The First Few Weeks: Performance in February.

At the time of writing, the system has been installed for a bit over a month. I got the installers back to correct some things that did not seem quite right to me - some kind of communication breakdown between the company and the guy who did the original installation.

I thought that now might be a good time to look at how the system had performed and how this compared to simulations.

What to compare against?

Before installing SolarPV, our supplier was British Gas and we were on their Standard Variable Tariff. This provided electricity at a flat (i.e. 24hrs per day) rate. At the time of installation, the price was 33p per kWh. I have chosen to make all comparisons as if we had continued on this tariff.

Another possiility would be to compare 2022-23 consumption and cost with 2023-24. I have not done this as, not only does the UK weather change dramatically from year to year, but also the tariffs have changed and the way we use our home (number of occupants, etc.) has changed.

Savings from solar generation

This is relatively easy to calculate. It is reasonable to value the energy from the panels (that we consume) at 33p per kWh. The inverter tells me that from February 3nd to March 2nd we have consumed, either at the time of generation or later via the battery, a total of 301.6kWh. This would be a total of £99.56 at 33p per kWh.

Actually a very small part of the power generated was exported (at a rate of 15p per kWh) but the amount was only about 9 or 10kWh so it does not affect the calculation very much.

Another correction that could be made to this calculation, if more accuracy were required, would be to correct for battery inefficiencies. A proportion of the energy consumed passes through the batteries and some power is lost (in the form of heat). For my batteries the charge/discharge efficiency is about 87% which, for greatest accuracy, should be factored into the calculation for any power that goes viia the battery.

Savings from time-shifting consumption

The other savings that we have made is from the change of tariff. Some of this is directly attributable to the system because it is done using the battery. Some of it is done simply by delaying the washing machine, dishwasher, and plug-in hybrid charging using timers. We also tend to shift meal preparation and cooking. These components of the savings could, reasonably, be said not to be attributable to the system. However, since we made the tariff change as a result of installing the systems and since we ARE actually making this savings, I am counting it here.

This saving would be quite difficult to calculate. Fortunately, there is an app called "Octopus Compare" (available for Apple and Android) which takes your actual smart meter reading and applies different Octopus Energy tariffs to them to see what the cost would have been on different tariffs. Fortunately, I was able to find one of their fixed tariffs which exactly matches the British Gas Standard Variable Tariff that we were on at the time of installation.

Looking at a typical day using Octopus Compare, you can see how we have been able to move our consumption away from the peak rate and towards the cheap rate. The blue line is the previous fixed rate (what we were paying with British Gas before we switched tariffs and started time shifting).

Using Octopus Compare, you can see that the savings for a one month period, which was £79.42.

Total Savings

The savings is going to change dramatically as the weather changes, partly because our heating use changes and partly because the panels produce more or less energy depending on the hours of sunlight and cloud cover. For these reasons it makes very little sense to extrapolate the savings from what we have experienced so far.

Predictions

Hovever there are some resources that can give us some kind of indication of what to expect. The Global Soar Atlas (GSA) Website allows you to put in the details of your installation to get an estimate of the amount generated at the actual geographical location taking into account the weather patterns. The figure it gives for February was 295.3kWh, which is pretty close to the figure we actually obtained between Feb 3rd and March 2nd which was 301.7kWh. The actual March figures look a bit lower than predicted, but we still have over a week to go. The annual figures are shown below.

If I compare the GSA predictions with our actual smart meter data for last year it looks like this:

This graph is a little misleading because it is not always possible to consume ALL of the power generated. A small proportion will be exported but, in my case, this has been insignificant so far. This may change in the summer. A more important thing to consider is that any power that is consumed after being stored in the batteries is subject to that charge/discharge inefficiency I have mentioned before. So, about 13% of any such power is lost.

Given that I am calculating my "savings" against a tariff that I am no longer on, I don't think these inaccuracies are particularly important (to me, at least). However, I am pointing out that one COULD calculate the savings more accurately if desired.

It is difficult to say what the savings from time-shifting will be going forward, but they are likely to continue to be substantial.

One thing that the above graph does show quite clearly is that there may be times, in the summer, when I generate more power in total than the amount that I use. It will be interesting to see whether my 15p/kWh export rate makes a significant dent in the cost of the electricity that I do have to import at night and in my standing charge.

Just a reminder: When you are looking at these figures, remember that our household is not very typical. In particular:

  • Our roof faces almost exactly due south. Many do not
  • The ideal tilt of the roof changes throughout the year. You probably don't have a choice in the angle of your panels to the horizontal - but whatever it is, it will be closer to the ideal at a particular time of the year
  • Our electricity consumption is a LOT more than the typical household. This is mainly because of our heat pump, but we also have a plug-in hybrid car which we dump about 10kWh into on most days.
  • Installing SolarPV inevitably goes hand in hand with switching to a new tariff and/or provider. The savings from shifting your consumption to take advantage of a new tariff can be quite significant.
  • We have chosen to set up our system so that we don't have to continuously tweak the settings. If you are the kind of user who would want to thoroughly optimise your usage with either tweaking or extensive automation you could quite possibly save significantly more that we do.

The value of a kilowatt-hour

Part of the quoting process when you buy a solarPV system usually involves some kind of calculation of how long it will take for the system to "pay for itself". This is a perfectly reasonable thing to try to calculate if you are trying to decide whether to "invest" on a system. The problem with actually calculating such a number is figuring out how much value to place on the power generated by the system. Often, the calculation is done using the current rate per kWh on the Standard Variable Tariff, say 33p. That is one value you can use - but in reality the kilowatt-hours that your system generates have different values. Some of them replace Standard Rate kWh and are "worth" 33p. But others are exported and are really only worth much less. What about the solar energy that you use during a peak rate period? Are these worth more? Or are they just worth 33p? I would not be on a tariff with a peak rate had I not installed solarPV, so I'm inclined to think that they are only "worth" the standard rate. My monthly bill from Octopus gives me an "average" rate per kWh (about 25p on my last bill) for my actual consumption. Perhaps this value comes into the calculation somewhere.

Update: We are now on Octopus Agile which, since the first week of March 2023, has been even less expensive.

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