Transportation update

It’s approaching sixteen years since I bought my first electric vehicle.

Things have moved on enormously from my first electric vehicle – a REVA G-Wiz. The G-Wiz is a small vehicle similar in size to a Smart car, but strictly was not even a car at all but rather a quadricycle. As a quadricycle it is limited by law in both weight and power resulting in a tiny underpowered vehicle compared to even a small car, however that was all that was available at the time. It was good for 30 to 40 miles according to time of year and speed and maxed out at around 50 miles per hour although 30-40 miles per hour was more achievable.

2007 REVA G-Wiz
2014 Vauxhall Ampera

In the summer of 2014 I bought a new Vauxhall Ampera. I’d initially seen the Ampera at a motor show and dismissed it as neither a full electric vehicle or a plug-in hybrid. However a few years later and now married with a young child I was looking for something bigger. Now I saw it as being more capable than the G-Wiz in size, range, and safety with the bonus of having an engine for the occasional longer trip. I’ve now owned it for over eight years.

The latest addition to the household is a 2020 Ford Kuga PHEV. This is similar to the Ampera in many respects with some variations.

2020 Ford Kuga PHEV
AttRiutereva g-wizVAUXHALL Ampera Ford kuga PHEV
Usable battery size10 kWh10 kWh10 kWh
Range30-40 miles30-40 miles30-40 miles
Seats2+2 seats4 seats5 seats
ConnectedNoNo (at least in Europe)Yes
Electrical capabilityLow (heavily compro-mised versus ICE)High (Same as ICE)Medium (Poorer than ICE but keeps up with traffic)
Electrical inlet connector3-pin caravan5-pin BS EN 62196 Type 1 / J1772 7-pin BS EN 62196 Type 2
ICE capabilityn/a (BEV)Conventional 1.4L Atkinson 2.5L (for economy)
Transmission Simple reduction between electric motor and wheels. No ICE or variable transmission ratio.Similar to Kuga but has additional clutches and brakes allowing transmission to either work as powersplit or separate into a series hybrid with a generator set connected only electrically to the motor and wheels.Powersplit – combines two electrical machines with torque trans-mission from ICE to wheels via both electrical and mechanical means. Lacks the ability to split the transmission into a separate generator and motor set.
Comparing my plug-in vehicles
Powersplit transmission

Both Ampera and Kuga use epicyclic or planetary transmissions to create a variable ratio between engine and wheels. Both gear together three shafts such that the speed and torque on any two shafts determines the speed and torque on the third shaft. Both have the wheels connected to one shaft, the engine to the second, and an electrical machine to the third. However Ampera and Kuga differ in that Kuga places the second electrical machine on the same shaft as the wheels, while the Ampera places the second electrical machine on the same shaft as the engine (which helps when splitting the transmission to create a series hybrid).

I’d differentiate the Ampera from the Kuga as describing the Ampera as a short range electric vehicle with an ICE for occasional longer trips, while the Kuga is more of an enhanced hybrid optimised more for those longer trips with a higher efficiency Atkinson engine. The Ampera on the other hand has greater electrical capability (uncompromised performance electric versus ICE) with greater transmission sophistication having the ability to switch between being a series hybrid or a powersplit.

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Battery Economics

Having recently spend around £600 replacing the internal batteries in my Powervault G200, I thought I’d look at the economics of that decision.

Powervault G200 storage

Originally I bought the Powervault to store the surplus electricity from my solar panels and then use that electricity later instead of buying electricity. I don’t receive any revenue for exporting electricity so to fill my battery from my solar surplus costs me nothing. I currently pay 35 p/kWh for daytime electricity. So, if I fill my battery from my solar surplus then I save 4 kWh (the capacity of the battery) x 35 p/kWh = £1.40 per day.

More recently I also charge the battery on cheap overnight electricity when there won’t be enough solar to charge the system fully.

I only get four hours of cheap power which isn’t enough to fully charge the battery. The battery charges at 0.8 kW so in four hours would use 4 hours x 0.8 kW = 3.2 kWh which at 7.5 p/kWh costs 24 pence. The system has a cycle efficiency of 80% so 3.2 kWh in gives 3.2 kWh x 0.8 = 2.56 kWh out. 2.56 kWh energy out is worth 2.56 kWh x 35 p/kWh = 89.6 pence. Having spend 24 pence to save 89.6 pence then I’ve made net savings of 65.6 pence per day.

G200 User Portal

A day like that shown in the portal with 4.75 kWh in savings is a bonus in terms of annual savings although it may accelerate the deterioration of the battery as it’s been double-dipped during the day with both overnight and solar charging.

If I assume that half the time the system is filled by solar and half from the grid then my annual savings are 0.5 x (£1.40 + £0.656) x 365 = £375.22. Having spent £600 on the new batteries then my payback time is £600 / £375.22/year = 1.6 years. The previous batteries lasted for 6 years although the new batteries were relatively cheap and might not be expected to last for so long. However even if the new batteries only have half the life of the prior ones then that still seems like an attractive investment.

My total investment in the battery storage is now £2600 – £2000 to buy it six years ago plus the £600 just spent on new batteries. That’s a payback of about seven years with expectation that the new batteries will extend the life of the system to nine years.

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Reviving the battery

Powervault G200

My Powervault battery is now six years old. Initially I used it only to store surplus electricity from my solar panels but more recently have also managed it from my HEMS to reduce my energy costs by charging it when electricity is cheapest to offset consumption when electricity is most expensive. However it’s become clear in recent weeks that the working capacity of the battery had significantly declined.

Powervault portal for G200 before battery replacement

My Powervault is designed to have a reasonable life – its 7.92 kWh internal batteries (6 batteries x 12 Volts x 110 Ah) are limited to 4 kWh to maximise the number of charging cycles. However after 6 years working capacity had dropped to less than half of that and I decided that the time had come for replacement. Declining battery capacity is illustrated above with the blue line showing battery charging between 00:30 and 04:30. If the battery had continued at constant power (about 750 Watts) then it would have charged 3 kWh (4 hours x 750 Watts / 1000 Watts/kW). However here the battery has ceased to draw 750 Watts from around 02:00 and thereafter power consumption has significantly declined – this may be about 1.75 kWh rather than 3 kWh.

These figures (1.75 and 3 kWh) are also energy into the battery – but you don’t get out everything that you put in. With a quoted round trip efficiency of 80% then 1.75 and 3 kWh in would deliver 1.4 and 2.4 kWh output versus the original capacity of 4 kWh.

I reached out to both my installer and Powervault themselves but neither got back to me with any proposal to investigate or replace the batteries so I resolved to do it myself.

Powervault G200 internals

Initially I did a little investigation by removing the top and side panel of the Powervault (after turning off the power). Inside I found six twelve Volt one hundred and ten Amp hour batteries arranged over three shelves on plug and socket connections – so fairly straightforward to replace. A search of eBay for deep cycle batteries of similar capacity indicated prices range from around one hundred to around three hundred pounds per battery (six required). I eventually decided to go for the cheapest as I wasn’t entirely sure of success and I don’t know for how long my six year old system will be supported even if battery replacement was successful.

6 x Numax CXV31MF Batteries 105Ah

The Numax batteries are sold as deep cycle batteries ideally suited for solar installations.

Powervault G200 installation.

The process for installation of new batteries consists of unplugging and removing all the batteries from top to bottom, and then installing new batteries from bottom to top – two on each of the three shelves. I found it necessary to undo the fixing screws on the back of one of the shelves in order to flex it upwards to create enough clearance to get the batteries on the level below out.

Powervault portal for G200 after battery replacement

It’s only been just over a week now, and it’s fairly dull weather at the moment, but it does seem that the replacement has been successful. The dashboard above for example shows both that the system has charged at full power for 4 hours overnight (3 kWh in) and also that 4.75 kWh has been saved (versus 4 kWh working capacity) as to a degree there’s some double-dipping going on with the two partial charges of more than half capacity (one overnight on cheap mains power and one during the day on surplus solar power).

Time will tell what the longevity of these cheap batteries is.

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Connecting to Home Connect

For some time now, I’ve been automating my wet goods (dishwasher and washing machine) using smart plugs to enable them to be started (or perhaps more accurately restarted) by the HEMS at the best times. However declining performance of the dishwasher prompted its replacement which now allows for improved capability.

The new dishwasher is actually quite similar to the the old one as I suspect Bosch and Siemens brands are different sides to the same coin but has a few differences:

  1. It has a cutlery drawer rather than a removable canteen for cutlery which I quite like but does mean less height in the now middle drawer. That means in turn that taller glasses now need to be washed in the bottom drawer previously the preserve of larger plates and pans.
  2. It is more energy-efficient using water-activated zeolite technology for drying.
  3. It’s a connected device using Home Connect. Home Connect provides an app to configure and receive notifications from devices, but also an API. Over the next few weeks I plan to use this API to my HEMS.

It should be relatively straightforward to replace the existing script file for the smart plug with one to activate the dishwasher directly via Home Connect.

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AA Breakdown Cover Discount

I’ve had AA breakdown cover for many years. The AA provide the reassurance of fixing and/or collecting my car if it breaks down. The AA are currently providing a discount to those who sign up via this link.

AA provides 24/7 roadside assistance and will try and fix your car on the spot. You can even track the mechanic right to your side. Just tap our free app and help’s on the way. 

  • Call AA out as many times as you need to, as long as it’s not a recurring problem with your vehicle
  • AA covers vehicles of any age including cars, electric vehicles, vans, motorcycles, campervans and caravans
  • Get up to 25% off at over 1,300 restaurants, pubs and service stations with the app
  • If you have an accident, AA can support you with its Accident Assist service

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Electricity rates

It’s now 3 years since I became an Octopus Energy customer. Each month I’ve been noting the average unit rate from my electricity bill. These average rates vary month-by-month as I’ve always been on a smart tariff where the price is different at different times of day, and thus my average rate varies depending on how my consumption is divided between different times of day

MonthYear 2018/19Year 2019/20Year 2020/1Year 2021/2Year 2022/3
November14.27 p/kWh8.96 p/kWh6.69 p/kWh7.37 p/kWh12.75 p/kWh
December10.17 p/kWh6.51 p/kWh9.61 p/kWh7.66 p/kWh14.26 p/kWh
January6.86 p/kWh9.74 p/kWh8.32 p/kWh15.36 p/kWh
February5.59 p/kWh11.18 p/kWh6.93 p/kWh11.36 p/kWh
March5.02 p/kWh8.85 p/kWh6.88 p/kWh
April8.35 p/kWh8.74 p/kWh7.36 p/kWh
May4.48 p/kWh11.50 p/kWh6.36 p/kWh
June7.03 p/kWh3.49 p/kWh12.52 p/kWh
July8.20 p/kWh4.40 p/kWh15.60 p/kWh (part)
End Agile / Start Go
5.92 p/kWh (part)
10.09 p/kWh (new Go with 7.5 p/kWh overnight)
August6.29 p/kWh (part)
5.99 p/kWh (balance)
6.15 p/kWh6.19 p/kWh 11.82 p/kWh
September5.63 p/kWh7.62 p/kWh6.30 p/kWh11.07 p/kWh
October6.86 p/kWh6.97 p/kWh6.83 p/kWh13.76 p/kWh

For most of this time I was on a very complex tariff called Octopus Agile which is directly linked to wholesale prices and, while historically that has been very good value, rising wholesale prices recently have caused that tariff to get increasingly expensive and so I’ve switched to Octopus Go which provides a 5 p/kWh inc-VAT night time rate for 4 hours. This is ideal for charging our electric cars and also the home battery if the next day’s solar production looks as if it will be limited.

Both of these tariffs are so-called smart tariffs enabled by smart meters. Some people can be very negative towards smart meters and indeed smart tariffs, but based on my experience it seems to me that if you have some flexibility to move electricity consumption to off-peak periods these can be excellent value for money.

I currently use the FiT revenue from my solar panels (about £700 annually) to offset my energy bills and am left paying £20/month towards household energy. I also earn a little income from referrals.

If you fancy an energy company that can provide excellent value for money, has good customer service, and that’s been recommended by Which? magazine for years why not switch with this link and earn an additional £50 credit?

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Making smart choices – smart tariff smart comparison revisited

Back in February I previewed a new online tool to help consumers choose an appropriate smart tariff in Making smart choices – smart tariff smart comparison. There are numerous price comparison sites that work for standard energy tariffs, but smart tariffs are generally excluded from such sites. Price comparison sites typically ask for a token meter read or guess consumption based on typical bills for similar homes, but this online tool takes actual half hourly consumption from your own electricity meter so its analysis is very sophisticated in comparison. As a research tool the tariff names are anonymised, although with a little thought they can be decoded.

Prior tariff versus nearest cost alternative

The above graph shows my prior Octopus Agile tariff being very competitive for an extended period but then losing top spot in January 2021 to an Octopus Go Faster tariff. Subsequently I’ve moved to a related tariff called Octopus Go (without the ‘faster’). The Go tariff offers four hours of electricity overnight at 5 p/kWh for a fixed period while the ‘faster’ derivative offers 5 hours at a slightly higher price and with choice of the discounted hours.

New comparison 6 months later

The newer graph above shows a widening gap over the last three months with the Go tariff being increasingly advantageous as rising wholesale prices force Agile pricing higher and higher. Since Agile is linked to the daily wholesale markets the price can rise (and drop) very quickly. Go on the other hand is not just cheaper but fixed for a year. July’s analysis shows Go being less than a third of price of Agile for my usage.

Go is ideally suited for EV charging. You could also get a sign-on bonus of £50 by clicking this link to move to any Octopus tariff.

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À la mode

I just made a change to the way my smart central heating controls works.

An example room with prior controls

Previously I had four modes:

  1. (Enable for) Summer
  2. (Disable for) Winter
  3. (Going on) Vacation
  4. Back from Vacation

These modes were the defaults from the Eve Thermo Electronic Thermostatic Radiator Valves (eTRVs). However I’ve thought for some time that there was some ambiguity around what mode the system went into when Back From Vacation was selected (Winter or Summer?) and that it would be more straightforward to have 3 modes as follows:

ModeTemperature measurementTemperature controlTemperature
set Point
SummerYesNoNot specified
Proposed new smart heating control Modes
An example room with current Modes

I think that this new arrangement is much more intuitive with the user just selecting which Mode they want to enter at the end of a vacation without the ambiguity of selecting Back from Vacation and then quickly following up by selecting Summer or (most likely) Winter.

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Heated debate

So here’s the question – is it better to heat the home continuously or to only heat it when you need it? You may think that’s obvious as I did but a recent debate elsewhere suggests that there’s a variety of opinion out there, so let’s explore the relavent issues:

  • Heat loss from the building
  • The efficiency of the heating appliance (boiler, heat pump etc)
  • The degree to which the building acts as a thermal store.

Heat Loss from the Building

Heat loss from the building

Whether we consider walls, floors, roofs or windows everything around the boundary of a building contributes to heat loss. However well insulated there will be a flow of heat through the structure (let’s assume brick for the moment). Heat will travel from hot places (usually inside the building) to cold places (usually outside the building). Insulation can help reduce the flow but the flow still makes place. One key way to reduce the flow is to reduce the internal temperature so, for example, if the outside temperature is 10 C and you reduce the internal temperature from 20 C to 19 C then you’d expect to reduce the loss by 10% – was driven by a 10 C temperature difference now driven by a 9 C temperature different which is 10% less than 10 C.

I’m an advocate of switching heating off when not needed – typically overnight when in bed asleep or during the day when out at work. Turning the heating off obviously immediately cuts the energy use, the temperature of the house then starts to drop as the heat flows out through the walls and other components of the boundary, the energy flow gradually reduces as the internal temperature drops towards the external temperature, and eventually the flow stops if the internal temperature reaches the external temperature.

The question is then how much heat energy does it take to get the temperature back up to a comfortable level and potentially are there issues with efficiency of the heating appliance.

Efficiency of the Heating Appliance

Any heating device will have an efficiency at which it coverts energy in the fuel to useful heat output. Typically these are less than 100% as some of the fuel energy ends up as waste energy rather than useful heat. The exception to this is a group of electrical heating devices known as heat pumps. Heat pumps as their name suggests pump heat from one place to another – typical from the outside air or under the ground – so as they heat the house they also cool the source and thus their efficiency is more than 100% since much of the heat output comes from the source being cooled and not from the electricity supply. However I’m going to consider a condensing gas boiler as one of the most common domestic heat sources in the UK.

Boiler temperature controls

A condensing gas boiler is more efficient than prior generations of gas boiler as it uses waste heat in the flue gas to preheat the water before it gets heated in the conventional main boiler. To get the preheating to work efficiently then the flue gas has to be hotter than the return water.

Typically the user sets the temperature if the hot water generated by the boiler (which may vary seasonally) – the flow temperature. The hot water is then pumped round through the radiators causing the radiators to heat the rooms and the water temperature to drop, the return water is preheated by the flue gasses and then heated in the boiler itself before going round again. Best efficiency is obtained by maximizing the extraction of heat from the flue gasses, which typically requires the following conditions:

  1. The flow temperature is below 70 C
  2. The return temperature is below 55 C
  3. The difference between the above is around 20 C

There are arguably two typical uses of the boiler for space heating: (i) relatively low demand to maintain the current temperature and (ii) relatively high demand to get the building to the required temperature. The latter would be expected to push the return temperature lowest and thus most likely to condense, so if the size of the load makes a difference at all then you’d expect high load to be more efficient. That’s good news for those who tend to turn their heating off at times, as the resulting peak loads shouldn’t have lower efficiency than continuous use.

Thermal Mass

The thermal mass of the building is its ability to store heat. Storing heat alone over time doesn’t really add to the energy requirements to heat a home, but it can make a home slower to cool down or faster to heat. You might imagine a stone cottage for example having a relatively high thermal mass. On the other hand a modern house have a stud walls internally (and even externally if it’s a wood-framed home) which have very little thermal mass. At the other extreme I’ve seen experimental homes with so much mass (such as earth banks) that it can take over a year to warm up.

The combination of the amount of thermal mass and the amount of insulation will determine the time constant associated with heating and cooling or how long it takes for the structure to warm up.

In extreme, if your thermal mass is enormous, then you may be able to heat your thermal mass enough in summer to heat the house in winter but you’d need to actively heat the thermal mass in summer – such as by diverting the output from solar panels – just having a big thermal mass without actively heating it would leave the mass at an average annual temperature likely below what is comfortable in winter.

How thermal mass works

So where does that leave me with the question of turning off my heating when not required? Well, it would seem that (i) a reduced internal temperature would reduce heat losses while the heating is off which should mean less heat input to get the air temperature back later and (ii) running the boiler at relatively high load to get the temperature back isn’t an efficiency issue as high demand promotes a low return temperature.

What about experimentally?

Effects of turning the heating off overnight

The above graphic shows a very typical winters day with the home occupied. The blue upright bars are half-hourly gas consumption readings from my smart meter. The blue bars show six hours of no heating overnight, an hour and a half of relatively high demand to heat the house up to temperature, and then lower demand (generally to maintain the temperature) through the day. The orange dashed line shows the typical level of heat input required to maintain the temperature – 2 kWh/half-hour. The green box shows the period overnight with no heating. If the home had been heated during this time that would have been 6 hours of heating at 2 kWh/half-hour = 24 kWh of additional heat input. The red box shows the actual heat input required to reestablish the comfort temperature – 2 kWh + 6 kWh + 4 kWh in each half hour respectively = 12 kWh. Over all I saved around 24 kWh – 12 kWh = 12 kWh overnight and more if the heating had been off during the day too. Since the data is taken from the incoming gas supply then the above savings include the effects of any variation in boiler efficiency between low heat demand to maintain temperature and high heat demand to establish temperature.

It thus seems that turning off the heating when not required saves energy compared to running the heating at a constant temperature both theoretically and experimentally.

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Smartening up

We will shortly have been in this house for six years. During that time I have created three smart control systems that improve my energy costs or efficiency:

  1. heating controls to minimise gas purchase
  2. self-consumption controls of the electricity generated by my solar panels to maximise value of self-consumption
  3. smart tariff controls to buy grid electricity at the lowest price

Heating Controls

Most homes have a single heating zone with one timer and potentially one thermostat controlling the whole house with perhaps some thermostatic radiator valves (TRVs) capping the temperature in specific rooms.

By contrast we have seven heating zones created by electronic temperature control valves (eTRVs). Each zone has its own timer. There is no central timer or thermostat. Each eTRV can summon the boiler on when cold rather than simply cap the maximum temperature like a TRV.

Some rooms also have links to other smart devices such as disabling room heating when the window is open or turning off heating early when there’s no movement in the room.

The intent is to save energy by only heating rooms that are in use.

Self-consumption Controls

These controls manage the diversion of any excess output from my solar panels rather than give that energy to the grid. The loads are prioritised as follows:

  1. Powervault storage battery (fully proportional)
  2. Car charger (stepped proportional driven by ImmerSUN relay output)
  3. Hot water (driven by ImmerSUN fully proportional output)

Last year these controls helped me to use over 90% of the output of my solar panels avoiding buying £100s of electricity and gas. The priorities are set to maximise value – #1 avoid daytime electricity use at 16 p/kWh, #2 avoid car charging at 5-16 p/kWh, and #3 avoid gas consumption at 2.96 p/kWh.

Smart Tariff Controls

These controls manage my electric devices for lowest grid energy cost. The controlled devices are:

  • Battery storage (Powervault)
  • Dishwasher
  • Electric car charger
  • Hot water heating (ImmerSUN)
  • Washing machine

The hardware that has this control is known as a Home Energy Management System (HEMS). My HEMS is based on a simple computer known as a Raspberry Pi. The HEMS uses foreknowledge of the electricity price and predicted solar panel output to determine when best to run the above devices. It was designed around a tariff called Octopus Agile which has 48 half-hourly prices that change daily, but is currently working with a simpler two-rate smart tariff.

deviceCentral Heatingself-consumptionsmart tariff
Battery Storage#1X
Central Heating BoilerX
Electric car charger #2X
Hot water heating#3X
Washing machineX
Devices controlled by smart systems

Most of these solutions are made up of commercially-available items that I have perhaps combined in a way not anticipated by their manufacturers. In particular:

  • I created a relay module to enable the gas boiler to be turned on remotely and programmed a series of logical rules for the Apple TV’s that act as the controllers.
  • I identified a way to prioritise different self-consumption devices by configuring their current clamps.
  • I built my smart car charger integrating various items of hardware and writing the ladder logic program that runs it.
  • I built the HEMS from commercially-available parts and wrote the software that runs on it to control my devices.
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