take the house away from Grid reliance for electricity — acting as a back-up during Grid outages.

The battery of Powerwall 2 will double the capacity of the smaller Powerwall 1 for less than double the purchase price. The initial price of the Powerwall 2 on international markets is $5,500 for a 14kW system, compared with $3,000 for the 7kW system. Tesla has aimed this unit at the mass consumer market and claims that it will be able to store enough electricity to power the lights, sockets and fridge in a four bedroom house for a whole day.

Storing Electricity: The Expert View

As with all these things, considering the cost/benefit analysis can be a complex process and has to start with some idea of how much electricity is to be stored. Perhaps obviously, storing a small amount of electricity is cheaper than storing a large amount.

We take the Grid as the default option, as it has no capital cost, irrespective of how much electricity we want to store, but has a ‘running cost’ being the difference between the sale and purchase prices — currently about 8p/kWh. It is then reasonable to suggest that any storage system needs to be cheaper than that to justify a capital investment.

Until recently, lead-acid batteries were the only viable option and a typical domestic-scale system could cost upwards of £10,000 — including racking, a shed to house it and disposal costs. If we assume that the batteries are storing surplus from a 4kWp PV array and that they will last 10 years, that equates to a ‘running cost’ of perhaps 52p/kWh, making the Grid look exceptionally cheap.

Solid state battery technology has advanced in leaps and bounds in recent years and the Tesla Powerwall is a result of that development. It will store up to 10kWh in a weekly cycle, which is a reasonable amount for a domestic user. A 4kWp PV array (i.e. typical domestic scale) will, in spring and summer, generate around 12kWh per day and the Powerwall is designed to store the peak energy for evening demand. It comes with a 10-year warranty. At a capital cost of around £2,500 (installed) that equates to a ‘running cost’ of about 13p/kWh — which still leaves the Grid looking cheap.

Thermal storage systems are lead by the ImmerSUN unit — this will enable the electricity produced by the PV array to operate an immersion heater in a hot water cylinder. All of these options have an efficiency factor and in the case of thermal storage we also need to consider losses from the hot water cylinder. This sort of conversion unit will cost around £500 installed and if it similarly lasts about 10 years will have a ‘running cost’ of about 3p/kWh — cheaper than the Grid, but is now energy that can only be used as heat, of course.

Electricity & Your Home

Fun facts for the energy nerd

  • Using an immersion to heat the water from 20°C to 70°C in a typical 150 litre hot water cylinder will require 8.75kWh
  • A 4kWp (16 panel) solar PV system will produce up to around 25kWh on a good day, so should be more than capable of providing all the hot water required
  • Approximately 50 per cent of the electricity generated by PV panels is exported to the Grid
  • The typical home consumes 9kWh of electricity every day
  • A kettle requires a supply of 3kW. The (current) continuous supply of the Tesla Powerwall is just 2kW — which means that it would struggle with powering a kettle
  • The average home uses 3,300kWh of electricity a year

Conclusion

Electricity storage is not purely a hardware problem. The Tesla Powerwall also contains software that determines when to store electricity and when to use it; something the Grid cannot do. It optimises its performance to provide the best result for the house, so the pure ‘running cost’ calculation might not be wholly valid.

If renewable energy has a future in the domestic market then battery storage will have its place — be it as a principal energy source, as back-up or merely for those wishing to safeguard themselves against an apocalypse! If you want the surplus electricity to be usable as electricity, then storing on the Grid is still the cheapest and most convenient option. But that may not always be the case.

History tells us that the Powerwall will be copied and improved. What the Powerwall has done is open a market and that has to be good for all domestic-scale renewable energy producers. Indeed Mercedes-Benz have a similar 2.5kW product available — and you can guarantee there will be more to come.

Our Sponsors