The problem with solar is that generation peaks through the daytime. That is often the opposite of your high energy demand periods of the morning and evening.
One solution to help is battery storage as it allows you to capture the surplus solar energy and store it to use during your high energy use periods.
Would a battery help us?
The best way to answer this question would be to review our solar generation and electricity consumption data on the Solis Cloud app.
1st January 2023 to 14th April
It was clear from the Solis Cloud app data that we had plenty of surplus solar energy (778kWh - 87% of solar generation) that was exported and could be used to reduce our current imported electricity use (224kWh). It would still leave plenty of solar energy to be exported for SEG. From a solar only install point I’m pretty happy with a 33% self use figure considering we aren’t always at home during the day to use the solar generated energy. One of our initial reasons to install solar was to make us more self sufficient and reduce our reliance on the national grid.
When I reviewed our daily electricity usage I often saw we used very little to no imported electricity during the day as solar was meeting our day time energy demand. The evenings were very different as we imported a high amount of electricity on both high and low electricity use days.
Shifting Energy Use
This winter’s Octopus Saving Sessions (National Grids ‘Demand Flexibility Service’ scheme) showed that shifting energy use during expected peak energy reduced the need to start up fossil fuel power plants to meet high demand periods. That’s a great reason to shift electricity use to lower periods.
Whilst a battery could allow us to store surplus solar energy to use later it could also allow us to shift our energy use to lower demand periods. This isn’t something I’d initially through of when installing solar, but helping the energy generation network reduce demand during peak demand is a good environmental reason too.
Theoretically we could charge a battery during a low demand period and store that energy to use later in the day. That would allow us to export excess solar generation during the day and through the evening peak period too, helping supply electricity during the peak demand evening period.
The Octopus Flux tariff actually promotes this idea by setting the day and high period export rates above the Flux period import rate. Under Octopus Flux you’d be (slightly) better off financially charging a battery at 20.39p per kWh as you’d get slight more exporting your surplus solar at 22.98p per kWh during the day. That’s a difference of 2.59p per kWh, which isn’t much. You could theoretically increase this to 16.19p per kWh during the peak period, but you’d need quite a big battery to have sufficient surplus stored energy to do so.
Our Octopus Flux tariff rates (April 2023)
For us using the flux rate for all imported electricity instead of the flux tariff low/day/peak unit rates would save 14.95p per kWh imported. For us that would be a saving of around £120 per year. Greater savings would be possible if you imported more electricity (our import electricity after 33% solar self use is circa 800kWh per year - 1200kWh total consumption). There would be a saving from exporting solar energy instead of using it ourselves but this would be quite low (circa £20 per year) due to the extra 2.59p per kWh exported instead of used.
The largest saving from Octopus Flux is the higher SEG export rates over Octopus Outgoings 15p per kWh. Octopus Flux SEG export rates would likely give us an annual revenue of around £750 per year, that would be an increase of around £300 per year over Octopus Outgoing .
Do you need a battery for Octopus Flux?
Whilst Octopus Flux doesn’t necessarily need home storage battery as solar only is eligible for the tariff, but it’s really design for solar and battery owners. With solar only you’re getting the benefits of the higher export rates, but you have the disadvantage of the higher cost evening peak period. Depending on your use during the peak period any export savings could be wiped out by the higher peak period energy cost.
For the first month on Flux without a battery our imported energy cost breakdown by period was:
* Flux - 15%
* Day - 60%
* Peak - 25%
Whilst it worked out an extra 2p per kWh when averaged over all three periods that took a bit of work on our part. To reduce energy cost we set the dishwasher and washing machine to run on timer overnight in the lower cost flux period. I have an Ecoflow portable battery that I’ve been charging during the day via solar, which we used to run the TV in the evening. My wife has also got to a point where when I highlight limiting electricity use in the evening peak period she’s not so happy about it, mentioning she may murder me if I keep mentioning it. If I want to live, or not be divorced then adding a battery isn't a bad thing!
A battery could do a lot of the heavy lifting and would make our life easier again. During winter when solar generation is low it could be charged during the low cost flux period, ensuring the lowest cost electricity, but also let us use that stored electricity at any time of the day so we wouldn’t no need to think about timers or delaying using the oven/induction hob to after 7pm when the peak cost period had passed.
Sizing a Battery
Sizing a battery is quite difficult as there is not a single battery size that will work for everyone as everyone situation and needs are different. Some people may want to cover all of their daily consumption, whilst others may want to cover energy consumption during higher cost periods such as only the evening.
Personally we want to cover all of our consumption. I started by looking at our total annual consumption, which is currently 1200kWh per year. That would give an average consumption of 3.3kWh per day. If we look at our imported electricity, which is 66% of total consumption as solar generation provides 33% of our electricity, it’s an average of 2.2kWh imported electricity per day.
Whilst this gives a good idea of average consumption it doesn’t give us an idea of the overall range of our daily use. This is where smart meter imported electricity data comes in handy. The graph of January 2023 showed how our daily imported energy varies greatly.
January 2023 imported electricity
It can be as low as 1.5kWh per day on a good solar day, but also nearly 5kWh on a poor solar day. The graph of imported electricity during January 2023 shows our daily imported electricity averaged 2.9kWh per day. It usually ranged between 1.5 to 3kWh but peaked up to 5kWh on some days.
This initially made me think a Dyness 5kWh battery would be sufficient, but there are factors such as Depth of Discharge (DoD) to consider. DoD is often up to 90% for a Lithium Ion battery, but it differs battery to battery. A 90% DoD reduces a 5.1kWh battery to just 4.6kWh useable capacity. Couple battery degradation which will over time reduce battery capacity further you could be left with 3.7kWh useable capacity after 10 years.
Similar to a mechanical device that wears out faster with heavy use, the depth of discharge (DoD) determines the cycle count of the battery. The smaller the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses. Partial discharge on Li-ion is fine. There is no memory and the battery does not need periodic full discharge cycles to prolong life. Battery University - How to prolong Lithium based batteries.
A larger capacity battery should theoretically allow a lower Depth of Discharge helping battery life.
One larger capacity battery I found was the 7.1kW Pylontech Force L2. The Pylontech 7.1kWh battery has a 95% Depth of Discharge, giving a useable 6.75kWh capacity (likely around 5.4kW after 10 years battery degradation). Our average imported electricity of up to 3kWh per day would be 45% of the capacity on a 7.1kW battery, but 65% of the capacity on a 5.1kW battery. Our worst daily imported electricity of 5kWh would be 75% of the capacity on a 7.1kW battery, but 109% of the capacity on a 5.1kW battery. That was a concern to us that we would fully discharge a 5.1kW battery on some days, and more often after years of battery degradation has reduced battery capacity. Even after 10 years of battery degradation the 7.1kW Pylontech Force L2 battery should not fully discharge on our worst imported electricity days.
Why the Pylontech Force L2 7.1kW battery?
There were several reasons why we chose the Pylontech Force L2 7.1kW battery:
1/ The higher capacity benefit reasons over the 5.1kW Dyness battery stated above.
2/ Pylontech batteries have a good reputation.
3/ It’s modular design allows future expansion by allowing us to add one or two further battery modules (10.65 to 14.2kW) should we need more capacity.
4/ It’s compatible with our existing Solis Hybrid inverter.
5/ It just looks much better than the rack type batteries
Pylontech Force L2 series specification
Summary
A battery install isn’t as cost effective installing solar, but it would provide non cost benefits such knowing we were helping reduce peak period energy demand. Reducing demand should hopefully stop the need for extra fossil fuelled power stations in high demand periods in winter.
A battery does make the Octopus Flux tariff much easier to use. That tariff should increase savings to around £1000 per year compared to solar only install saving of around £600 with Octopus Outgoing. A battery should pay for itself in circa 14 years, but Solar and battery together should pay for themselves in circa 13.5 years. Compared to solar only install payback period of 12.5 years it wouldn’t be too different.If unlike us you installed solar and battery storage at the same time it would likely cost you less which would result in a faster payback too.
Another reason to install a home battery would be if we were to change to an electric vehicle in the future. Sadly if you go with an Octopus EV tariff you can’t be on a SEG export tariff like Octopus Outgoing or Flux. The SEG export payment would therefore drop to 4.1p, which makes exporting excess solar energy back to the grid less cost effective as a means of paying back the investment in solar. A home battery would allow us to store excess solar energy rather than export it to the grid at such a low rate.