Living with Bert-the-Battery for 5 months

We’ve now had our Wattstor 6 kW battery (‘Bert’) for 5 months. The first chart shows our daily kWh averages and how they are changing as summer tails off into autumn.  Our daily average demand (red) has remained relatively consistent at 7.5-8 kWh a day other than June when we had 2 weeks away.


The PV generation (yellow) is dropping sharply with the shorter days and lower sun in the sky which (in our case) disappears behind two trees in the adjacent field in the afternoon. The average surplus kWh we have exported to the grid each day (green) is falling and the average we have had to import from the grid (black) is now rising sharply. September was the first month we have taken more from the grid than we exported to it.

For many happy days in high summer, the PV panels and Bert met all our demand and we imported nothing from the grid at all. But in September, the daily import needed from the grid rose sharply once we were cooking supper after dark and Bert could no longer reach 100% capacity from the day’s sun.

The second chart shows the proportion of our daily electricity demand which came from each source and how that is changing month to month.


The grid provided only 4% of our electricity in June and July but in September was up to 33%. The proportion of energy we take from the solar panels directly versus from the solar via the battery is also changing. In the last two months we have taken a higher proportion from the battery than directly from the sun. Again, we think this is because our consumption peaks in early evening (like most of the population!) when we are cooking (mainly on electric) after the sun has set.

Thierry Twizy above Usk Reservoir

Thierry Twizy above Usk Reservoir

On the days we need to charge our Renault Twizy, the solar PV (4kW) is rarely able to consistently provide all of the power required (2kW) for a few hours so the rest is taken either from Bert-the-battery (if he’s full enough) or the grid.

Fortunately, our Twizy only has a small battery 6kWh (same capacity as Bert) and operates at very low energy (140 Wh/mile). In this country, you’d need a huge South facing property with maybe 8-12 kW of PV panels to be able to charge a full-sized electric vehicle every day and still have surplus Wh to fill a domestic battery. And, as discussed previously, we now think it would be horribly complex to double one’s electric vehicle battery as domestic storage. The two usage patterns are too often in direct conflict.

About Alison Kidd

Research Psychologist
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