Skip to main content
Observer Ethical awards Winners 2011

YouGen Blog

Solar security: grid connection and batteries give the best of both worlds

Posted by Nick Roach on 20 January 2012 at 9:49 am

We’re pioneer microgenerators who installed PV in 2006 before the feed-in tariff or the spurious ‘when do I get my money back’ argument appeared on the scene.

We live in a rural area with no mains gas, with overhead power cables flapping around in ever more frequent storms; we are solely reliant on electricity for power – and to power oil-fired central heating.

To ‘secure’ hot water, we installed solar thermal in 2009 with a solar-powered pump, i.e., independent from mains power.

As all solar PV owners know, they are as useful as a chocolate fireguard when there’s a power cut because the system shuts down (if it didn’t exported electricity could cause serious injury to people working down the line). So when a YouGen newsletter from Cathy Debenham arrived heralding a battery back-up system, this appeared to offer a solution.

After several false starts, we managed to locate an installation company, which was as committed to the concept as we were; there are a few outstanding issues, but the system is up and running.

The system Cathy highlighted came from SMA, whose units are very widely used in homes throughout Germany & Scandinavia.

The system provides battery power to the house in times of power failure, or may be manually switched on to use battery power when required.

It  comprises two separate, integrated boxes of tricks: unit one isolates the  home from the grid whenever there is a power cut or supply is momentarily disrupted, and engages battery power. The second unit regulates power flow into the home, and any solar PV power that is being generated at  the time continues to be the first choice, but is supplemented by battery power instead of grid power. The changeover takes milliseconds, ensuring  computers, TVs etc don’t go into death mode.

Unit one senses when grid power is restored, the process is reversed, restoring grid power once batteries are isolated safely.

In daylight, the excess electricity generated by the solar PV is used to recharge the batteries, rather than being exported to the grid; at night, mains electricity tops up the batteries ready for the next grid failure.

For our home, the back-up system provides battery power for around four days, but we shepherd our power use, which others may not.

As with all installations, there are compromises to be recognised and accepted, eg: 

1. both units remain on standby (around 40w between them) in readiness for grid failure and battery power flow,

2. space is required for unit installation & battery storage. 

The greatest compromise is cost – the final bills haven’t arrived yet, but I am reckoning on around £5,500 + 5% VAT. To our way of thinking, this represents savings exchanged for 20+ years of energy security in a rural area with a dubious national grid, which fails around four times a year.

Failures which are increasing due to the lack of investment in rural areas, which isn’t going to get better any time soon, and the increasingly worrying phenomenon of metal theft – several sub-stations in our region (SE) have been plundered for scrap metal in recent months; they are easy targets.

In an increasingly uncertain world, we have taken the initiative to secure the power supply for our home and home-based business. We’re happy to provide more detailed information to anyone interested in this type of system, and equally happy for people to view the installation.

Our suppliers:  PV battery back-up: RS Eco Company Ltd. Solar thermal (independent of mains): Solartwin

…and for the record, we have no financial interest in either of the two companies mentioned above; we merely think they are a very useful additions to solar PV.

Picture: SMA

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

Like this blog? Keep up to date with our free monthly newsletter


8 comments - read them below or add one


HiltingburyComment left on: 2 February 2012 at 2:02 pm

Dear Chris, Thanks for your comment. Because I was told that this back-up solution was not MCS approved I decided to go with an Aurora 2 string inverter instead of an SMA, so I probably have no way forward from here?

The logic used to justify the non MCS approval was that this is supposed to be about a FEED IN Tariff. I think that in the case you describe, your FIT meter would be clocking up generated units even when your system is isolated from the grid (albeit because of a power cut). Those units would not be eligible  for the FIT. Given that all this is thought up by committee that sounded perfectly plausible to me.

report abuse

Rudge Energy

Rudge EnergyComment left on: 29 January 2012 at 10:40 pm


Just spotted your comment. This system IS MCS approved, as the generation meter that you claim your FiT is installed on the output of the Solar PV inverter. It will ONLY clock up units of power that are generated by Solar.

The total generation meter MUST NOT be installed on the output of the backup inverter, as the batteries could be charged by a number of sources, including grid mains or a generator.


report abuse

Rudge Energy

Rudge EnergyComment left on: 29 January 2012 at 10:34 pm

Hi Nick

We have installed a number of the SMA Backup or Island units and besides one that decided to ignite itself, has proved to be a very successful system.

On a backup system, SMA provide a separate contactor box that does the safety G83 disconnection, which enable the qualified installer to reprogram the existing SMA grid tie inverter to carry on operating during a power cut. 

This facility is jolly handy over other less expensive (but functional), Victron or Outback solutions, as it will enable the PV system to continue to both supplement the local household grid and keep the batteries topped up in daylight.

For most premises, this kind of setup will be over the top, as urban grid is not prone to power cuts quite the same as rural areas, and one has to remember that the battery set will need replacing every 10 years or so. An added £1000 or more bill every decade needs to be justified! 

If anyone wants one of these systems installed, ask if the installer has the capability to modify the programming of an inverter. I needed to go to SMA in Germany some years back for a training course. 

We cover the South West UK for this kind of installation if required.

report abuse


MicroNickComment left on: 24 January 2012 at 9:59 am


reply for AnnePan001:

Designed for retrofitting (my PV installed 2006) or at new installation, and does not effect certification - have a word with my installer; they know the gory details better than I do..

I'd offer to show people the system and explain, but restricted to location of enquiree.. but happy to supply supplementary info if you need it.




report abuse


annepan001Comment left on: 23 January 2012 at 5:43 pm

This is the sort of solution I'd like to have installed too, if it's safe and effective, and can be retrofitted to my 3.6 kWh solar pv! Looking forward to hearing whether it can be included in an MCS system. 

I, too, remember the 3-day week, and would like some peace of mind, at least wrt energy supply with the forthcoming economic tsunami!


report abuse


MicroNickComment left on: 23 January 2012 at 3:33 pm

Hiltinbury - things have moved on - have a word with the supplier in my blog who can fill you in on the latest. His link is featured.

I too am old enough to remember the 3-day week, and came to the same conclusion.

report abuse

Cathy Debenham

Cathy DebenhamComment left on: 23 January 2012 at 3:20 pm

Interesting - I'll look into it and report back

report abuse


HiltingburyComment left on: 23 January 2012 at 3:09 pm

I considered this product as part of my studies into our solar pv system which was installed on November. I was told, quite categorically that it was not MCS approved and that the installer would not be able to give me an MCS certificate for my system if this SMA back-up product was installed. Being old enough to remember the 3-day week, and foreseeing an even more turbulent future I think it is a shame that such a logical approach cannot be followed for anyone claiming the infeed tariff. Of course I could have been misinformed?

report abuse

Leave a comment

You must log in to make a comment. If you haven't already registered, please sign up as a company or an individual, then come back and have your say.