Storage - The Future

FOLLOWING the Government’s review of the Feed in Tariff the payment rate has dropped considerably so its important to use as much of your own generated energy as possible. Some people will think it’s all over, there is no money in it anymore, but the reality is everybody needs energy.

The customer profile will change from one who is looking for a return on investment on their money, to one looking to secure their future energy and reduce their outgoings in the future.

Traditional financial models calculate financial return based on the FiT and the projected price increases, but how do we know what will happen to the price? For energy the most financially secure, and reliable way, is to have your own energy generation. This can be Solar PV, solar thermal, wind or hydro power.

With all the current publicity about possible government cuts many people will think there is no point in installing a renewable system now, but with all the uncertainties and the latest technology offerings, there has never been a better time to invest in generating your own energy.

A 4kw peak PV system will supply about 3500kwhrs/year, that’s about the same amount of energy that an average family 

of four will use in a year.

Of course with a typical system the generation and consumption don’t always happen at the same time, so the grid is there to take the excess and deliver the shortfall. Here you can see the generation of a 4kw peak system so far this year. The exported energy is shown in yellow, the imported energy is shown in red. The energy used as it is generated is shown in green. Even in October more was generated than used by the building.







With a 4kw PV system a typical summer day produces much more energy than you need (shown in yellow) but to meet the demand in December and January would need a much larger PV array. Below you can see energy generated on a typical February day and a typical June day.


But what if you had storage? This could be storage into batteries or storage by dumping the excess into heating water. Either of these techniques makes more use of your own generated energy and can reduce peak demands on the grid, and even better save your daytime generated energy for your own use in the evening.

All these are ways of storing your own generated energy for reuse later.

Whatever happens to the Feed in Tariff in the future, once technologies that generate and save energy are installed it future proofs the building and the occupants against rising energy prices, and perhaps even an unreliable supply.

Currently using your own energy saves, depending on your tariff, around 15p/kwhr, over 10p more than the current Feed in Tariff rate, so using your own energy is a reliable, secure, and predictable investment for the future.


We do not suggest there will be a traditional payback or return on investment – this depends on your priorities. Depending on what storage system you choose you may recover the cost within 10 years. Most storage batteries will last longer than 10 years, in fact most are warrantied for 10 years, so these will payback within their service lifetime at current costs.

The way I like to consider it is compared to a comfortable armchair, there is a payback, but it is longer than is currently fashionable and not necessarily monetary. Primarily you do have the ultimate satisfaction that you are making the most of your own energy. What you will get is that satisfying feeling of comfort and security that you feel in winter when sitting in your favourite armchair next to a cosy warm log fire, with a nice cup of tea!

Energy Diverters. 

One method of storage is energy diversion into heat. The diverter we use is the Powerflow which diverts the exported 

amount of energy into an immersion for heating water, or could be a towel rail, storage heater or what ever you wish. 

Some energy diverters offer more than one output so diverted energy can be supplied to two or even three appliances. An 

important point here is that the reaction response time of the device must be fast enough to avoid importing more than 

necessary – for example sudden changes in demand such as switching the kettle on or the fridge starting, will result in unnecessary import unless the diverter can respond quickly.


Integrated Storage Inverter

Some inverter manufacturers are now producing inverters with storage batteries built in, they use the same inverter to feed your house from the PV and the built in battery. Illustrated here is a Sunny Boy Smart Energy 3.6kw inverter with a 2kwhr lithium iron battery built in.




Battery Storage.

Another method is store electrical energy in batteries for reuse later. After the sun has gone in the evening stored power can be recovered and delivered to supply the demand. If you have sufficient PV output during the day it may be worth installing a larger capacity battery.

There are two basically different types, DC coupled and AC coupled:-

            a) DC coupling basically uses a battery and via your existing inverter supplies you with electricity when the export has ceased and you start importing from the grid. Using your solar saved energy to supply the load when the PV stops generating - eg. at dusk or during the day when the sun goes behind a cloud.. Prices start from around £2500 installed.

            b) AC coupling uses a battery and is independent of your PV system and does the same as the DC coupled, but in addition has the future possibility of running as a backup should the grid fail. Prices start at around £2500 for self use storage installed. Storage with backup capability will add a considerable amount depending on the length of time (battery capacity) you might need to rely on for your own supply.

Battery technology is changing and typical storage batteries are Lithium Ion, (Expensive but small), Lead acid AGM (cheapest but old technology and bulky) Aquion salt technology (New, expensive and bulky but recyclable). 

From our early tests it appears that for maximising the use of your own solar energy cost effectively you require about  2kwhr for every 2kw peak of solar panels, eg. a 4kw peak solar system would benefit from 4kwhr of storage. This will utilise all of the storage capacity from around February until November. There will not be enough surplus generated electricity in January and December to contribute much to recharging. Adding more storage gives greater capability in the summer when generation is greater, but adds little to the winter months when daylight hours are shorter and little surplus is generated.

The PowerFlow Sundial is an AC coupled storage system than can be easily added to an existing PV system reguardless of what make of inverter is installed.  See recommendation guide below.

Interesting little video clip can found here.

Below is a Powerflow Sundial with two SMA Sunny Boy inverters, one on the original 2010 system and one added to increase the generation when storage was added. There are two Sundials one above the other.


Tesla and Solar Edge supply a StorEdge system called the PowerWall, this is a DC coupled system. Tesla and SMA also offer a Powerwall system that can be retrofitted to any PV system using any make of inverter. Greenshop Solar Ltd is a Tesla Approved Installer. The system pictured at the top of the page is a Tesla Solar Edge system with a connected PV system of 8kW peak.  

The Tesla storage system is developing fast with more invations to follow next year.


Tesla House - 8.4kW PV and Tesla Powerwall 1 Storage + Export Limitation to 5kW (Network Requirement) showing operation on a sunny December day.


SMA have been supplying the Sunny Island and Sunny Backup system for many years. The Sunny Island systems will operate as "self use" system storing the energy during the day for use later, or as a backup should the grid fail. The Sunny Backup is primarily a backup system keeping the batteries full incase of power failure. A diagram of a single phase system is below, three phase systems just use three coupled Sunny Island inverters using a a three phase switch box. 

The system pictured here is a Sunny Backup capable of 5kw continuous and 8kw peak to the building. This system is grid connected with 4kwp of PV. The batteries are in a chest below. The system has been installed for over 5 years in a rural house subject to power cuts. 

Another long establish manufacturer of stand alone, and backup systems is Victron, these range from small 1.5kw up to large 3 phase systems. Below is a Victron grid connected backup system with lead acid batteries.


Storage system Guidance

The BRE National Solar Centre guide to battery storage systems can be found here, this explains the possibilities and the difference between AC and DC coupling in more detail.