May 2016 to Feb 2017 – Solar Storage Data

Spring is in sight and we have just done the analysis of February’s performance with our solar PV (4kW) plus Wattstor battery (6kWh) system.  The longer days and the sun being higher in the sky have started to make a significant difference despite much of February being gloomy and wet. Some of our evening cooking and lighting (once the sun has set) is now supported by Bert-the-battery rather than from the grid.

Chart 1 shows the daily Wh averages for each month since May 2016. Our daily consumption is staying relatively steady at just over 8kWh/day. (It does vary somewhat day to day depending on the significant effect of charging our Twizy which also has a 6kWh battery to fill. We try to charge the Twizy when the sun is contributing the most to avoid either taking from the grid or depleting Bert-the-battery in order to fill another battery. In the winter, that’s hard to manage.)


Chart 2 shows the percentage of our daily electricity consumption which comes from each source – the solar panels directly, the solar via the battery or directly from the grid. In February, 31% of our consumption was covered by the solar and battery combined – the best performance since October. The battery alone contributed 12.5% of our consumption. Because the lead acid batteries spend most of their time on absorption as opposed to bulk charging (to protect themselves), we aren’t capturing as much of the solar into the battery (for later use) as we would like. As Chart 1 shows, the actual solar generation was up to 50% of the overall consumption but the amount of the solar generated power which the battery could absorb in real time is restricted so roughly 25% of it was passed to the grid for others to use. We try to view this in a generous spirit!


We now look forward to the long sunny days of March (typical weather here in Wales of course..).



About Alison Kidd

Research Psychologist
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6 Responses to May 2016 to Feb 2017 – Solar Storage Data

  1. John says:

    I see you had the same problem we had with the Wattstor / Victron system. From the charts it would seem that generation was severely curtailed.

    • Alison Kidd says:

      Generation has never been curtailed (other than by winter sun levels!). The problem has been the way Victron protects the lead acid batteries means that we send the majority of the solar we are generating to the grid because of the severely curtailed amount that the battery will take once it switches to absorption charging which tends to be at around 85% capacity. The solar day rarely lasts long enough to reach 95%-100% capacity at that low rate.

  2. John says:

    Are you sure? One would expect solar output to trend upwards in May, June and July, you seem to be going the other way. This suggests that output from PVs is not happening, why is this happening?

    • Alison Kidd says:

      Our solar panels were producing an average of over 13kWh/day throughout May June and July last year. May just happened to be a slightly sunnier month than the other two last year. From previous years’ experience, we would expect those 3 months to be roughly the same. My comment about Spring was reflecting the fact that our solar output in February was almost twice that of January – probably as a result of increase in day length as well as the sun being higher in the sky.

      • John says:

        Your average generation figure for May/June seems odd. Our experience with a 4kw pv system indicates output ranging from 12 to 30 kWhr per day, your average seems very low. Peak output during this period is heading for 4000 watts, are you sure your battery can take this charge rate? If not, and your demand is as low as you suggest then the system may be curtailing generation, hence the low average daily output figure.

  3. Alison Kidd says:

    Your panels may be better oriented than ours, which are roughly WSW with hills (and until recently, serious trees!) behind. Our generation meter shows that we have produced a little under 3000kWh per annum over the 6 years we have had the panels. You are right that our lead acid batteries will not charge at anything like the rated output of the panels. They seem to be limited to about 50 amps, or roughly 1200 watts at 24v, so I have no reason to think that our solar output is being significantly curtailed – just that we export more than we need to the grid!

    We would be better served by a battery technology that could handle a charge rate closer to the panels’ rated output, could tolerate deeper discharge levels, and would take higher charge when closer to fully-charged. On the other hand, we do have a thirsty little electric car – a Twizy – that can soak up our spare 2kW on a sunny mid-afternoon!

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