Solar #8: 12 Months Review

 Introduction:

• The system has continued to work reliably and needed no intervention from me
• All the points in the 6 months review still hold
• The system production is broadly in line with the quote
• The GivEnergy battery disabled software issue results in non-optimum scheduling
• Some of my discoveries may be of interest to someone specifying a new system:
• Battery charges at a slower rate when cold
• Battery losses are around 10% of the 'round trip'.
• ROI based on the 1st 12  months of data is...

I will expand on these points below.

 

Solar Production Through the Year:

I Used the Period 1-Apr-24 to 31-Mar-25 for Analysis in this Post.

It meant that I had a set of 12 complete months of graphs and it misses the 1st 2 weeks or so when I was working out the scheduling. It does mean that the rather gloomy Mar-24 was replaced by Mar-25 that was pretty much wall to wall sunshine!

 

Seasonal Variation in Solar Production

When I was sold my solar system I was given vague assurances that I would still produce solar power in the winter. While this is true, in winter production is much less and, of course, this is the time of year when you probably want it the most.

 

I thought that a comparison of solar production around the summer and winter solstices and around the spring and autumn equinoxes would be useful for others, for example in sizing a solar system. By way of refence our system has 14 x 400W panels and faces SSE (See these posts on Solar#1:Our House is Not Suited to Solar Panels? and Solar#3: Supplier Selection for more details on our installation)

 

Summer: June 2024   Max: 36.2kWh   Min: 8.8kWh   Ave: 23.0kWh	Autumn: Sept 2024 Max: 24.5 kWh Min: 1.5kWh Ave: 13.0kWh	Winter: Dec 2024 Max: 5.18kWh Min: 0.65kWh Ave: 1.86kWh	Spring: Mar 2025 Max: 24.3kWh Min: 5.0kWh Ave: 16.0kWh

 

My conclusion to this is that, with a battery and flexible tariff, the scheduling needs to be robust to cope with varying weather/solar production, unless you are OK with adjusting the schedule each day in line with the weather forecast. In the summer, when/if most days production is higher than consumption (for us this is true between the spring and autumn equinoxes) then this issue is much reduced (do minimal top up of the battery at cheap rate, to last until the sun rises, and then power the house + fill the battery from solar power and still have excess to export before getting to peak rate with the battery at 100% charge).

 

Variation in Consumption:

 

Consumption across the Year.

variation was around +/-40kWh (excluding months when we were away on holiday). During the winter months the grid replaces solar production as the energy source. The variation in battery as a source is as much to do with adjustments to the schedule as to solar production (eg when my smart meter was not functioning and I worried that I would be on a  fixed tariff).

 

Consumption varied much less than solar production over the year, with the highest in December (403kWh) and the lowest in November (250kWh). In May, July, November and January we had holidays lasting a week or so. The seasonal variation is around 360kWh/month +/-40kWh, for months when we were not away for more than the odd day. There is actually more variation in consumption day to day, depending on our activity, than across the seasons:

 

Winter Month's Consumption: Includes a 7 day holiday	Winter Typical Day's Consumption: At home Dishwasher runs at cheap rate from ~2am. Minimal solar production Scheduling conundrum - how much battery charge to you need to keep for evening peak rate consumption?	Winter Holiday  24hr Consumption: Away from home Battery activity: charging at cheap rate and discharging at evening peak rate - clearly causes a rise in what the inverter counts as 'home consumption', but must, in reality, be inverter/battery losses.
Summer Month's Consumption: Includes a 7 day holiday	Summer Typical Day's Consumption: At home Dishwasher runs at cheap rate from ~2am. Minimal solar production No scheduling conundrum - battery is full for evening peak rate	Summer Holiday  24hr Consumption:   Away from home

 

Production Vs Consumption

It seems rather elegant that consumption and solar production (after losses) for the year roughly match at around 4,000kWh. Of course this is for just one size of house (4-bedroom detached) and one lifestyle (retired couple).

 

While autumn and spring solar average production roughly match consumption, this equivalence does not survive further scrutiny:

• Summer production averages 23kWh, well above average summer consumption of about 10kWh
• Winter production averages 1.8kWh, only just enough to make up the average battery losses of 1.1kWh (see details below) and well below average consumption of 13kWh.
• Daily variation is significant - minimum daily summer production (8.8kWh) can go a little below average consumption, but winter maximum production of 5kWh is still not enough for a day's consumption.

 

Consumption changes with the weather as well:

Dark (cold) days: more lighting, tumble drying and cooking

Bright (hot) days: less lighting, more clothes washing/less tumble drying and less cooking (more salads), more time spent outdoors...

...so a mixed bag, but in general we consume around 25% more electricity in winter. I am intrigued that, in the above charts, an empty house consumes more in the in winter than in the summer. I have no explanation for this!

 

Battery Disabled Issue

I have a post on scheduling brewing and I will cover this topic in more detail there. The issue is still not resolved in the latest GivEnergy software update. It does mean that we pay a little more for electricity each year than we need to and it makes setting up a robust discharge schedule difficult, eg it might be worth adjusting the discharge schedule with the seasons and, where practical, to change it with daily usage (eg holidays).

 

Typical Winter Day Consumption and Battery

This shows the problem of trying to build one schedule that is robustly optimised for all winter days - given the variation in solar production and home consumption

Note Chinese date format (yyyy-mm-dd) - the same 'typical' winter day as in the set of 6 graphs further up the page.

 

The issue can be seen in the 'Winter Typical Day Consumption' graph above.

1. The battery has been (almost) fully charged at night (cheap rate).
2. During the day the GivEnergy ECO mode kicks in and uses what solar energy there is (2.3kWh on that day),
3. but has to augment this with the battery - using over 7kWh - most of the battery's capacity.
4. After 4pm (peak rate start) we manage to export 0.6kWh to the grid before the battery is exhausted by 5-30pm
5. and we start to use the grid (before 7pm and the end of peak rate). The battery then finds a bit more power at around 7-30pm when it looks like we are cooking.

 

The issue is that, in winter, the ideal schedule, is very dependent on the timing of any in house consumption and the amount of solar production:

• If we are out all day until after peak rate ends and there is lots of sunshine (eg a winter walk ending with dinner by the fireside, in a pub) then exporting at peak rate is a good thing to do.
• If we stay in all day (eg a wet and cold day when we get jobs done and cook dinner for friends) then solar production is low and peak rate consumption is high.

How do you design a schedule that is robust to these eventualities, and does not rely on daily human intervention?...

 

...see up coming post coming on scheduling (currently brewing).

 

Battery charges slower in winter/when cold

I noticed in the winter that my battery did not reach 100% charge during the 3hour Octopus Flux cheap rate period (2am to 5am). The specification of the battery/inverter is a maximum charge power of 3.3kW and the battery capacity is 9.5kWh, so it would seem logical that full charge could be reached, starting from 4%, between 2am and 5am. In the summer this is true, but in the winter and spring it does not fully charge. There seems to be a slow charge rate of ~2.6kW that will sometimes rise later to around the specified rate:

 

Summer: 100% charge is reached at 5am - gradient is 3.4kW	Winter:88% charge is reached by 5am - gradient is 2.6kW	Spring: the gradient rises from 2.6kW to 3.44kW at 3-25am

Note: GivEnergy app dates are in Chinese format of yyyy-mm-dd

 

I contacted the GivEnergy Support desk about this and got this response:

 

"Our firmware will restrict the battery if the cells are below 20 degrees, what you might see from the temp sensor is the BMS board temperature not the cells. The cells are often a few degrees lower than the board so I would just judge off the temp we give as there are 5 sensors at play. One for the board and 4 for each packs quadrant of the cell housing.

 

The battery will restrict for numerous reasons, but they can all be classed as stopping efficiency loss and long term degradation of the battery. As a cold battery cannot hold as much or efficiency charge so its best to restrict the rates"

 

This fits with the temperatures seen in the app: on the 'spring' (12 March 2025) battery temperature chart there is a rise in temperature and then an inflection point at 3-25am about 38.6degC and another rise in temperature. I assume due to the faster charging rate.

 

Battery temperature chart while charging at cheap rate on 12-Mar-25 (same date as right hand graph above): Note temperature inflection point at around 3-30am when charge rate increases

 

So what is the impact of this? With Octopus Flux tariff I have 3 hours to charge the battery at cheap rate. In the summer it is not too critical if I reach 100% as the sun can do the rest - I end up exporting solar energy later in the day at about the same rate that I pay for cheap rate (~15p/kWh). In the winter the cheap rate charge is more valuable; I often never reach 100% battery charge as by the time the (weak winter) sun has risen the battery has been discharging to run the house, breakfast, kettle, etc. In winter the 88% charge achieved effectively misses the opportunity to store about 1kWh of energy bought at cheap rate (~15p/kWh) and means that I have to buy this at the regular rate (~25p/kWh) a loss of about 10p/day or £3/month.

 

Note: my battery is installed in the garage, so a bit warmer than outside in the winter. The garage is part of the house - with 2 walls and the ceiling connected to other rooms in the house.

 

Solar production Vs quote:

The annual solar production was 7.0% higher then the installer quotation (actual: 4508kWh Vs quoted: 4212kWh). However, losses were over 100% higher than quoted (actual: 403kWh Vs quoted 181kWh). The actual net production after losses is 1.9% higher than quoted (actual 4106kWh Vs 4019 kWh).

While it is encouraging that solar production, after losses, is a little more than the quote I think that:

• The difference is in the noise - eg I expect that the variation due to weather, year to year, is greater.
• The quotation was not explicit about the operating conditions; for example, my take is that it assumes that the battery is only charged from solar power. If that is the case then I am working my battery harder (to profit from the Octopus Flux tariff) and so perhaps should expect greater losses.
• The GivEnergy app data is just the battery losses (ie the difference between what you put into the battery and what you get out); I cannot find any data on solar production losses (& perhaps this does not matter - I am only interested in the output power and the app gives me this)
• The quotation implies that the losses are solar power production losses, so perhaps the quotation completely ignores battery losses.

 

Battery Losses:

The losses in the battery amount to 403kWh over the year, or 1.1kWh per day or round trip. 10.9% of the power that was put into the battery in the last year (3673kWh) was lost.

 

The cost of 1kWh (either cost to buy from Octopus or lost opportunity to sell to Octopus) is, on average, between ~15p and ~25p. So over a year these losses amount to around £80 of electricity purchased / could not be sold (note: this is already accounted for in my ROI calculation).

 

I assume that all of these losses go into heat energy. For me this is all lost in warming the garage. Perhaps it is worth considering having the battery and inverter mounted in the house and actually using the heat energy:

• 1kWh is worth around, say, 7p (gas an 90% efficient boiler) - so over, say, 8 months when the heating is on, would be ~£17/year
• It would be uncontrolled heat - looking at the app you get most of the temperature rise during charge and discharge - so cheap rate and peak rate.
• The inverter and battery are nicely made, but not so beautiful that I would want them in my house - the custom cupboard to put them in would need decent ventilation and the cost would negate some/all of the £17/year saving.
• Some users comment on their worries about high operating temperatures limiting the life of the inverter/batteries. I have no idea if this is true, but it does sound reasonable. In the house, in a cupboard would significantly raise the summer operating temperature, compared to my garage.

 

Return on Investment (ROI), based on the 1st 12 months of data:

...This will have to wait for another post when I have received my April bill from Octopus!

 

Conclusions:

The system has worked reliably and performed pretty much in line with the quotation.

 

If I were buying again then there would be aspects that I would ask more questions about when selecting a system:

• How do I set up a charge/discharge schedule for my selected tariff?
• How does this schedule cope with variation in supply (eg winter/short gloomy days Vs summer/ long bright days) and demand (eg cooking and washing at peak rate)?
• What is the slowest charge rate (eg in cold weather) and is this fast enough to fully charge the battery in the cheap rate period?
• What is the loss in kWh that I should expect if I fully charge and fully discharge the battery every day.

 

UK (North of England) seasonal variation means that:

• Solar power is not a serious consideration for home heating in winter (conversely, it should be considered for air conditioning/cooling in summer, but not a North of England issue, yet)
• Battery storage of cheap rate electricity should be a serious consideration for winter home heating, but should account for slower (lower power, or lower rate of energy, storage) battery charging in the cold.
• Year round solar production on our (a typical?) 4-bed detached house roughly matches consumption (lighting, cooking and washing) for a retired couple, but this masks large daily divergence between consumption and solar production.

 

The garage seems like a good location to install the system - minimising both summer and winter extremes in temperature, but does waste the, not insignificant, energy loss as heat.