As discussed previously in 'Adding a Solar Battery' solar generation peaks during the daytime and drops off towards the evening. Sadly for many people like us solar generation doesn't match our consumption profile. The home battery allows us to store the solar power generated during the day and use it later in the evening when solar generation doesn't cover our consumption..
Our Battery Performance
Since installation our home battery has performed well. We've found we generally use around 25% to 30% of the home battery capacity during the summer months. This is because solar generation still covers most of our consumption during the day and into the evening. We generally use 40% to 50% of the home battery capacity during the winter months.
On a good solar day the home battery can be charged just by solar generation. During the early summer months the home battery would likely be full before 8am as generation can already be around 2kW by this time. In the summer months the home battery stays full majority of the day and slowly drops in the evening and night.
Early June 2023 - solar charging
Early autumn can still provide good solar days but the days start to become shorter. It's noticeable that the battery would be fully charged later in the morning (circa 9am). The solar generation also drops off earlier so there is less solar generation available to power our house. The home battery is key here as it can provide power to cover our evening use as its fully charged.
Early September 2023 - solar charging
The above examples show good solar days where solar generation was 15kWh to 25kWh, which is much more than the normal consumption by our home. As the seasons cause the days to close in and the weather also turns this causes solar generation to drop. Solar generation on some winter days can be between 2kWh and 5kWh at best. If we solely relied on solar generation to recharge our home battery on some winter days we may run out of battery power as our house consumption could be higher than the solar generation stored each day.
Charging the Home Battery during the low cost period
One benefit of smart energy tariffs such as Octopus Flux, Octopus Go and Intelligent Octopus Go, as discussed previously in 'Smart tariffs', are the low cost energy periods where the cost of electricity is much lower than the standard daytime rate.
By charging the home battery overnight during the low cost period we are ensuring there is sufficient charge in the home battery to cover our house consumption throughout the whole day as our 10kWh home battery covers over average daily home consumption.
As can be seen in the graph below recharging the home battery has moved to the early hours of the morning (between 0030 & 0430 in this example from Octopus Go EV tariff). The battery then covers the house consumption from 0430, slowly dropping until it recharges if there is any solar generation available. The battery once again covers the evening consumption once solar generation has stopped. There is a noticeable drop in the battery % due to the use of the high powered appliances such as the oven and washing machine in the evening.
Mid October 2023 - grid charging
Another benefit of charging the home battery from the grid during the low cost is that it can lower our electricity bill. This is because the home battery is being charged when the electricity unit rate is at its lowest, and we are storing that electricity to use later in the day rather than importing electricity at a higher unit rate in the daytime.
By charging the home battery during the low unit rate period it means we import very little energy at the higher unit rate. The consumption data on our smart meter clearly shows we import nearly zero electricity from the grid during the higher daytime cost rate period. The small blips during the day are from our home battery reacting to changes in our consumption.
Half hourly energy consumption from recharging the home storage battery
Half hourly energy consumption from recharging the home storage battery and EV
As shown in the example below 98.4% of our electricity consumption (which covered both the house and the EV) was at the 7.5p per kWh low cost rate on Intelligent Octopus Go. Our import consumption using daytime rate electricity of 31p per kWh covered just 1.6% of our electricity consumption.
This would mean our average electricity unit rate was just 7.9p per kWh for both the house and EV. The standard unit rate, at the time of writing, is 28p per kWh, which shows how big savings can be achieved from charging a home battery and EV during low rate electricity periods.
Electricity consumption by unit rate - 1st to 10th October 2023
That's not only a huge saving on the unit rate for us (which helps to offset the cost of installing the home battery), but it also means we are helping the grid by reducing our peak period consumption to practically zero. We are therefore helping the grid reduce peak period power demand every day, which may mean fossil fuelled power stations may not be required to be turned on if peak period electricity demand is lower.
What does reduced solar self use consumption mean?
By paying to import electricity in the low cost period rather than rely on solar generation to cover our home electricity use many would think it's costing us more money. Why pay would you pay for electricity when you're getting 'free' solar power?
Firstly, there is no such thing as 'free' solar power. Solar power still has a cost to it, whether that's the initial cost to install solar in the first place, or it's the SEG export value. The way I see it every kWh of solar generation has a value, that value is dictated by your SEG export rate. In our case the SEG export rate is currently 15p per kWh (Outgoing Octopus Fixed tariff).
As the SEG export unit rate of 15p per kWh is higher than our low cost period import unit rate of 7.5p per kWh, it makes sense for us to always charge the battery from the grid during the low cost period rather than from solar generation at 15p per kWh during the day.
This means the amount we export should be a high percentage of the monthly generation because if we don't self consume a kWh of solar generation we are able to export it at double the cost to charged the home battery. We should export enough solar generation every month, both summer and winter, to cover the cost of our home electricity consumption (excluding EV charging) because exported solar is worth double that of the cost to import electricity during the low cost period.
From an environmental point of view shifting our electricity consumption to a lower demand period, such as early morning, means the imported electricity is likely to be of lower carbon intensity. It is more likely to be generated from low carbon sources such as nuclear and wind. Storing the electricity to use later during higher demand periods rather than importing high carbon intensive electricity generated from gas or coal is better for the environment too. It just happens it's also better financially for us too.
The home battery and EV have changed how much electricity we import, but it's also changed how we manage our electricity use too. Gone are the days of a single rate tariff. Smart tariffs are helping us change how we use electricity but also help us utilise more low carbon intensive electricity but changing when we import it but not when we use it.