Homes use electricity after dark – but PV arrays can’t generate power when there’s no sunlight. Because of this, a typical installation will use solar energy during the daytime and switch to taking power from the grid in the evenings and nights. However, there is a solution to maximise the power that could be generated during the day: store it in batteries and use it later.

Using batteries to maximise the energy generated during the daytime

A typical home won’t use as much power during the day as it does in the evening – during the daytime, people are mostly out at work and there’s generally less need for light and heat – so there’s a good chance the PV array has spare capacity. If this is fed into a battery and stored, then the home can call on this reserve before it needs to switch to the main grid.

Batteries can be added to existing PV installations or introduced at the design stage. Technically, it makes more efficient use of the system to feed the DC generated by the panels as directly into the battery cells as possible because adding batteries later means they are usually charged from the inverted AC and that introduced some losses but the principle remains: make the most of the power generation capacity and use it when it suits us.

While batteries for solar storage can be similar to the typical ‘wet cell array’ found in a car, there’s an increasing trend to use the high tech cells more often found in mobile phones – just scaled up significantly. Either kind needs to be broadly maintenance free, able to discharge enormous currents and hold a charge for a reasonable time. There are some downsides to battery technologies: because they rely on a chemical process, they can be affected by temperature and some designs must be kept above freezing in order to meet their stated capacities. Equally, they shouldn’t get too warm – but that’s generally not as much of an issue as the cold limits in places like the UK. As with all rechargeable cells, they gradually wear out and the typical lifespan of a solar battery is between 5 and 10 years before its storage capacity is depleted sufficiently to warrant replacement.

Combining PV panels, batteries and the grid

In an ideal situation, a home would generate enough energy during the day to meet three needs:

• Power the home needs during daylight;
• Fill the batteries;
• Send excess capacity back to the grid

This is a balancing act, of course: if a very large array were coupled to a substantial bank of batteries, we could generate enough power to keep the home ‘off grid’ permanently and still have some energy left over to sell back to the national system. However, even with an exceptionally energy-efficient home it’s unlikely to be a realistic proposition: a PV array would have to be large enough to cater for the depths of winter when its efficiency and generating capacity is at its lowest and at the very time the home places the largest demand on energy.

In the majority of cases, the balancing act is to use home-generated energy for as much of the time as possible, sell any surplus energy if possible and only buy what’s required to ‘top up’ as and when it’s required.

• With a battery-assisted system, you will want to be sure your batteries aren’t ‘feeding’ the national grid and that you’re not using ‘their’ power to charge the cells when you could be doing it for free the next day. This needs an intelligent management system and it’s beyond the scope of this series to examine the technology of that in too much detail. Suffice it to say these do exist and are field-proven.

Is it worth selling power back to the grid?

Sadly, the price you buy power is rather more than the price you get when you sell it. At today’s prices, these are roughly £0.34 per KWh to buy and £0.05 per KWh when you sell it. Of course, the biggest saving is the energy you no longer need to buy: each KWh you generate and use internally is another £0.34 that you’re not spending with the electricity company and that really adds up. It’s not uncommon to find people saving £80 a month on their electricity bill with even a modest PV array.