Monthly Archives: June 2021

Farewell Agile, Hello Go

For the last few years I’ve been a user of, and advocate for, Octopus Energy’s Agile tariff. This unique tariff in the UK is linked to half-hourly wholesale electricity prices and gives the user 48 half-hourly electricity prices each day. As the price of electricity varies considerably from one half hour to another, customers like myself could save quite a lot of money by shifting consumption around such as by charging the car or running the washing machine at different times.

Example Agile electricity bill – average 4.48 p/kWh over the billing period

However in recent months wholesale electricity prices have been high. My understanding is that this is from a combination of factors including aging power stations being offline for maintenance and Brexit-related issues around market access and trading. The result of this is that while there’s still variation by time of day in the wholesale markets the pricing is always relatively high. The Agile tariff also applies a multiplier to the market prices to cover Octopus Energy’s costs which drives the retail price even higher. Thus there really doesn’t seem to be a financial benefit to such extreme agility compared to some more conventional tariffs. (I’m not criticising Octopus Energy here – they are completely transparent about how this tariff works)

As a result of this a few days ago I switched to another Octopus tariff – Go. Go is more like a traditional time-of-use tariff – like Economy 7 in the UK – except that Go provides a shorter cheap window of 4 hours not 7 hours and a deeper discount. Go’s headline pricing is a cheap 5p/kWh from 00:30 to 04:30 with a higher standard rate that varies by region and gets adjusted from time to time. My standard rate is 15.96 p/kWh fixed for 12 months.

My electricity consumption is managed by the Home Energy Management System (HEMS) which has been optimising my energy costs for some two and a half years. Having decided to move away from Agile then I need a quick change for my HEMS to work with the new Go tariff. My quick solution (a whole two lines of code) is to edit the Agile costs on the fly each day to replace Agile costs with Go costs in the 4 cheap hours – a softy of hybrid of Agile and Go. Additionally I’ve used an existing configuration file to apply a price cap at which the battery can be charged from the grid preventing grid charging above the 5 p/kWh price as it makes no economic sense to charge the battery at the higher Go price to avoid buying electricity at the same higher grid price. A side-effect of this (which I quite like) is that daytime consumption is still managed in a grid-friendly manner via the Agile price even though I’m physically paying the Go price. (I may have to think about this further if the Agile price starts to drop below 5 p/kWh)

The immediate effect of the standardised Go price at night is that the behaviour of the HEMS for EV charging, dishwasher, washing machine and water heating has all standardised too. All these now start at 00:30 except for water heating which now never happens from the grid since mains electricity is now always more expensive than gas. Dishwasher and washing machine may also be scheduled for during the day if anticipated solar production is high enough, while EV charging and water heating may also happen from solar in a closed loop manner.

Battery charging is a little less standardised at night. Battery charging now varies in a range of 0 to 4 hours overnight varying with anticipated solar production the next day. In the last few days I’ve seen both bookends – no hours of battery charging when the day ahead will be sunny and 4 hours of charging when the day ahead will be more mixed. During the cheap window, if the battery is not charging, then the battery is not permitted to discharge. Yesterday was one of the mixed days.

Electricity consumption on Go

As can be seen from my smart meter data above, almost all the electricity consumed was at night in the cheap window, so my average electricity cost for the day will be very close to 5p/kWh.

Battery behaviour on Go

The battery charging and discharging is shown by the blue and gold lines above. Initially the battery discharges to avoid paying 16 p/kWh to the grid, then it charges for 4 hours at 5 p/kWh, then there’s some further discharge until the sun rises, in the morning there’s some sun which charges the battery in a somewhat variable manner, then in the afternoon it’s sunnier and the battery reports some ‘Grid Power Out’ which is actually power available for lower priority self-consumption devices, and finally the battery discharges through the evening. There would have been some opportunity to charge the battery more during the afternoon but the battery was already nearly full.

The ImmerSUN gives probably the most complete overview of the home, albeit at only hourly resolution as follows:

  • Purple is the electricity consumption (home, battery, dishwasher, washing machine etc)
  • Red is bought electricity (58% of total)
  • Green is generated electricity (42% of total)
  • Blue is electricity used for water heating (which is all from generation and part of the 97% self-consumption)
  • Teal is electricity export (which is minimal at 3%)

In conclusion the change to Go seems to be saving me a significant amount of money compared to current Agile prices, and the hybridisation of the tariffs seems to be working well from a control perspective giving me the financial benefits of Go and should deliver the grid-friendly behaviour of Agile (although I’d need poorer weather to demonstrate that).

If you are a GB resident and would like to switch to Octopus Energy (who have incidentally been Which-recommended for years) then we can each earn £50 credit from this referral link.

Improving the Home Area Network

While generally the network at our smart home works well, I have in the past had some issues with inability to connect between devices. Many of the smaller smart devices use Bluetooth (and in particular Bluetooth Low Energy – BLE) because battery devices lack enough energy capacity to run WiFi with adequate battery life, but we have had some issues with WiFi-connected devices. Intermittent WiFi issues included:

  • connection between all HomeKit hubs (2 x Apple TV + iPad)
  • connection to an external HomeKit WiFi bulb or WiFi smart plug
  • connection from iPad to Raspberry Pi HEMS

After some head-scratching I concluded that the issue relates to my WiFi network or indeed networks. Like many people I have dual band WiFi – 2.4 and 5 GHz. The 2.4 GHz is supported by more devices, carries for a longer distance, but can carry less data; while the 5 GHz can carry more data, but is supported by fewer devices and has less range. Devices with 5 GHz capability can generally choose either frequency, but many cheaper devices are 2.4 GHz only.

It seems to me that devices on the 2.4G WiFi network can reach each other, hardwired ethernet devices in the home, and the external internet. Similarly devices on the 5G WiFi network seem to be able to reach each other, hardwired ethernet devices in the home, and the external Internet. However devices on the 2.4 GHz and 5 GHz WiFi networks don’t seem to be able to reach each other. I couldn’t find any setting in my router that might join or separate these WiFi networks.

The solution that I came to was Powerline adaptors. Powerline adaptors extended a wired ethernet connection over the existing mains electrical wiring of the home rather than require new dedicated cables. Typically these Powerline adaptors are sold in pairs – one to be wired to the router and one to a remote device – but it’s possible to pair additional units. Indeed I currently have three units from two different manufacturers all interlinked:

  1. In my study connected to the router.
  2. In the lounge connected to the Apple TV.
  3. in the airing cupboard connected to the HEMS (as illustrated above).

The effect of this is to put the Apple TV on the wired ethernet with the result that the iPad (however connected to the internet) can reach it as can the external HomeKit WiFi bulb on 2.4 GHz. Similarly, with the Raspberry Pi hardwired, then the iPads can reach it regardless of their internet connection, rather than only when the iPad was also on 2.4 GHz.

The result seems to be a significant improvement in robustness and it didn’t even cost me anything as I had two pairs of Powerline adaptors already from prior projects.

Powerline adaptor

From back to front:

  • Mains socket incorporating USB power supply for Raspberry Pi HEMS
  • Black USB power lead to HEMS
  • Powerline adaptor connecting HEMS to Powerline network with mains socket
  • Yellow Cat 5 ethernet cable to HEMS
  • Mains plug with ‘Do not unplug’ label supplying power to HEMS-driven relays and RF Solutions Mainslink radio transmitter to car charger.

From top to bottom:

  • Raspberry Pi HEMS (black box) incorporating relay HAT
  • 10-way junction box (white box) typically employed for wiring central heating controls
  • RF Solutions Mainslink radio transmitter
  • immersun solar diverter
Raspberry Pi as HEMS
Smart controls – HEMS and immersun.

Payback time

It been over a year now since I last reviewed what return I was getting on my investment in energy smart technology – solar panels, battery storage etc – so I think an update is due. This time I’m going to take the input data from my immersun system – one year of data from start of June 2020 to end of May 2021.

Diverted – this is where the immersun sends any surplus solar electricity to my immersion heater to make hot water. In 2020/1 we diverted 1056.6 kWh to hot water saving gas at 2.82 p/kWh. However the gas boiler isn’t 100% efficient losing heat both via the flue to the outside world and also via the hot water pipes to the home rather than hot water. If we assume 80% efficiency at the tank then 2.82 p/kWh as gas at the boiler is 4 p/kWh as heat in the tank. 1056.6 kWh at 4 p/kWh saved £37.25.

Exported – this is where I’m unable to use the solar power that we generate and it overflows into the grid. I’m not paid for Export so this is worth nothing to me.

Generation – this is the energy that we generate in the solar panels. I’m on the UK’s legacy Feed-in Tariff (FiT) scheme which pays me to generate electricity. In 2020/1 I was paid 14.65 explicitly for every kWh that I generated. I also received deemed (rather than metered) Export which paid 5.5 p/kWh on 50% of the kWh that I generated (which is where the ‘deemed’ part comes from). 5.5 p/kWh on 50% is equivalent to 2.75 p/kWh on 100% of the Generation making my revenue 17.4 p/kWh per kWh generated or £693.51 on the 3985.7 kWh that I actually generated.

Imported and House – these are respectively the electricity that I buy from the grid and that which I used within the home including appliances and car charging, some of which will comes from my own solar panels. The difference between House and Imported is the electricity that I used from my solar panels which would otherwise have been bought from the grid. If I assume that each kWh that I use from my solar panels avoids buying a kWh of electricity from the grid at 16.36 p/kWh (current Energy Saving Trust value for the average UK electricity price) then I avoided buying £423.81 of electricity by using the output of my solar panels.

Diverted1056.5 kWh*£0.04=£37.25
Exported338.6 kWh*£0.00=£0.00
Generated3985.6 kWh*£0.17=£693.51
Imported4748.9 kWh*-£0.16=-£776.92
House7339.4 kWh*£0.16=£1,200.73
Return on smart energy investment @ 16.36 p/kWh grid price

With an investment of £8,670, £1,154 represents 7.5 years to pay back the capital invested.

I’m actually on a smart tariff so my electricity cost in this period at 8.05 p/kWh was significantly less than the UK’s average 16.36 p/kWh. This lower price will arguably reduce the value of the energy generated by the solar panels for self-consumption, but equally the ability to maximize the value of a smart tariff is itself a saving.

Diverted1056.5 kWh*£0.04=£37.25
Exported338.6 kWh*£0.00=£0.00
Generated3985.6 kWh*£0.17=£693.51
Imported4748.9 kWh*-£0.08=-£382.29
House7339.4 kWh*£0.08=£590.82
Return on smart energy investment @ 8.05 p/kWh grid price (excluding the tariff benefit itself)

Using my actual average energy price rather than the higher UK average grid price pushes down the return by over £200 (£929.29 versus £1,154.56). However the costs of buying the imported 4,748.9 kWh falls by £394.63 through the tariff benefit, increasing the annual return to £1,333.93, and reducing the payback period from 7.5 to 6.5 years.

Thus, had I invested in this technology at one time back five and a half years ago and shortly after we moved to this house, then we’d have been in sight of payback with 1 or 2 years left. In practice of course I’ve made the investments at different times (solar first five and half years ago, battery around a year later, smart tariff later still), so my payback will be achieved a little later.

A snapshot of the ImmerSUN diverting to hot water

Some other statistics:

  • Of solar panel output:
    • 91.5% replaced bought energy (self-consumption)
    • 65.0% replaced bought electricity
    • 26.5% replaced bought gas for water heating
    • 8.5% was exported to the grid
  • Of incoming electricity:
    • 54.4% was from the grid
    • 45.6% was from the solar panels (“green contribution” in ImmerSUN’s terminology)