Last August we took a look at some of the decisions that led us to build a low energy home. Now that we’ve been through our first full calendar year, it’s time to reveal our energy bill and see how things are working out. I have separated out figures for the heat pump versus everything else. But first a bit about our setup.
As an all-electric house, it was important to move to an Economy 7 meter which happened a few months after we moved in. This gives us separate Day and Night rates for our electricity. Unfortunately smart meters are not available in Northern Ireland yet so there are currently no options for half-hour metering, demand side response or agile tariffs.
The best thing we can do then is to make sure as much of our energy usage is pushed into the night rate, a 7 hour window which runs from 1am to 8am while we’re on GMT and 2am to 9am while on British Summer Time.
All our electric vehicle charging is done on the night rate and our Siemens smart appliances include a Delayed Start function which has allowed us to set our washing machine and dishwasher to run during the cheaper rate too (we rarely need to use the dryer as clothes dry easily in the warm atmosphere in the house).
The graph below shows almost a month’s worth of data from the excellent OpenEnergyMonitor (with more sensors there’s a specific heat pump measuring system available too, check out this video).
The dark blue lines here represent the amount of energy we used on the cheaper rate, and it’s easy to spot the 5 nights that the EV was charged during this period.
Across the year an average of 36% of our consumption was at the night rate, with the peak in December at 48%.
Pricing & Loads
We are on a 100% renewable tariff and while the electrons from the wind farms aren’t actually flowing to our house, I believe choosing a tariff like this encourages the generators to invest in more renewables. And as we’re running our heat pump, appliances as well as charging the car at night, we genuinely are using lots of wind power. Around 50% of all electricity generated here is renewable already, with targets for 80% by 2030, so just like with an electric vehicle, an electrified heating system will get cleaner and cleaner over time.
Here are the details of our current fixed rate tariff (including VAT);
- Day Rate £0.1699 / kWh
- Night Rate £0.0784 / kWh
- Standing Charge £3.54 / Month
On top of the normal fridge and freezer loads (both A++ appliances), we also have lots of security cameras, a 19″ rack with a server, 48 port PoE Switch, NAS, 6-Zone Whole House Audio amplifier, and MVHR (even our septic tank has a 24/7 draw), so our base load is high. The lowest figure I’ve seen from our energy monitor is around 725 Watts, but more usually the base line seems to sit around 900 Watts. So that’s around 22kWh per day before we turn anything else on. I’d like to look into this base load in more detail this year and try and reduce it where possible.
Heating, DHW & Controls
The house is block construction with a 150mm cavity, double glazing and has a B85 EPC rating which should hit an A once we add a solar PV system. Heating is from this 11kW Hitachi Yutaki M Monobloc Air Source Heat Pump which is sited at the rear of the house at the north facing garage gable wall.
Our Loxone home automation system controls the heat pump via a TCP/IP Modbus Gateway (ATW-MBS-02). Both upstairs and downstairs use Roth underfloor heating fed from three manifolds around the house, using Loxone valve actuators.
Loxone brings three, core heating / climate control functions to the system. The Intelligent Room Controller is accessible from the UI, and is used to view the current room temperature, change the set points and timing etc. Then the Intelligent Temperature Controller works in the background, grouping our rooms and UFH loops, controlling the flow temperature required in order to achieve the temperatures demanded by the room controllers. Finally the Climate Controller turns the heat pump on and off, while also considering the outside temperature readings from our weather station. The heat pump itself then does its own decision making around when to power up to boost the flow temps as required.
Inside the house, every Loxone light switch has a built-in temperature (and humidity) sensor which we use as the room thermostats. The Loxone system can average values across multiple sensors which is handy in the open plan barn for example.
The Loxone system has been programmed to heat the house more on the night rate too, using the thermal mass of our block construction, tiled floor and internal stone wall to release the heat during the day. We have experimented with some of these strategies over the last year, tweaking things to move as much as possible into the night rate, whilst maintaining comfort levels during the day.
The heat pump also provides our hot water into a Joule 300 litre DWH (Domestic Hot Water) tank which is installed below the stairs. There may be a little energy in the ‘Everything Else’ part of the data below, for the rare occasion that the system turns on the immersion heater if the heat pump cannot refill the tank quickly enough.
Water heating uses a surprising amount of energy and warming 300 litres from an ambient of say 16° to our 45° target uses over 10 kWh. The heat pump’s COP (coefficient of performance) means we get around 3 units of heat for every 1 unit of electricity consumed, so we would only use around 3.3kWh in this example. A ground source heat pump has an even higher COP than our air source unit.
The system temporarily raises our hot water target temperature to 55° every morning at 6:00am, while we are still on the cheaper night rate, getting things ready for the morning showers. The target is then set back down to 45°. The stainless steel tank is rated to lose around 1° every 4 hours so, if unused, should take almost 40 hours before it cools back down to our normal 45° target.
The Loxone system also allows us to monitor the status of many of the heating’s parameters, including the compressor’s power, pump speed, flow rate, alarm status, outlet and inlet temperatures, DWH temperature, as well as the positions of the 3 port valve and motorised valves on the manifolds. It’s still a work in progress, and we need to get a better graphic, but it has proved to be very useful.
Annual Heating & DHW Usage & Costs
While we have exact energy usage figures for the heat pump, I currently have no easy way to break down what was on the night rate versus day. So I have used the annual average split in our bills for this (64% Day, 36% Night). This should be the worst case scenario as the heat pump is biased to run over night more than through the day.
- Total Annual Heating & DHW Energy Usage – 5,345 kWh / £732.05
- Monthly Average – 445 kWh / £61.00
- Daily Average – 15 kWh / £2.01
It’s worth pointing out that I began recording this data around 6 months after we moved in. Don’t be disheartened if you are seeing high energy costs with your heat pump and you’ve just finished building, as often energy consumption can be higher as the house dries out. Likewise, don’t judge your heat pump bills from a single month as there is a large variation across the year. See the right hand column in the table below for our breakdown.
In terms of single days, the highest energy consumption for the heat pump was 27th November at 39.4 kWh, while the lowest was an order of magnitude less at 3.9 kWh on 16th July.
Total Annual Energy Usage & Costs
The table and graph below shows our total energy used last year in kWh, broken down into 2 values, separating the energy discussed above that was used by the heat pump for space heating and hot water, from everything else.
You will notice an increase in consumption in March where you would have expected it to fall as the weather continued to warm up. This was because our son came home from uni and our daughter and son-in-law came to live with us too (between house moves). So we went from 2 adults to 5 overnight, with all the associated extra energy consumption (damn that PS4).
So here are the energy consumption figures for the first full year in the new Automated Home;
|% of Annual Heating & DHW|
- Total Annual Energy Usage – 16,276 kWh / £2,273.60
- Monthly Average – 1,356 kWh / £189.47
- Daily Average – 45 kWh / £6.23
Remember, we aren’t burning anything, we buy no gas, heating oil, coal, wood, petrol or diesel. So that £189 (average) is our single monthly bill for all our heating and hot water, all our cooking and washing, all our Lighting and entertainment plus our ‘fuel’ for the Electric car.
Comparisons to Last House
The latest figures I could find showed that 68% of homes in Northern Ireland use oil as their primary method of heating (and amazingly 72% still use solid fuel as a secondary method). In our previous home, 900 litres of heating oil would last us around 4 to 5 weeks over December / January. The current cost for that one fill is around £440 – over half our annual heating cost in the new house.
The old house was a similar size, 275 sq m / 3,000 sq ft (including the integrated garage) and while I don’t have detailed data, I do have access to my old bills and they show we averaged 5 x 900 litre fills of heating oil each year. In energy terms, that 4,500 litres represents approximately 46,500 kWh and on top of this we used about 10,500 kWh of electricity annually too, giving us these comparative totals;
- OLD Total Annual Energy Usage – 57,000 kWh / £3,985
- OLD Monthly Average – 4,750 kWh / £332.08
- OLD Daily Average – 156 kWh / £10.92
In monetary terms then, we are spending almost half what we did in the last house. But the savings are only half the story, as it’s the increase in comfort levels that are most noticeable. The last house was built in the late 90’s and its traditional construction employed basic levels of insulation and no airtightness measures. The new house is warm night and day, in every room, we always have hot water on tap. It’s hard to quantify the quality of life improvement this brings. Stepping onto warm tiles on a bathroom floor in the morning is pretty, pretty good.
Perhaps more importantly, we have reduced our total energy consumption by more than 70%, along with all the associated CO2 savings and air quality improvements (check out this data from our Airthings Wave Plus).
The latest figures I can find (2019) show that 330 grams of CO2 was produced per kWh of electricity generated in Northern Ireland. Assuming a COP of 3.2 that means less than 100 grams of CO2 per kWh of heat. That’s around half the CO2 emissions of a mains gas boiler and around a third of an oil fired heating system (source). In fact our bias for running the heating at night, coupled with the extraordinary levels of spare wind energy in NI, means we may be doing better than that.
The heat pump has a 7 year parts warranty (3 years labour) so hopefully there will be no major surprises any time soon. However, maintenance is the one aspect of the equation that we have no data on and it needs to be considered.
Some of the negative heat pump stories I’ve read mention that the UK and Ireland have climates that are not suitable for air source units. We have not seen any of these issues. The pump may reverse on a few days a year to defrost itself but in general the efficiency of the system is superb (and we’ve seen temperatures as low as -6 so far).
Follow Jan Rosenow for some great factual information on heat pumps and the energy transition in general.
I’ve been gathering the data for this post over the last 12 months and the last third of 2021 saw incredible energy cost rises. Whether you are using electricity or burning gas or oil, prices have seen unprecedented increases over this time. While we’ve been lucky to be on a fix-rate tariff, this will end in the summer.
We had to cut our Solar PV plans during the build to stay within budget, but we hope to buy a system, perhaps this year, to try and off-set some of those price rises. Once installed, costs for the brighter months should be substantially reduced. A friend’s nearby 4kWp system produced 524 kWh in June 2021 for example, which (assuming the same 87% self-consumption rate as our previous home) would have almost halved our bill for that month from £164 to around £86.
A domestic battery is still an interesting option too, either to store solar production or buy cheaper electricity at night for use during the day. As energy costs rise, payback times for these technologies decrease. And it’s not always about the money either, for some there will be a moral imperative to try and further reduce their emissions. Using an EV for V2G (Vehicle to Grid) and V2H (Vehicle to Home) is going to be an interesting option in the future too.
Some UK blogs talk about receiving £400 a month from their RHI scheme as an incentive to buy a heat pump. However, Northern Ireland currently offers no such help to encourage homeowners to do the right thing. Let’s hope things change here in the near future
It’s important to point out that these savings are not just because we have an electrified heating system. The insulation and airtightness measures in the new house are vital to the success here. As is the involvement of the experts we employed – Daly Renewables and Epitome Living. Most of the complaints I read around heat pumps are from projects that did not have a properly designed or fitted system.
While there are plenty of more energy efficient homes than ours, we are delighted with the performance of the building and its cleaner, quieter, low carbon heating system. We’re enjoying a much warmer, more comfortable home with endless hot water, for around half the running costs of the oil fired house, all while producing far less GHG and air pollution than before.
If you are building or renovating and going the extra mile with your insulation and airtightness measures, then take a serious look at electrified heating.
Can you afford not to?
Remember to check out our Instagram to follow the project, read the rest of the Automated Home 2.0 blog posts and find the links to all the products we’ve used in our self-build.