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

January 5th was a good day to demonstrate the battery’s contribution to the home on a day on which 9.75 kWh was generated. The battery at this point in the year is configured to charge on cheap overnight electricity as well as store excess PV electricity (when there is any). The day also included evidence of water heating and car charging after I returned home at lunchtime.

Over all only 2 kWh of power was purchased during the day at full price, while 12 kWh of cheap night time electricity ran the dishwasher and washing machine, did some car charging (I charge for an hour which leaves headroom for later solar charging), and contributed 5 kWh to charging the fixed battery.

During the relatively sunny December day, as PV output rises, the battery starts to recharge and then, once the battery is charging at its maximum rate, remaining PV output is used to heat water (although described as ‘Grid Power Out’ in the chart above).  At around 12:30 I arrived home and put the car on charge (the blue spikes) and made lunch (the brown spikes).  The blue spikes of car charging occur as the chargers turn off intermittently to allow available power to be assessed.  As PV output falls EV charging starts to require limited support from the battery (the rising brown line through the blue spikes) until eventually all EV charging stops and the system returns to a combination of battery charging and water heating.

As the sun sets the brown line rises again as the battery takes on the load of the house. Green spikes indicate boiling the kettle before 18:00, cooking dinner from shortly after 19:00, and a further kettle boil at around 23:00.  Apart from these import events totalling less than 2 kWh, the house continues running on stored energy from the battery through into the next day when the cycle restarts.

At some point I assume that it will be necessary to curtail the overnight charging of the battery so that it doesn’t miss out on day time charging from PV as a result of being full. That may be responsible for some of the blue cycling with the car charging associated with the rising brown line – if the battery is already full then the car charger will be enabled at lower levels of power generation potentially causing the battery to partially support the car charging.

As you may recall my storage battery has a usable storage capacity of 4 kWh which, with the quoted round trip efficiency of 80%, would mean it takes in 5 kWh of energy to make 4 kWh available for output later.

We’ve been away for a couple of days, leaving the house drawing a low amount of power in our absence, however yesterday the battery reports having discharged 5.9 kWh despite its 4kWh capacity.

I’ve already had some days when throughput was greater than capacity, but that’s generally been when the battery charged fully overnight and then partially recharged from solar PV during the day. However this was not the case yesterday where, although there’s a dip in demand associated with limited daytime generation on a dull winter’s day, there’s no evidence of battery charging.

There is a characteristic of battery behaviour that may describe some of this which is described by Perkert’s law. Peukert’s law describes a relationship where battery capacity reduces as current increases according to a power relationship. Since my battery is discharging at about 350 VA that much less than its 1200 VA maximum capability so it may appear to have more capacity.

However it seems to have both greater input and output energy than its rated usable capacity suggests (input energy remains higher than output energy) so it would seem that there’s something else going on here; whether that’s an early life effect, a genuinely larger capacity than nominal, or some other effect is unknown.

The battery has a nominal capacity of 8.8 kWh to provide is usable capacity of 4 kWh, so it’s possible that some characteristic of the control software is making more of the nominal capacity available than would normally be the case.