Are Solar Batteries Worth It?

One of the things that's been on my 'radar' lately has been battery storage for our solar panels.

Think about it - For much of the year in the Britain, a decent-sized solar array on your roof probably produces more energy in daylight hours than you can use. So the 'spare' electric goes back to the grid. But what if you could store that spare power until you could use it? That could save you some money and reduce your carbon footprint.

The way to do this is to install a solar battery storage system.

The batteries can be Lead-Acid or Lithium-Ion but the latter are supposed to be the better choice (in terms of weight and battery life). However, they are expensive - Despite the cost of Lithium-Ion batteries coming down dramatically (> 80% since 2010) - with prices in the UK currently starting at £3000 for a 6.5 kWh battery.

So, is it worth it at today's prices? Well, I did some rough calculations for our own panels.....

Example 1:
If I take a Solax 3.3 battery, costing £4000 for 3.5kWh with a 10 year warranty. I'd need to get £400 a year's worth of stored electric out of it just to break even (Assuming the battery only lasts as long as it's warranty). If I assume it stores the maximum 3.5kWh every day, it will give me 1277 kWh a year. Based on the price per kWh from my electric company of £0.1617, that gives me a saving of £204.40 per year. Just over half what I'd need to break even. But it would cut our carbon footprint by 0.69 tonnes.

However, assuming the battery stores 3.5kWh every day is a bit optimistic. Even in the height of Summer you can get days were the panels generate less than 3.5, that gets worse in Winter. So, the savings in electric and carbon would be even less (maybe half?).

Example 2:
Tesla Powerwall, costing £5500 for 13.5 kWh with 10 year warranty. Here, I'd need a return of £550 per year to break even.  However, the larger capacity of the battery means I could get an increased return. Maybe as much as 2/3rds of the energy we generate of around 2000 kWh a year. This would give us a return of £328.40 a year and a saving of 1.08 tonnes a year of carbon.

An improvement, but still only around 60% of what I'd need to break even.

Bottom-line: Solar battery prices would need to halve before they become cost effective in the UK. I'm guessing that's not too far off now.