One of the features of the way my HEMS works is the inverse relationship between electricity demand and electricity price – electricity demand increases as the price falls and falls as the price rises. This effect helps to contribute to my low average cost for electricity – 5.63 p/kWh ex-VAT most recently (Sep/Oct 2019). The effect exists because my different controlled loads (battery, car and water heating) each need different times to achieve full (approximately 8, 5 and 2 hours respectively).
This chart is illustrative only because day-to-day needs for the different loads may change, and also the electricity price varies half-hour-by-half-hour and day-by-day but typical recent behaviour is shown. The actual rules for each device may be summarised as follows:
House baseload – support house from battery when cost greater than 10 p/kWh (limit adjusts automatically to reflect pricing on the day), otherwise take baseload from grid.
Storage battery charging – charge from the grid in cheapest 5 hours over entire 24-hour period.
Car charging – charge for a total period of so many hours within a window between arrival time at home and departure time the next day, and also any hours during day at equivalent price or lower.
Water heating – charge for the cheapest 2 hours when electricity price falls below gas price (did not occur on day illustrated below).
These same loads may also be enabled in priority order when the solar panels are in surplus. Priority is:
One question that gets asked periodically is how green are technologies like solar panels given that there much be some emissions created in their manufacture and ultimate disposal.
It’s probably no surprise that the rooftop solar PV is considerably less carbon-intensive than any fossil fuel (1/20th of coal for example) but that it beats biomass or even utility scale solar (I.e. solar farms) may be a surprise. My understanding is that the recent switch of emphasis in the UK from rooftop solar (with elimination of FiTs) to solar farms is driven more by investment costs than ultimate carbon-intensity.
Source: Intergovernmental Panel on Climate Change (IPCC) as quoted by Spirit Energy
The height of summer is perhaps an odd choice of time for a post about heating, however we’ll get to that. Regular readers may recall that our smart heating controls enable the boiler when any radiator demands heat via its smart valve (eTRV) and disables the boiler when the last room is up to temperature. That rather begs the question what happens to the control logic when the batteries go flat. The HomekIt rules are not sufficiently sophisticated for the author to set that behaviour via the program, and I’ve seen no default behaviour described on-line.
Today the inevitable happened and a battery did go so low as to stop operation. The behaviour of the heating was to force the boiler on. Most rooms did not heat up as their own eTRVs recorded them already being sufficiently hot, although the bathrooms and cloakrooms did heat up (they generally lack eTRVs) as indeed did the room with the flat battery (my daughter’s playroom). Other symptoms included an icon in the Home app that would not grey out when disabled like other room eTRVs and the the boiler repeatedly being re-enabled even after manually disabled through the app.
Replacement with fresh batteries immediately restored normal operation.
Although any more heat input is unwelcome on a day as hot as today, we have at least demonstrated that the system is failsafe in a flat battery condition – I’d rather that the system heated up with a flat battery to prevent freezing damage in winter at the cost of some discomfort in summer due to excess heat.
It’s 265 days since my records indicate an earlier battery change, albeit for most of that time the heating hasn’t been on (so no power needed for valve movements). It’s over a year now that I’ve been using Ni-MH cells in these valves. The cells are slightly lower voltage than the recommended cells (1.2 versus 1.5 Volts) so do create spurious low battery warnings, but apart from that they seem to work well with adequate life before recharging. I thus anticipate continuing with their use of a means of reducing battery waste. I’ve had no cell failures to date.
Yesterday provided a good example of my HEMS in action as the electricity price dropped quite low due to stormy weather conditions. Normally at this time of year the HEMS isn’t doing much with the storage battery as daytime solar output is enough to fully charge the battery, but yesterday low pricing was enough to automatically enable both battery charging and water heating overnight. Car charging was due to run anyway driven by the demand for an hour of charging, but battery charging and water heating was triggered by the low price rather than a needed to take power for a pre-defined period of time.
The screenshot above from my phone shows the HEMS’ plan for the the early hours of the 9th. The first price column shows one hour of car charging at the cheapest price. The second column shows half an hour of water heating as the electricity price has fallen below 3.5 p/kWh when it is assumed to be cheaper than gas. The third column shows four hours of battery charging when the electricity price is below 5 p/kWh.
The above image from the HAN side of my smart meter shows the energy consumption of the house varying through the night in response to these requests from the HEMS – battery charging at the widest point, car charging above that for an hour, and water heating above that for 30 minutes.
Finally this image shows the energy consumption versus price data for the same period shows how the action of the HEMS increases electricity demand as the price drops. Indeed on this day there was virtually no consumption at any other time.
For August 9th as a whole I paid 52 pence for 7.547 kWh of electricity. Taking off the 21 pence for the standing charge leaves 31 pence for the electricity kWhs alone, an average of 4.11 p/kWh.
I recently signed up for a free home energy survey. After various questions about my home – so not much of a survey – my visitor alighted on spray foam insulation in the loft as a technology that should be of interest to me. Claims included:
Spray foam insulation would dramatically reduce my heating bills (“save upto 50% on future fuel prices”) – a sum amounting to hundreds of pounds of savings annually.
Spray foam insulation would result in a better EPC rating and thus increase the resale value of my home ( “.. Eco measures are adding up to 14% on property values.. “).
Given that the salesman attached some credibility to the UK’s national EPC scheme which requires an assessment of each home before sale as evidence for the buyer, then a good start point would be to see what the EPC said about my loft insulation. According to my most recent survey from September 2015 (so almost 4 years old) my home, at that time, had 100 mm of loft insulation between the floor joists with a recommendation to increase to 270 mm thickness by placing another layer of insulation across the top of the floor joists which would reduce my energy consumption by £219 over 3 years. £219 over 3 years amounts to £73 annually. In fact, since the survey was completed, I added the additional insulation recommended in October 2016 so that saving has been made and 1 point improvement on the EPC scale obtained. According to Which? magazine quoting the National Insulation Association “100mm of spray foam insulation is equivalent to around 170mm of loft insulation” so my 270 mm of conventional insulation may provide more insulation than the proposed spray foam insulation. My £73 annual saving from the EPC is far from the hundreds of pounds annually being claimed by my guest. Thus I don’t think the claimed savings are justified.
Then there’s the question about what the impact on my EPC rating would be. The extra 170 mm added post-2015 survey added 1 EPC point to my house rating. That’s not a big increase.
Then there’s the question about what the value of that 1 point on the EPC rating would be. According to the EPC certificate itself the indicative cost of adding 170 mm of insulation is £100 to £350 pounds. I actually paid £300. It’s not reasonable to believe that any additional value added to the home exceeds the cost of doing the work for such a small scale job. The salesman actually left a written claim that “Reports now showing that Eco measures are adding up to 14% to property prices” – I suspect that you’d need a lot of Eco measures (well beyond loft insulation) to add 14% to property prices which would be around £70,000 for my home!
Then there’s the overall question of the cost. Bearing in mind that my current insulation cost £300 and may be equally if not more effective than the spray foam then what was the spray foam quotation for? Discounted to £7,658 with marketing testimonials after the usual call to his boss to get “approval”. Which? reckons £2,500 – £3,000 for a 3-bed semi, whereas I am blessed with a substantial 4-bedroom house so it might conceivably be double the price; but for any investment on that scale I’d want to see competitive prices not just one.
Finally the question arises if I really wanted to spend £7,658 on energy efficiency would more loft insulation be best value? Our old friend the EPC has a proposal in that sort of price range, and one that it believes delivers greater savings. And that proposal is.. floor insulation for my solid concrete floors. The EPC suggests that this can save £132 annually – 81% greater savings than that extra 170 mm of loft insulation albeit at up to 20 times the price. However I already ruled out floor insulation on grounds of poor return. (My guest wasn’t persuaded as to the value of floor insulation, but a check of EPCs on similar neighbouring properties by different surveyors all identified floor insulation as a greater benefit than more loft insulation)
So for me the answer is clear – I don’t see any value in spending £7,658 on alternative loft insulation. For you the answer might be different – you might be planning to convert the loft into dwelling space in which case you want insulation against the tiles/slates rather than at floor level. However if you just wanted storage space then other solutions like a shed, storage unit or skip (!) could easily prove to be better value.
Today I’ve further refined the wiring of the relays on the HEMS. At the time that I’d originally wired it I didn’t have small enough flex, or indeed multi core, which created an unnecessary number of cables (one per used relay) of over large size (and thus difficult to insert into the terminals). During the week I acquired some smaller gauge multi core allowing me to wire all three relays with a single cable containing one live feed and three switched live returns.
Of the 5 incoming / outgoing cables at the bottom (left to right):
Incoming mains (live / neutral / earth) from mains plug
Live and switched live to / from ImmerSUN output relay to activate car charger.
Live and switched lives to / from HEMS to activate car charger and water heating.
Red – live to contacts
Green – switched live for car charger direct – charge in response to price
Black – switched live for car charger indirect via ImmerSUN relay output – enable proportional charge in response to surplus PV
White – switched live for water heating – heat in response to price
Outgoing mains (switched live / neutral / earth) to RF solutions radio transmitter to activate car charger, and on the second cable clamp..
Outgoing switched live and neutral to ImmerSUN Boost relay input to enable immersion heater.
The revised wiring diagram looks like this..
All of this still leaves one unused relay on the HEMS (HAT #3) and one unused proportional output on the ImmerSUN (#2; available for future expansion.
Initially even my smallest boot lace ferrules would not fit into the terminals on the HAT. Fortunately, once the ferrules has been crimped around the new cables, and flattened by squeezing in pliers, then the ferrules could be persuaded into the terminals.
I’ve been on my dynamic smart tariff for some months now, so I thought it would be a good time to see what I’m actually saving. My actual tariff rate changes each half hour, but for the purposes the supplier calculates the weighted average of what I’ve actually paid for the invoice. In principal that should be monthly, but I’ve had some bills combined over more than one month.
6.29 p/kWh (part)
5.99 p/kWh (balance)
Over the course of the last few months my electricity price has reduced very significantly. I suspect that this is down to a combination of several factors including:
With the development of my HEMS (including its control of the battery storage) I’m getting slicker at optimising my purchase price
As we move into the summer the energy price is dropping with reduced demand and more renewable power available.
For reference the Energy Saving Trust reckons that the average UK price for electricity is 15.75 p/kWh on a flat rate tariff, or 19.0 on days and 9.1 p/kWh on nights for Economy 7. Thus my average electricity price in a month always beats their day rate and often beats their night rate. Of course the EST figures are the average market rates, so both better and worse deals will exist with different suppliers.
The 7.03 p/kWh for June 2019 seems to have been received with incredulity elsewhere so here’s the relevant part of the bill..
I’ve never felt so engaged with my electricity supply, and am very pleased to have made the move from the dual-rate Economy 7 tariffs that I’ve used for around 30 years.
After a series of quite detailed posts, I think that the time has come for an updated high level overview of what we have.
We moved to our early 1970s house almost 4 years ago bringing with us our electric vehicle. The house had already been refurbished with new double-glazed windows, had cavity insulation (although that wasn’t recorded on EPC so must have predated the prior owners), and a token level of loft insulation. The existing gas boiler was arthritic, couldn’t heat the whole house, but was quite good at heating the header tanks in the loft! We had gravity-fed gas hot water (i.e. no thermostat or pump on the cylinder) which was completely obsolete, the cylinder dated back to the building of the house and had no immersion heater (although we had the wiring for one). So what did we do?
We substantially increased the loft insulation to reduce heat loss.
We had a modern condensing gas boiler installed to improve efficiency.
We updated to smart controls using eTRVs to set both temperature set points and schedules at room level. I built a smart interface to the boiler so that heating can be enabled remotely. I programmed a series of rules into Apple Home allowing the smart thermostats to enable the boiler when any thermostat wants heat and disable it when no thermostat wants heat. Some rooms also have additional rules linking heating to open windows or movement sensors. All of this reduces heat losses by only heating rooms that are (or will be shortly be) in use.
We installed our own solar panels given 4 kWp generation. (I also own a small share of a solar farm although there’s no contract that I’m aware of between that farm and my home energy supplier)
I invested in an immerSUN to maximise self-use of our own solar by enabling loads when surplus solar is available.
We switched to a green electricity supplier so when we need to buy electricity it comes from renewable sources.
We bought a small storage battery 4 kWh to store some of our solar production for use later in the day. Subsequently I can also use it in winter to buy when the electricity price is relatively low to avoid buying when the price is relatively high.
We chose a dynamic smart tariff to buy electricity at the lowest price based on market prices established the day before. The prices change each half hour and are established in the late afternoon on the day before.
We replaced the old hot water cylinder with a modern insulated one (to reduce heat loss) with a low immersion heater (to allow more of the water volume to be heated).
Our principal water heating is now by diverting surplus solar electricity proportionately to the immersion heater, that’s backed up by the gas boiler which is enabled briefly in the evening for water heating in case the water isn’t yet up to temperature, and when the electricity price falls below the gas price I can enable the immersion heater on full power.
All accessible hot water pipes are insulated.
Electric car charger:
I built my own electric car charger that takes an external radio signal to switch between four settings 0, 6, 10 and 16 Amps to help me adjust consumption to match to availability of output from my solar panels. (Subsequently such products were developed commercially with continuously variable current limits, but the limitations of my immersun and on/off radio signal don’t allow me to go quite that far. Having said that my car only does 0, 6, 10 and 14 Amps so I would gain no benefit from a continuously-variable charger paired with a 4-level car).
Smart electricity controls:
We have two systems for smart control of electricity:
The immersun to maximise self-use of our solar electricity by proportional control of loads.
A HEMS to manage the purchase of electricity (when necessary) at the lowest price by maximising consumption when the price is lowest.
When both systems want to enable loads (because the bought price is low and we have a surplus from our own panels) then cost is prioritised, so we’ll buy from the grid any demand not being met from our own panels.
Both systems are linked to 3 devices:
Battery storage. The immersun is configured to work alongside the battery storage with the battery storage as the top priority to receive surplus solar PV. The HEMS can switch the status of the battery as required to charge from the grid when the price is lowest, or to discharge when the price is highest, or indeed to revert to default behaviour.
Car charger. Second priority for the immersun after battery storage.
Immersion heater. Third priority for the immersun after car charging.
I have no firm plans for the future. I’m toying with adding to the HEMS various features including:
Making the display switch between GMS and BST as appropriate (it’s all UTC at the moment).
Edit configuration via the web interface rather than a virtual terminal.
Control a domestic appliance. Our washing machine was replaced relatively recently, but the dishwasher is playing up a little and may be a candiadte for HEMS integration where the optimum start time is selected to deliver lowest energy price.
Late last year I started adding the ability to optimise my electricity price by shifting some electrical loads around in response to a dynamic electricity tariff. My electricity price changes half-hour-by-half-hour and day-to-day, with the prices for the day ahead published each afternoon. I already had the ability to manage the same electrical loads to maximise use of the output of my own solar panels for some 3 years. The first load smartly controlled to follow my electricity costs was my electric car charging, but I have subsequently added optimisation of water heating and storage battery behaviour.
However, time has revealed an occasional issue arising when both the bought electricity price was low and a solar suplus was available, so both sources sought to enable the car charger; but in practice the vehicle didn’t charge. The issue here is that the car charger does a sanity check on the radio signal indicating that it should be operating, which fails since the combination of two signals driving a common radio transmitter can lead to excessive duration of the ‘on’ signal which fails the sanity check.
My chosen solution is to disable one of the two signals sources when the other wants the car charger on. I’ve chosen to make the price signals via the HEMS the master, so when the HEMS wants to charge the car HAT #4 opens so that the ImmerSUN no-longer has influence over the car charger, and HAT #1 is used to control car charger behaviour. When the price is relatively high HAT #4 remains in the normally closed position and HAT #1 is open allowing the ImmerSUN to control charging behaviour via its output relay to use any surplus solar. (HAT refers to HArdware on Top – accessory circuit boards that mount on top of a Raspberry Pi. In my case board with 4 output delays. HAT #3 is currently unused.)
As regular readers will know, I shift my major loads (including battery, car and water heating) around to exploit the cheapest periods on my tariff. Yesterday I had a conversation with someone who was very keen to do the same with a fridge. I’ve never considered doing this a fridge on two grounds:
Is it safe to mess with a fridge or freezer if that risks compromising the temperature and thus the safe storage of food?
Is there enough saving to be worth even considering this?
Although I don’t explicitly measure the power consumption of the fridge, we can make some assumptions from data that I do have.
According to the US Department of Health, a fridge can be safely left for 4 hours during a power cut, but you should avoid opening the door. That period at least corresponds to the duration of the early peak (and thus potentially the highest cost power), but I still don’t think that this would be worth the effort.
The above illustration shows my home drawing 168 Watts overnight. That load is all the standby loads in the house (TV, DVD, oven, microwave etc), assorted phone and iPad chargers, cordless phone power, WiFi router, alarm clock, central heating controls, smart plugs and the fridge-freezer.
For a couple of periods overnight the consumption dips to around 114 Watts, a difference of 54 Watts. I can think of nothing on my list of loads that might cycle on/off except the fridge-freezer so potentially that’s the near-continuous 54 Watt load. It’s conceivable of course that the fridge-freezer load cycling between two levels, but the lower level cannot be more than 114 Watts. 114 Watts is 4 p/hour at my highest electricity cost of 35 p/kWh or 1 p/kWh at my average rate of 9 p/kWh. This doesn’t seem a worthwhile saving to me.