Introduction
The wait for solar panels to be installed gave me time to choose an electricity supplier and a tariff.
My Existing Supplier is Octopus - Should I Change?
- I am not interested in switching supplier every few months so to get the best offer, so I ignore special introductory offers and focus on the regular rates available to all users
- I do want low rates, but the rate does not need to be the lowest, just close to lowest
- Good, friendly service for any issues is important; issues will arise and I wanted to be looked after.
- I had been with the Coop who, a few years ago, gave up doing energy and passed on all their customers to Octopus. This 'relationship' had been fine over a number of years; rates reasonable, minimal effort on my part and friendly communications.
And Which Tariff?
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Octopus Flux is the Tariff that I signed up for and the one that I used for my return on investment calculations |
- Good Return on Investment (ROI) for battery storage (eg buy cheap rate electricity at night and sell back at peak in the evening) - roughly 10p per kWh difference between buying at cheap rate and selling at peak rate (when I did the ROI calculation in summer/autumn 2023).
- Set up and forget - no need to tweak scheduling each day (eg Vs options with ever changing rates that track the wholesale energy market)
- We are free to use power at home for our convenience, when we want without significant penalty (eg we can decide when to cook based on when we want to eat, not on the changing electricity price).
Another Octopus tariff option is Intelligent Flux where Octopus take control of the battery charge and discharge. This sounds quite clever, but I worry about the complexity and also the website did not give any return on investment figures. My decision was to keep it simple and go with Octopus Flux. With this up and running and acting as a baseline over, say, 12 months I could then consider other tariff options.
How Was the Sign Up Process?
It is important to note that we had a working smart meter, at this point, sending electricity consumption meter readings back to Octopus every 30mins. The sign up timeline was:
- Solar panels commissioning completed and left on battery charge/discharge cycle in Friday 8th March
- In full operation on Sat 9th March
- Installer e-mailed me the MCS certificate/number and Distribution Network Operator (DNO) notification letter on Monday 11th March
- Tried to sign up for Octopus flux on Mon 11th March - website crashed - 3 attempts
- Tues 12th: 1 more attempt and called Octopus - sounds like a website issue and my request has actually been submitted.
- 23rd March: export and Octopus flux now set up and working:
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Getting Paid by Octopus for my Solar Generation, Much of it Exported at Peak Rate via the Battery |
How Do I Schedule the GivEnergy Inverter for Octopus Flux?
For this setup I used the GivEnergy Cloud interface for the inverter (https://givenergy.cloud/login). There is an app for both iOS and Android, but (at the time of writing) these do not give access to all the scheduling features that are available on the cloud interface.
My Setup:
- Fill the battery at cheap rate 2am to 5am
- In the winter this needs to be a 100% battery fill
- In the summer this can be a lower %age fill as the solar power will usually fill the rest.
- The cost of cheap rate import is slightly less than day rate export, so, in theory, you actually gain just over 1p/kWh by charging from the grid at cheap rate as opposed to letting the solar complete the battery charge. However, I think, that this comes with additional losses and heating/wear of the inverter/battery (just my unproven theory).
- My take is that it makes little difference to the final ROI, but I would be interested to hear from anyone who knows more about losses and inverter/battery wear.
- The GivEnergy scheduled charge does the above.
- After 5am (day rate) home use power source in priority order of:
- Directly from solar
- If not enough solar, from battery as well
- If not enough battery, from grid as well
- An use excess solar above home use in priority order of:
- Top up battery
- If battery full or charging at max power then export to grid
- Good news: the GivEnergy ECO mode does all of the above.
- Peak rate, 4pm to 7pm. This is the tricky bit, objectives are:
- Do not import at peak rate unless there is no other possibility
- Export everything that you can without breaking rule #1
- Freedom to use power when we need it, eg what if we want to have the electric oven on between 6pm and 7pm? I don't want to delay tea so that we can cook at a lower electricity unit cost.
- The last and first points make it tricky - you cannot just dump all the stored battery energy to the grid at 4pm. It is important to have a staged discharge and keep some back right up until 7pm.
- After 7pm it makes little difference if you have managed to export the last kWh from the battery at 22.885p / kWh and then import at the day rate (24.66p / kWh) or if you keep back the odd kWh to supply the house in the evening.
- You do want a (close to) empty battery at 2am to start filling it at cheap rate, so you have a margin of error of 7hours minimum domestic use so that, even if you are on holiday, any residual battery charge that is left at 7pm is used up by 2am. For us we can run as low as 200W when we are on holiday, so a margin of error of 7hours x 200Watts = 1.4kWh.
- Givenergy provides 10 discharge periods, so by dividing up the 180minute peak rate period into 10 x 18mins (or 9 x 20mins for simplicity) mean that you can discharge a step at a time and leave, say 1kWh at 7pm. However, there is one problem, a peculiar behaviour on charge and discharge that means that this (currently) does not work in all situations...
...more on this in another post. In the end I used just 3 discharge periods (as can be seen from the graph below) - again more on this in the future post.
- So what is the ROI? - see 3 examples of recent Flux tariffs below (I will use the July '24 tariff in the following text):
- "Summer" ROI: here my assumption is that there has been enough daylight up until 4pm (peak rate start) that the battery is 100% full. On bright days, during the day, after the battery is full, we export at the day rate (at 14.85p/kWh). After 4pm get to sell all the cheap rate electricity that we bought in the night before (2am to 5am) at peak rate - a difference of 9.3p/kWh. We also programme our dishwasher to run between 2am and 5am (roughly 2kWh at about the same rate saving). Roughly speaking 10kWh (9.5kWh battery + 2kWh dishwasher - losses - baseline battery 'reserve' charge (set to 4%, but seems to vary between 3% and 6%)) at 9.3p/kWh rate difference is approximately a 10kWh or £9.30 return on investment each day.
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System Power Graph showing Typical Summer Day. Charge up to 50% at cheap rate, using solar power to get to 100% change and then discharging to the grid in 3 steps during the peak rate. |
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Solar Generation Energy Graph for typical Summer Day. |
- "Winter" ROI: The battery capacity (9.5kWh) is around the same as our daily consumption (~10kWh). In winter, my assumption is that solar production is minimal and we only have enough battery charge from cheap rate to run the house. Discharge at the peak rate can only be minimal if any. When we are at home we use an average of around 11kWh (including inverter & battery loses - see upcoming post), but the typical range is 8kWh to 16kWh. Solar power production can be as low as 3kWh on gloomy days (based on my experience so far from March to September - I assume we will go even lower in the next few months). So in these situations the return on investment is the difference between cheap rate import and the day rate import that we are avoiding, ie 9.03p/kWh difference giving around a £9.03/day return on investment.
- Note: this is the return for using the Flux rate over a regular fixed import and export tariff (eg at the day rates below). While this depends on having a battery, the battery has an additional return not covered here, when the solar system is producing more than is being consumed at home, the battery can charge. When there are peaks in demand (eg kettle on), or dips in solar production (eg cloud cover) then the battery is used as opposed to importing ie comparing a solar system with and without battery the battery removes both import and export at day rate. This return on investment for the battery is true of all import/export tariffs and is not calculated here.
Octopus Flux Tariffs Have Changed in the Last Few Months:
From 1st July 2024
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Import |
Export |
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Day rate |
22.586p / kWh |
14.85p / kWh |
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Flux rate (02:00 - 05:00) |
13.552p / kWh |
4.816p / kWh |
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Peak rate (16:00 - 19:00) |
31.621p / kWh |
22.885p / kWh |
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Standing charge |
49.119p / day |
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Battery profit (summer)= 22.885-13.552=9.333 p/kWh
Battery profit (winter) = 22.586-13.552=9.0340 p/kWh
These figures are used for the above post.
May 2024
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Import |
Export |
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Day rate |
24.66p / kWh |
14.85p / kWh |
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Flux rate (02:00 - 05:00) |
14.80p / kWh |
5.26p / kWh |
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Peak rate (16:00 - 19:00) |
34.53p / kWh |
24.99p / kWh |
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Standing charge |
49.12p / day |
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Battery profit (summer)= 24.99-14.80=10.19 p/kWh
Battery profit (winter) = 24.66-14.80=9.86 p/kWh
To April 2024
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Import |
Export |
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Day rate |
28.44p / kWh |
17.44p / kWh |
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Flux rate (02:00 - 05:00) |
17.06p / kWh |
6.06p / kWh |
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Peak rate (16:00 - 19:00) |
39.82p / kWh |
28.82p / kWh |
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Standing charge |
49.77p / day |
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Battery profit (summer)= 28.82-17.06=11.76 p/kWh
Battery profit (winter) = 28.44-17.06=11.38 p/kWh
Over the last year the battery ROI has been going down from over 11p per kWh to only just over 9p per kWh...
...this was identified in the risks for this investment (See Solar Panels Investment Case)
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