I’ve now completed 6 calendar months with the battery storage system. Those six months cover January – June so might be considered representative of the year as a whole. I have real-time monitoring of solar panel output and house consumption / export so I have clear visibility that the battery is working and storing energy, and then discharging energy through the evening as I see the house with near zero electricity consumption sometimes through into the early hours of the morning (less if we’ve cooked our evening meal on electricity). So, environmentally it’s doing its stuff, but what is it saving financially?
There are at least three ways to assess that, so let’s see how they look.
Firstly, from the capacity of the battery (4 kWh), and assuming that it’s filled daily one can calculate a benefit. That benefit would tend to overestimate benefit in winter when there may not be enough energy to fill the battery, but equally could understate saving some days when the battery goes through repeated periods of charging and discharging during the day. One could imagine the heating cycle of the dishwasher, for example, perhaps causing some discharge of the battery during the day if the washing machine load isn’t met from the solar panels, but then then re-charging before the evening and thus its daily throughput being higher than it’s capacity as some of that capacity is used more than once per day.
So, if the battery stores 4 kWh and there are 365 days in a year where each kWh not bought is worth 11.5 p/kWh then the saving might be 4 kWh/day x 365 days/year x 11.5 p/kWh = £167.90.
Secondly, let’s look at electricity savings. If I compare the first 6 months of 2017 (with a battery) to the first 6 months of 2016 (without a battery) then electricity purchase has reduced by 824 kWh (38%). Thus the saving could be 824 kWh/six-months x 2 six-months/year x 11.5 p/kWh = £189.52.
Of course, as my chart shows, there are other changes that potentially impact electricity use between those 2 time periods, so that might be an overestimate.
The third and final way that I’ve analysed this is to look at the data about how the output of the solar panels has been used. In 2016 I used 44% of the output of the panels to replace bought electricity, and a further 28% of output to replace gas consumption. In the first half of 2017 however I used 63% of the output of the panel to replace bought electricity, but only 24% of output to replace gas consumption. The reduction in use for water heating reflects the prioritisation of the battery over water heating as a kWh of electricity purchase avoided is much more valuable than a kWh of gas purchase avoided. Against 2016’s full year generation of 4,192 kWh that gives a saving of only £87.84 which is much the lowest of the 3 figures. However this only accounts for more efficient use of the solar panel output, and not winter savings from shifting energy purchase from day to night time when it’s cheaper.
Whether any of these figures represents a saving over the life of the storage system entirely depends on the lifetime of the system, the life of the batteries inside it, and the replacement costs of those batteries