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Should I buy a device to store my solar generated electricity?

Posted by Chris Rudge on 13 September 2013 at 10:18 am

Q: I have solar PV panels on my roof. There are now some devices becoming available that enable one to store the surplus electricity to use at another time, for example after dark. They are expensive to purchase though. How do they work; are they a good investment; and are there any models you would recommend?

A: The devices you are talking about are 'self use' systems, designed to store solar power generated during the day and make it available at night (thus avoiding the need to buy power from the grid).

To understand why you might want to install a self use system, it is first useful to remind ourselves of the principles of the feed-in tariff scheme.

Feed-in tariff benefits are based on three tiers:

1) Basic FiT payments which get paid for every unit of power generated, whether you use it or not.
2) Export payments which are are currently paid on a 'deemed' basis. Under this tier, 50% of generated power is assumed to have been exported and payouts are made on that principle.
3) Using as much PV generated power within your house without affecting your FiT payments.

Until recently, homeowners have generally seen it as a no-brainer to go for tier three and try to use as much generated power as they could for themselves. For many people, this meant investing in devices such as the ImmerSun and SolariBoost which use the excess electricity to heat water in your cylinder.

However, there is another option which only the well-heeled have been able exploit until recently: storing the excess generated power to be used after the sun has gone down.

Though there have been a number of custom built devices based on existing off grid equipment and specialist contactors, the technology to achieve this goal has only become more widely available in the last couple of years.

How does it work?

For the vast majority of self use systems, the installer will need to split off your high power circuits from your consumer unit and have them operating on grid only. Ideally if the self-use unit runs your lights, central heating boiler and some low current circuits, this will make the best use of your stored power. Simply connecting the incoming power to the consumer unit will potentially rapidly drain the batteries within minutes of sundown as soon as you put the kettle on or have an electric shower. The batteries will need replacing very soon at that rate.

How much space do they take up?

There are an ever increasing array of self-use systems coming onto the market. Some are good, a few look very doubtful! They will need to  have a control box with pure sine inverter and a set of rechargable batteries. This all takes up space. The batteries are heavy and ideally need to be accessible and in a vented location if based on lead acid technology to ensure hydrogen build up cannot occur.

Most of the systems we have installed are in the customer's garage, which is an ideal location. Battery sizing is important as the larger the battery, the more you can store. In addition larger batteries generally have a longer life. Some low-cost self-use units have cheap, low-capacity batteries which will not do the job they promise. 

Which system should I choose?

One of the first units to hit the shelves in the UK was the Nedap Power Router. Made by an established Dutch electronics company, this diversion from their usual automation trade proved successful in Europe a few years back, and is growing in popularity in the UK, especially since their unit costs have come down. The identifiable green housing is a one-box solution, with solar PV inverter, battery charging and monitoring and control circuitry all in one neat housing. On top of this, they have a built in optional internet interface which allows you to be interactive with what your power is doing. Quite apart from the fact that this is a great one for gadget geeks, it was, for some time, the only system available for installers such as ourselves to offer.

To install one of these as a retrofit onto an existing PV system you will have to replace your existing inverter with the power router. Set up is quite straightforward, and it should include web monitoring, to help you get the best out of the system.

The big let down of this unit's design is that your total generation meter, on which that all-important feed-in tariff payment is based, has to be installed on the output, after the power has been stored in the batteries. However, since powr is lost in the charge, storage and power conversion process, you will potentially lose up to 20 per cent of FiT generation payments per year. Not so good if you are relying on these payments to cover an investment. Otherwise, the Power Router can still be seen as a flagship for domestic self use.

The Victron Power Hub is an alternative to the Power Router, and our current favourite. Although it does not have the comprehensive built-in internet interface that allows you to manage and view your usage, the cost is far less and it has real advantages on a retro-fit on an existing PV system.
1) You keep your existing PV inverter.
2) The total generation meter is fitted to record all the solar output ensuring you get paid for all the you've generated.

The Power Hub is simply retrofitted into existing PV systems. You just need space for the unit and a set of batteries. 

Victron only released its official Power Hub early in the summer, and we are getting lots of interest in this lower cost device that offers the same storage options as the Power Router, but without the price tag. As with any battery system, the battery set included will affect both short term and long term performance of your system. Victron systems have a recommended battery set for inverter/ Power Hub size combinations.

Harking back to their off-grid and marine roots, the Power Hub also works well as a backup system, which will switch in within 20ms (milliseconds) during a power cut. This application, which ensures your battery system only runs essential services such as lights and the boiler, is ideal as it will keep your house alight for hours when everyone else is in darkness. If your house is in a remote rural location with a weak grid connection, this unit really is the one for you as it will provide energy security with the self use option as well.

There are also a few other emerging self use devices coming onto the market. I've not actually seen any yet, but having heard from customers who tell us that their device's batteries are low capacity or, in one case, the subject of misleading national newspaper advertising, it becomes apparent that it really is crucial to do your research before committing. We were approached recently to see if we wanted to attend product training on one device, but after some probing we found the training was simply how to sell it rather than technical specifying. Already it seems that the rogue traders whom we thought had gone from renewables have come back with a new angle. 

If you are interested in getting a self-use system installed, and indeed there are a few good reasons why you should, make sure you make contact with an established local installer. Just as with any other product, going with the first cold call offer or with a company who approaches you while you're out shopping will never get you the best system, or indeed the best price. Make sure you get a few quotes before committing.

Currently, due to battery costs, the installations are not cheap and the financial payback will be long. However, we do have one customer who pays £15 a month for electricity, which gives him a cosy feeling of energy security as power prices continue to rise.

More information

The YouGen guide to solar electricity

The YouGen guide to feed in tarriffs

10 tips for choosing a good solar PV installer

Picture: Power Router


About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

If you have a question about anything in the above blog, please ask it in the comments section below.

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6 comments - read them below or add one


lisakingComment left on: 21 September 2020 at 9:08 am

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JenniferJComment left on: 12 September 2020 at 8:25 am

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richmcComment left on: 30 May 2015 at 2:07 pm

By way of an update, I've just had a Wattstor system installed, this is Victron based and has reduced my electricity import by 86%,. The system I opted for will give me 6KWh of power that is used during the evening and over night and so far to around 9.00am. This easyly covers my background useage of 500-600Wh the system also has a Solar iBoost that once the batterys are charged uses the solar to provide hot water.

These systems are becoming more economical and what was once an area where one would shrink at the cost is becoming a no brainer and will soon be a must have.

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Hines Darrel

Hines DarrelComment left on: 21 February 2014 at 12:02 pm

Only if you are not using and you want to store electricity you can buy devices. Or you can use capacitors to store electricity because capacitors are specially made to store current by storing electrons.

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Rudge Energy

Rudge EnergyComment left on: 1 October 2013 at 12:00 am

This new emerging area of power storage for the purposes of self use, which will involve regular connection and disconnection to the grid does vary between unit types.

The UK DNO (such as Western Power), regulations have slightly different standards to the rest of Europe (except Belgium, which is the same as us), with regard to backup generation systems taking over from the grid either in power cut situations or for self use. Whereas the rest of Europe require that Live and Neutral (on the main AC suppy) is disconnected completely, the UK also require the Earth to be disconnected from the grid as well. These devices by themselves do not have to comply with the G83/ G59 standards as they are not designed to export power onto the grid, as a PV inverter does.

The Nedap indeed does not completely comply with the UK regulation in this aspect, as the logical Euro standard requires all earthed equipment to be bonded together to avoid voltage differentials to be possibly built up between exposed metal surfaces, etc. However, both the Victron and Sunny Island units can easily be made to comply with the UK by an external contactor, which can be installed to disconnect an earth connection if required. SMA actually supply a dedicated kit for the UK to carry out this task, with the Victron, a generic relay can be purchased from an electrical wholesaler.

Summing up on the above, all the units which are designed and certified for the new German market which is enjoying a subsidy to get people to store their daytime generated power to avoid loading the grid, have been installed around the UK. Some will have the earth bypass relay or change the household earthing system to TT (an earth stake), to keep strictly within the DNO standards, some will not, depending on how capable the installer has done the installaion work. 

Regarding cost benefits, though the electronic heart of any of these systems have been subject to reduced costs over the last few years, battery costs have remained largely the same though we are now starting to see LIon battery technology  costs starting to fall as electric car storage gets more widespread. Cheap batteries indeed do not last very long as we have found when testing some lower cost brands. 

For many of these systems, the customer is looking for not just the self use aspect, but the added security of continued power supply during grid power failures which appear to be on the increase again. I would suggest that, like most  technologies based around renewable power, will not work out on a financial model without subsidy (such as FiT or RHI), and like the early days of PV we were installing systems before the FiT started, people installed systems for personal choice and a need rather than simply to enjoy the benefit of Government schemes. The same principle applies here.

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SunGift Energy

SunGift EnergyComment left on: 26 September 2013 at 8:49 am

This is a very interesting topic and something that will definitely become the norm as we move away from an in-efficient distributed electrical grid to localised generation and consumption of electricity.  At Sungift we have looked into the grid connected battery based self use options in the past but we have never really seen them as a viable option as of yet. Our main concern with the Victron and the Nedap Power Router units is that when we enquired with Weston Power if they had been approved for connection to the grid the answer was no.

The concern by Weston Power was because of the possibility that the inverter, that doesn’t have G83 or G59 approval, could run in tandem with the grid and what effect this would have. This is more of a concern with the Victron unit as it has multiple relays that could both operate at the same time. The other concern was that the change over relays do not disconnect the PME earth that is supplied by the grid so there is the possibility that even though the live and neutral are disconnected there still is a path to the grid through the earth and then the neutral in the event of fault conditions. This would obviously cause a dangerous situation for people working on powerlines that had been turned off. There are obviously ways around this by changing the property over to a TT earth system but then this has its own additional problems associated with it.  Also it was noted that during a period of disconnection from the grid ( when running off the Victron or Needap unit and on-site batteries )that the PV inverter, although producing electricity at 50hz as it would be tied to the 50hz from the Victron or Needap unit, could drift away from being synchronised with the grid due to inaccuracies in the internal clock and also fluctuations in the national grid frequency. When the reconnection is made to the grid the PV inverter does not get its usual 3 minutes to access the grid conditions and synchronise with it as it is mid production. In these instances it is unknown how it would react to suddenly being connected to the grid out of phase, but more than likely it would cause an under or over voltage that would destroy the PV inverter.

As well as this there is the cost aspect which seemed prohibitive. Traditional battery technology is inherently expensive due to the precious metals needed to produce them. Cheap batteries just don’t perform or last very long. Last time we looked at this it just didn’t work on paper cost wise. It would be great to see a example of cost and payback if you have one you could show us.

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