Bye bye bills

One year after the PV system and battery installation it’s a good time to review the setup and performance. You can see details of the setup in the original blog post.

Overall, the experience has been very positive. SolarEdge have a nifty API interface so it’s quite easy to analyse data. The system produced 6MWh of electricity over the 12 months, as per the graphs below. Production in this case includes self-consumption on site and export to grid (export was set up a few months after the original installation so we missed out a bit initially).

Self-sufficiency in 2022 was 65%, rising to 81% this year, giving an overall annual figure of 75%. In simple terms, this means that with our yearly demand of 4050kWh we only needed to import 25% from the grid. As expected, most of the production happened in spring and summer months, dropping off during winter. April, May and June have been excellent, whilst the wet & cloudy July was a bit of a disappointment. Pairing the battery storage with PV system definitely allowed us to maximise the use of electricity generated on site and minimise imports.

Our energy provider is Octopus. Initially we were on a fixed tariff, but that ended before our PV installation so we stayed on a default standard tariff to start with. Cost of electricty had gone up dramatically back in October, from about 17p we used to pay on a fixed tariff to 34p/kWh. Daily standing charge also increased slightly from 45p to 49p. We switched the tariff again in spring to Octopus Flux, which is tailored for solar and battery owners. There are three sub-tariffs for both import and export (see below), with a daily standing charge of 52p. Our worst bill was in December when we paid £115, the best bill was in June when we were credited £132. Over the year we have pretty much achieved zero bills, which is fantastic.

Some clipping to production is happening on really sunny days when we are self-sufficient and battery is fully charged, as the export capacity is limited to 3.68kW. This ‘lost’ power generation could be mopped up in future if the local network got upgraded (to allow bigger export capacity), or by charging up an EV (future next car?).

Unlike Tesla Powerwall, the current set up with SolarEdge does not work as a power backup. This is a bit of a shame as power cuts are a fairly regular occurence in our rural location, particularly in winter. However, SolarEdge are launching a new gateway hardware that will allow this, so we’ll probably look into it in future.

The last 12 months has really made us think of the energy we are using in the house, and time of use too. We’ve slightly changed our habits, for example running washing machine or dishwasher only when there’s enough power, either generated or in the battery. The battery also generally allows us not to draw power from the grid during peak hours. Speaking of which, it was fun to take part in the Octopus ‘Saving Sessions’ over the winter months, which rewarded customers for negawatts, i.e. energy not used. It was a brilliant initiative and proved that demand reduction can play a part in reducing those pesky peaks in the UK grid. Below are graphs of typical winter and summer generation and usage patterns, one from last December and one from May. We have a base load of around 100W, the main additional daily loads are hot water generation (HW cylinder with integral heat pump) and cooking. It is clear that the hot water heat pump is working more often and for longer periods of time in winter – this is due to lower ambient air temperatures and the resulting worse coefficient of performance (COP). Weekly electricity demand for the hot water heat pump varies between 10kWh in summer and 55kWh in winter.

Having lived with the system for a year, we couldn’t be happier with it. It has pretty much eliminated our electricity bills and radically lowered our carbon footprint. SolarEdge app estimates an equivalent saving of 1.1 tonnes of CO2 over the last 12 months. That’s got to be a good thing.