What Do You Mean Our Batteries Don't Work in a Power Cut?

Last summer I installed a 14kWh battery system alongside my existing 7kWp home solar PV system. All went to plan until one day we had a letter from SSE saying they would be turning off our power one Saturday for 'routine maintenance', or something like that.

'We'll be OK,' said my husband with a broad smile of satisfaction, 'we've got solar with battery backup.'

'Hmmm...,' I said sheepishly, '... actually we won't be OK because our batteries don't work in a power cut.'

'What do you mean, they don't work in a power cut? What's the point of having batteries if they don't work in a power cut?'

Well indeed...

The immediate assumption that most of us make when considering battery storage is that if we are going to install a system, it must – and it will - provide backup in the event of a power cut.

In reality, the baseline functionality of a residential battery storage system is to utilise excess solar on site, and possibly also to allow for avoidance of peak time grid costs (i.e. buy electricity cheap at night under an Economy 7 tariff, store it and use it in the day when costs are high). Providing backup is not included within the baseline functionality of many systems.

And in fact there are very good reasons not to include backup, specifically:

• the cost of installation will increase by about £750-£1,000, due primarily to the need to provide the necessary 'islanding' to stop electricity from being exported to the grid in a power cut and injuring those working on the grid;
• unless you wish to cater only for 'scheduled power cuts' of which prior warning is made, backup capability will permanently ‘block’ some battery capacity which would otherwise be available for excess solar storage and peak charge avoidance.

When I analysed the payback of our system with and without backup, I decided that back-up wasn't justified. Momentary loss of power happens a lot in our area, and we are forever resetting the oven clock, but long-lasting power cuts are few and far between.

And the truth is we survived that Saturday power cut without spending £750, without charging our iPhones, and presumably without boiling the kettle, although I suspect we did get out the camping stove... To set against this we were denied the smug satisfaction of being able to keep the lights on whilst all about us were plunged into darkness.

If you do want solar with battery backup you need to make sure it's well specified...

Everyone's situation is different, and whilst my choice - so far - has been not to get backup, about 50% of our customers are getting backup with their storage system. If you do want to include backup with your battery system, there are a number of issues you should consider:

• which loads are you going to backup?
• how long are you going to back them up for?
• do you want your solar system to continue to function in a power cut?
• does the backup need to be seamless, i.e. are you requiring an uninterruptible power supply (UPS - saves resetting the oven clock)?

Solar panel backup is a complex subject and in our view merits a guide in its own right. We have therefore written a 10 page guide on different systems, set-ups and things to be aware of.

If you would like help in designing your system, please get in touch. We can provide assistance with modelling and system design, and provide a free quotation. Please call us on 0118 951 4490.