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Update on Timeshifting to Minimise Heating Costs.



When we first decided to self-build in 2014, Jan and I visited quite a few passive house builds and talked to various experts;  we soon decided that a low energy approach was broadly the way to go for our build.  One of these experts, a passive-house evangelist called Seamus O'Loughlin, emphasised that a conventional heating approach (where boiler demand is based on some central thermostat set point) doesn't work well in a passive house, because the time constants of a high-thermal capacity low energy house are a couple of orders of magnitude longer than those anticipated by conventional CH control systems.  


At the time this seemed a controversial assertion, but because I have done some mathematical modelling professionally, I was able to and decided to do some time-dependent heat-flow modelling and control strategy simulation of how our designed house would behave and this very much supported this assertion.  I have already covered a lot of detail of my CH approach in previous posts and discussions, but it’s probably worth summarising some key headlines to set the context for my changes to our heating strategy: 

  • We were cash-flow limited during the build phase, so had to make various cost-benefit trade-offs on our build, like most members here. I based these on a general net 10-15 year payback, and it was clear that we wouldn’t be able to achieve a true zero-input passive house largely because of design compromises owing to planning restrictions and our plot size and orientation.
  • However, we would be able to build a low-energy house that would need generally low levels of supplemental heating for maybe 6 months a year, with overall heat losses an order of magnitude less than a conventional build, and the thermal capacity of the heated fabric be many factors more.  
  • We decided to go all electric in the house with wet UFH embedded in the ground floor slab only.  Cost benefit trade-offs didn’t even support installing an ASHP, though I did future proof the installation to simply the later addition of one if the cost numbers changed.
  • I decided to adopt a simple but unconventional strategy for heating the house: calculate the total heating requirement for the coming day daily at midnight; this is based on actual averages for energy use, average house temperature and forecast average external temperature for the coming 24 hrs. This allows me to dump as much of this heat into the house fabric as practical at the cheapest electricity rate, and for us this is in the 7 hour overnight off-peak window on our E7 tariff.
  • We used to get some spill-over into peak rate top-up in the coldest months, but a year ago I added an oil-filled electric radiator on my 1st floor landing, and one in my son’s 2nd floor bedsit controlled by my Home Automation System, with these scheduled to come on in the overnight E7 window to dump extra heat in the upper floors.  This simple addition reduced the thermal layering from ground to second floor, and almost  eliminated  the need for daytime slab top-up.
  • In practice we have roughly a 1°C daily ripple on overall winter house temperature. Because using a daily forecast computation does have some intrinsic prediction error, this can add typically less than  0.2°C day-to-day ripple on top, but any longer term drift can be corrected by the daily feedback.
  • I have RPi3B running NodeRED attached to some digital thermometers and 4 GPIO controlled solid-state relays (SSRs) to control the time of the UFH pump and Willis heater, plus the 2 × SunAmps for DHW.  This was very cheap to implement, and basically has no monthly or annual maintenance. 

With the current Electricity price hikes, we have decided:

  • To trim our house temperature set-point back from 22.3°C down to 21°C  
  • To hard limit automatic heating of the slab to the cheaper 7-hour off-peak window.  (We can still do peak by request in one hour chunks if we want to.)
  • To use electric oil-filled radiators overnight to do any additional top-up.  I can automate this through my Home Assistant (HA) that runs on a separate RPi4 and do this using MQTT via WiFi connected powered/metered sockets.

This strategy currently limits heat into the house to: 

  • ~21 kWh through the slab and 
  • ~7 kWh through the two radiators.

28 kWh is enough to maintain overall house temperatures so long as the external temperature is at ~7 °C or higher, and it clearly isn’t the case at the time of posting.  The house needs about 2½ kWh/K, so with the average daily external temperature at zero today this is 17½ kWh too little to maintain house temperatures.  The long term Dec / Jan average where we live is about 4°C, so to maintain temperatures in this case we would need an extra 7½ kWh/day. (This last year, we had 26 days where the average external temperature was 4°C or below and only 2 where  temperature was below zero or below.)


So what happens when we underheat our house?  Simple: it slowly cools down, and very slowly.  For example, in the last 5 days of cold-spell, capping the heating has dropped the average house temperature from 22.3 down to 21.3°C, and given an average of -1°C for today, it will be down to our new target of 21°C by tomorrow .  At this point I will need  to add more heat or to accept that the house temperature will fall further.  I will definitely need to add another 7kWh or so extra radiative capacity for overnight topup.  We will play it by ear over the next week or so.  I can either accept that I will be paying £0.38/kWh for extra peak period top-up during these really cold spells, or let the average temperature fall a little further if we find it comfortable enough (wear a thicker jumper, etc.)


This approach works well for us because our house is so insulated and it has a huge amount of thermal capacity within the heated envelope.  If we accept a small heating ripple then it really doesn’t matter that much when we heat within the day and so we can time-shift our demand to make use of the best tariff rates: currently over 85% of our electricity use is at the off-peak cheap-rate price. This latest exercise of clamping the heat output to 28 kWh when the maintain level is closer to 40 kWh underlines that the heat budget for and given day can be off by 30% or so and the net temperature drift is still on 0.1 °C or so; the time constants of the system are of the order of a week rather than days or hours.


By way of a contrast my daughter lives in a pretty large but conventional 1990s house.  When her heating goes off in the evening, the living room temperature drops maybe 4-5°C within an hour.

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Isn't that the one where the roof is a made of tiles stuck together with plaster of Paris and part of it collapsed during the build and had to be re done?

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19 minutes ago, ProDave said:

Isn't that the one where the roof is a made of tiles stuck together with plaster of Paris and part of it collapsed during the build and had to be re done?


Yes. One layer is brittle to point loads, three layers plus all the other stuff on top spreads out the loads.

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20 hours ago, ProDave said:

Isn't that the one where the roof is a made of tiles stuck together with plaster of Paris and part of it collapsed during the build and had to be re done?


From memory, it happened while they were filming, and you hear it in the background.

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Back to the OP topic, we've averaged 80% off-peak electricity use over this last cold spell.  This time last year it was typically just under 70%.  We still have ~12 kWh Peak use per day, so that's around £820 p.a. excess over off-peak rates at current OVO tariffs. With battery prices at around £120 / kWh (and still falling steadily), that's around £1½K for 12 kWh + inverter control, then you have to wonder when battery-based time-shifting of usage will become cost effective.

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>>> Day night shift of 1-2 kWh hotel loads is the most that should ever be cost effective


Possible to explain a bit more?

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You spend money on a battery and you know what this amount is


It lasts a certain number of cycles - say 10 years of daily charge/discharge


If you use 100% of the battery capacity every day then you can work out how much each charge/discharge costs. The cost per shifted kWh is low.


If you only use 10% of the battery each day, or you only use it once every 10 days, then the cost per shifted kWh is higher.


If you do the math then the first 1-2 kWh of "always on" stuff that you shift into the cheap rate period (be this self generated PV or some artificial time of use tariff) has a good payback



10 years = 3650 cycles

3650 x 1 kWh = 3560 kWh

1 kWh = £1000 

Shifting cost = £1000/3560 per kWh or you're spending 26p/kWh to move that electricity from one rate to another


Only worth doing if the cheap rate is at least 26p/kWh lower than the high rate.


If you try to shift more than this then you get into diminishing returns. Big batteries rarely pay off. It's only the load that your'e guarantees to shift day in day out for a decade that really moves the numbers.


Most people would be better off going larger on their PV by the £5k it costs for a battery (e.g. installing 8kW not 4 kW); spilling most of it as export in the high season; but having double available in the shoulder seasons.


Not that most people ever do the maths.



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I agree with your basic analysis and that we need to be fairly clear about RoI decisions which should inform such decisions.  In some cases equipping a new build was easier because man choices had to be evaluated and executed in line with the overall build program plan.  The options where realistically yes or no; there wasn't a sensible defer option.  Examples here for us include the decision to a warm loft to create a 2½ storey layout; the adoption of MVHR, the use of SunAmps for DHW.  In the case of ASHP, I deferred to decision to install the ASHP itself, but instead I put in the Willis heater as an interim solution and also buried insulated piping and power circuit that any future ASHP installation would need.


However we have completed our build and moved in at the end of 2017.  So now we have an established status quo with known characteristics and run-rate costs.  Any future material change should be based on an informed cost / benefit trade-off.  However, one issue that really complicates all this is that the whole environment is still very dynamic, and any trade-offs need to take an honest reflection of this: just extrapolating out the status quo can be a real mistake.  For example: 

  • Components such as batteries and ASHPs are still on the steep part of Engineering Experience Curve and so will be experiencing real capability improvement and price/performance drops year-on-year, so it can often make more sense to defer decisions year-on-year if you are going to get a better / cheaper system as an end result. 
  • Take the case of an ASHPASHP units cost roughly 5 times the gas-boiler equivalent, with a comparable gearing on installation costs.  This is because we are still on the 'low volume, high price' end of the price curve.  Even at current high energy prices, I can't make the case to install and ASHP and demote the Willis to fallback.  This might not be the case in a year's time.
  • Battery technologies and chemistries are still rapidly evolving, so I double that current Li-based systems will remain in pole position for house-based systems for long (other than perhaps for V2G solutions).  I also think that UK Energy Providers will be forced to offer new time-of-day pricing models to create demand-side leveling as the percentage of renewable increases.  This will all help, but for us the installed price of powerwall-style batteries would need to fall by a factor of 4 or so, for numbers to add up, as I can for do load shifting on roughly 80% of my high winter-based load by scheduling without any local storage.
Edited by TerryE
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6 hours ago, TerryE said:

Components such as batteries and ASHPs are still on the steep part of Engineering Experience Curve and so will be experiencing real capability improvement and price/performance drops year-on-year,

People have been saying that for a decade now, does not seem to be happening in the UK.


Here is a bit about lithium batteries.



Here is a bit about PV



I can't find any data on heat pumps at the moment, but shall keep looking when I remember.


I think one of the problems is that because we like high price houses, heating systems are considered a high mark up add on.

There was a small estate of new houses built in St, Agnes about 12 years ago, nearly all of them had ASHPs fitted.  Those houses were no more expensive than similar 'gas powered' ones built in Newquay, or Redruth, once the St. Agnes premium had been paid (the snob premium).

Much of the high prices we pay are because of incentive schemes to implement domestic renewables, a recent report highlighted the low take up of the BUS scheme.  The only scheme that was successful was the original FiTs on PV, but in reality, very few people could afford to spend between £8000 and £12000 on a PV system to get the 41p/kWh and the deemed 5.6p export.

Realistically, I doubt that domestic generation is every going to compete with grid power and I say that as someone that would love to live off grid for the technical challenge.  To get close to being self sufficient in electricity, I would have to halve my winter usage to around 600 MWh for December, January and February, which may be possible, and cover my house in PV, and my 5 neighbours houses.  Or just buy in 90% of my power at 15p/kWh, which I currently do, and wince when I see the 10% at nearly 60p/kWh.


Edited by SteamyTea
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5 hours ago, SteamyTea said:

People have been saying that for a decade now, does not seem to be happening in the UK.

What is the domestic gas boiler penetration? I would guess of the order of 60%. ASHPs about 1%. If the government is serious about its energy targets then this ratio needs to collapse. 

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5 hours ago, TerryE said:

What is the domestic gas boiler penetration?

Pitifully small.

I think the relevant detail is hidden in this spreadsheet.



Sheet S.1 show median costs of installing a kW of ASHP is £980, with lower quartile at £770 and upper quartile at £1710.  What that actually covers I have no idea.

As a comparison with combustion technology, small biomass is coming in at a median of £590/kW (LQ 480, UP 800)


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