inflated savings

We had our solar PV system installed in January 2014, and we’ve seen a drop in our external electricity consumption since then.  But how big a drop?  Our electricity company provides a handy annual summary.  Here’s the relevant figures from the first year:

So we almost halved how much electricity we bought in last year (with PV) compared to the previous one (without PV).  Excellent!

But note the start date for the comparison: 9th October.  That means we had the solar PV for only 9 months of the period.  So it should be even smaller this year!  Here’s the relevant figures:

And yes, there’s nearly a 20% drop this time.  Great.

But hang on a minute!  The first chart says we used 4323 kWh last year, whereas the second chart says we used 5611 kWh last year!  That’s a difference of 30%, over the same time period. Huh?

I wondered if it was because the figure is an estimate, and maybe they underestimated our consumption last year (and by a lot), and now have better figures.  So I checked the electricity bills themselves.  But the bill dated 26 Sept 2014 is based an actual meter reading, not an estimate.  So they only had to estimate about two weeks usage.  (The bill from the same period a year earlier was an estimate, however.)

Because we have to send in quarterly PV meter readings to claim our generation tariff, we now make a note of all the different meter readings.  So I know that our actual consumption 10 Oct 14 – 9 Oct 15 was 4576 kWh, very close to their estimate (this year, at least).

We had a new electricity meter installed in March 2014, to cope with the PV system’s generation, but that was in place when last year’s figures were estimated.  The only thing I can think is the estimating system has got very confused somehow, due to our sudden change in usage pattern.  I will have to check next year to see if the figures are equally bizarre, or have settled down.

This is not the first time I have had perplexing information on electricity bills, though (for a different location).