Let's imagine that we have a heat pump which isn't going to draw from the grid for 4-hours out of the 24. And we'll make the maths easy by assuming that the house has a daily heat loss of 100kWh.
We'd need a water cylinder capable of storing 4/24 x 100kWh which comes to 16.7kWh
Now suppose your heat-pump is such that it can deliver water at 50°C max, and that you're prepared to allow your radiator temperature to fall to 35°C by the end of the 4-hour period. Your Thermal store will exhibit a 15°C drop whilst delivering that 16.7kWh.
The calculation:
1 watt of power for 1 sec = 1 Joule
Thus 1kWh = 1000 watts x 60 secs x 60 mins and therefore 1kWh = 3,600,000J (3,600 kJ)
16.7kWh = 60,120,000J (60,120kJ)
We need to find the mass of water which will hold that energy.
Energy required = Specific Heat Capacity x Temp loss x mass in grams where the Specific Heat Capacity of water is 4.184J/gram/°C
Change the formula around: Mass (grams) = Energy / (Spec Heat x Temp loss)
= 60,120,000 / (4.184 x 15°C) = 957935 grams
Convert to kilograms = 957.935Kg
1Kg of water is 1 litre capacity, so we need a tank holding almost 1000 litres 1 litre of water occupies a cube with each side being 100mm.
Conclusion:
Assume that this tank is a cylinder with an internal diameter of 800mm (about the largest domestic hot water cylinder you could buy!)
Volume of a cylinder : V=πr²h
Change around the formula to find height (h)
h = V/(πr²)
Our cylinder would then need to be 1.98m tall.
Add at least 40mm of high quality insulation around it, plus the stainless steel strong enough to contain 1 metric-tonne of water. That gives you a tank almost a meter across and the height of the ceiling in an average home.
Feel free to play around with my figures. I just wanted to make sure that the required formulas were presented in this Topic.
Their firm proposal is to implement a process to be called Consumer Led Flexibility (CLF).
That will allow 3rd parties to act as Load Controllers. Households who sign up to do so will have their heat-pumps turned off during the early-evening demand peak as required by national statistics.
The Load Control Agent will then credit the householder's Energy Account for providing that demand-reduction.
It's therefore worthwhile considering how a house might remain heated during that period by storing energy in advance.
I'll add some first hand data here, albeit a single data point made through observation.
Conveniently, our house recorded a daily heat loss of around 100kWh on the coldest day last winter, so fits nicely with @Transparent's example.
Last year we ran our heat pump on the Octopus Cosy tariff, and operated a switch off policy for the 3h peak window between 4-7pm. The temperature drop experienced was 3-4C in that 3 hour window, and was pretty unpleasant without some alternative form of heat. A 4h switch off may translate to a 5C heat loss for us (21C down to 16C).
In terms of water volumes, our system with 12 radiators (some quite large), lengthy (over 10m) 28mm primaries and a 50L volumiser has a total system volume of just under 200L (190L by my estimates). Substantial thermal mass, but nowhere near the 1000L required to store sufficient heat by @Transparent's calculations to help offset that heat loss.
Of course this only applies on the coldest days of the year, and during the milder months of spring and autumn, heat loss will be significantly less and a lengthy switch off may be practical without losing so much heat as to be intolerable, but I do not believe the above strategy is a practical strategy for most homeowners for the coldest 3 months of the year. We could certainly tolerate a 3h switch off now whilst OATs are still in double digits.
I should note that we were able to operate such a strategy last winter, even if pretty intolerable, mainly because we were able to recover that 3-4C of heat loss pretty quickly due to having a massively oversized heat pump in our property. A correctly sized heat pump would simply not have the capacity to recover a 3-4C heat loss on the coldest days of the year in a reasonable amount of time. See here for a recent discussion of set backs and recovery thereof:
In my limited experience, it doesn’t seem like a viable national strategy to me.
This post was modified 4 weeks ago 5 times by Old_Scientist
Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.
In my limited experience, it doesn’t seem like a viable national strategy to me.
I have to agree with that. Although our house keeps heat quite a bit better than yours, I think, I got fed up of operating on one of the modes that my heat pump will operate in (a mode with a fairly high degree of room influence) because of the temperature swings, so early on switched to pure weather comp which for my house is much more stable.
The swings with which I got fed up were after an hour to two switched off, so three hours would not be tolerable.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
The SSES Team has not yet suggested any particular time-frame for which HPs would be switched off... and this would probably be a decision to be made by Elexon as their preferred administrator of the scheme.
However, I think it's a fair assumption that it would need to be 3hrs-ish due to the nature of the demand peak.
I have raised a good many objections. Many of these fell outside the scope of the recent consultation subject of Governance, and DESNZ requested that I send those in anyway. I did so in a 15-page separate document.
Sticking with the proposal of it being heating appliances which are switched off, I assume there is concensus here that a heat-pump would be less efficient as it is switched back on? It would have to 'run harder' in order to make up for the heat lost from the dwelling during the off-period.
Let's assume it was an ASHP which was normally operating with a COP of 3.0 (yes, I realise many can't achieve that!) is it possible to provide a reasonable estimate of the extra electricity it would consume during the time taken to raise the temperature back to the normal setting?
That's a critical calculation for the CLF Scheme to be viable for the typical consumer.
The 'compensation' he gets paid for having taken part in Demand Reduction must cover not only the electricity which would normally have been used during the off-period, but also the extra that is needed to recover the temperature whilst running less efficiently.
Let's assume it was an ASHP which was normally operating with a COP of 3.0 (yes, I realise many can't achieve that!) is it possible to provide a reasonable estimate of the extra electricity it would consume during the time taken to raise the temperature back to the normal setting?
It depends entirely on how quickly you want to recover!
If for example you want to recover in the same time that you switched off for (eg switch off for 3 hrs recover for 3 hrs), then you need to operate at twice the delivered power (which is unlikely to be possible unless the heat pump is well oversized, given that this will presumably happen at min temps).
Power output from a radiator (sensible assumption IMHO because its the majority case) is proportional to (Trad-Troom)^1.3. So if Trad is say 45C and Troom is 20C you need to raise Trad to 20+1.7*25=62C for the duration. Thats 17C higher, making it roughly 50% less efficient (very, very, very, very rough rule of thumb 3% per degree).
So during the recovery period you will consume 3* as much electricity as you 'saved' (2* as much because of twice the energy plus a further 1* as much because of the loss of efficiency.
So if its a fairly typical 7kW house operating at a COP of 3 it will normally run at 2.3kW and during recovery it will, in principle, be drawing ~6kW (which is only going to work if the heat pump is a well oversized 12kW model).
Not sure how much this strategy will help with peak management TBH unless there is a very significant element of randomisation of timing.
In reality what will happen is a much slower recovery probably spread over 12hrs or more, but it will likely seem quicker because the air temperature heats up before the fabric does and its an asymptotic recovery curve.
It would be better to give people fixed periods during which they were incentivised not to use their heat pump, corresponding to high demand periods. They could then plan accordingly by adjusting their WC curves to suit the reduced operating hours, as they do at present. Forced switch off could then be reserved for true emergencies.
Im coming to the conclusion that using switch off for demand management, other than in an emergency/very exceptional case, is a non starter.
Do you really think the switch off will be for 3 hrs, or is it more likely to be either an hour (which is the typical duration for the Octopus saver session) or even 10 mins when everyone puts the kettle on during the cup final.
This post was modified 4 weeks ago 16 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
I'm minded to copy the above post from @jamespa directly to the manager of the SSES Programme.
They are unaware of the science behind heat-pumps, and are instead concentrating on creating regulations.
Feel free to pass on my comment above. Even if it weren't a heat pump, to recover in the same time as the switch off would require double the power because you are trying to supply twice the energy per unit time. This is basic thermodynamics. The better plan of course it to have advance notice of the switch off so you overheat before partially pre-empting the recovery boost, thus both mitigating the effect on the occupants and spreading the load.
Sorry but I cant see how you can design a system to control heat pumps without knowing at least something about both heating and heat pumps. To design regulations you first need to know what you want to achieve and some idea how you are going to achieve it otherwise you risk designing regulations that can only be met by violating the laws of physics, or regulations which solve the wrong (or a non-existent) problem.
What's the problem that they are trying to solve (a bit more specifically than 'grid overload').
As I see things, what will happen with electrification is that the baseload will increase quite dramatically, from 400W per household currently to between ~2-4kW (2kW heat pump, 2kW car charger on average), but the peaks will originate from the same causes (with a bit of an uplift for induction hobs). The grid design has to accommodate the increased baseload come what may, so the task becomes to fold the peaks into something not much more than the baseload by attenuating the baseload during the peaks or spreading the peaks.
There are short peaks (a kettle, grill, toaster) and long peaks (oven, hob, maybe dhw cylinder although that could be timed for night).
The short peaks are easily dealt with by switching off or turning down either heat pump, car or both. A short attenuation will have no material effect on either of these. Problem from kettle, toaster and grill fixed, with absolutely zero inconvenience to user (so cup final interlude problem solved).
The long peaks are a bit more difficult because attenuating the baseload during them does have an effect. I would guess that attenuating the car charger was probably more acceptable to many than attenuating the heating, but of course not to all. This is a decision that needs to be left to the householder, with a default if they don't make a choice. With this flexibility I cant see a material detriment to anyone. Obviously some tech is needed to make this happen.
This post was modified 4 weeks ago 10 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
That's a crucial question which the SSES Consultations haven't properly addressed.
The crux of their argument is that we need to reduce demand during peak times. But I haven't seen any data from DESNZ which backs that up.
Here's my histogram comparing the amount of electricity we require at an instant of time (in GW), the available generation capacity which NESO have available to call on and the Offers to Connect made by National Grid (the Transmission Capacity)
There would appear to be enormous quantiites of electricity available to meet demand and meet Net Zero. The overall picture doesn't give us any idea of why Demand Reduction is required.
The above graph was valid up to April'25.
At that point NESO commenced a Connections Reform process. All those with Offers to Connect had to reapply. Priorities are being assigned to Applicants according to how quickly the connection can be made. That differs from the previous system, which was "first come, first served"... whereby applications were lodged pre-emptively to reserve a place in the queue.
NESO have yet to publish the outcome of Connections Reform. They may receive legal challenges of course.
The SSES Team told us in an online webinar on 16th September that they wanted to achieve 10GW to 12GW of 'consumer flexibility'.
I had in front of me the latest (July'25) DUKES within which Chapter-5 concerns electricity. It contains this diagram:
The 30% of Domestic consumption could be challenged because our single-phase use contributes the most to Energy Industry Losses.
However, let's ignore that for the moment and put the 30% figure into the histogram I used above.
I've removed the 'Offers to Connect' green columns because they'd be off-the-page.
But it's fairly obvious that achieving 10GW to 12GW flexibility would require all domestic consumers to take part in the Consumer Led Flexibility programme. And if CLF is only offering a facility to turn off heat-pumps, then we'd all need one of course!
The maths doesn't stack up.
... which means I still don't know what problem we're trying to solve.
to recover in the same time as the switch off would require double the power because you are trying to supply twice the energy per unit time. This is basic thermodynamics.
I don't think Heat Pump control electronics would attempt the recovery in the same time as the switch-off.
My understanding of HP control systems tells me that it will increase the Flow temperature and rate. That lowers the COP... ... which means that a higher proportion of the recovery heat will be coming from electricity and less 'from the air'.
Consequently, the overall process will increase the quantity of electricity we must generate nationally.
I'd like to draw that as a diagram. But I'd need to have a good estimate for how much the COP would fall during the recovery period, and an idea of what a (theoretical) typical ASHP would do with Flow Temperature and Rate.
I don't have such figures.
That's where I'm hoping @jamespa@cathoderay and others here might be able to provide some reasonable approximations.
As I see things, what will happen with electrification is that the baseload will increase quite dramatically, from 400W per household currently to between ~2-4kW (2kW heat pump, 2kW car charger on average), but the peaks will originate from the same causes (with a bit of an uplift for induction hobs). The grid design has to accommodate the increased baseload come what may, so the task becomes to fold the peaks into something not much more than the baseload by attenuating the baseload during the peaks or spreading the peaks.
I agree with that thinking.
Let me add two further points:
1: NGET tell me that they've modeled the overall effect of the entire country switching to EVs. If that were to occur overnight(!) we would need to generate and transport an additional 10% electricity.
That's nowhere near as much as we might suppose.
But let's note that NGET's model assumes that properly-smart EV chargers would apportion the load. Their 10% figure can't be valid if we all try to charge EVs at the same time!
Since generation from renewable sources isn't evenly-spread geographically, there would need to some aspects of Nodal Pricing involved in delivering this solution.
Nodal Pricing (for consumers) was rejected by Claire Coutinho MP in March 2024, when she was Secretary of State for Energy.
Government seems to consider Nodal Pricing to be a political football. Ministers appear to be unaware of the underlying science.
2: Around 90% of the grid exists at the 11kV level of the Distribution Grid. The bulk of that copper/aluminium cable is in rural areas of course. During 11kV upgrades, copper is being predominately used close to coastal areas, due to salt in the air.
Here's the diagram showing the levels which exist for most of England and Wales:
and here's an area of England on which I've picked out one 11kV Feed from a Primary substation:
We get to read headlines of National Grid promising a further £30bn of grid upgrade investment. But let's note that's all at the 400kV level (and 275kV in Scotland).
Upgrading the 11kV level to handle more throughput on demand would be astronomically expensive - more than GB could afford.
Think "HS2 on steroids" and imagine the political fallout if Government/Ofgem/NESO took us in that direction. 😲
Fortuitously, there is already enough spare capacity on the 11kV cable network to support electrification of heat and transport to meet Net Zero.
... but only if we time-slice its use!
That suggests to me that the SSES programme has set its heart on tackling the wrong problem.
However, this should be good news for technologies such as tepeo's ZEB.
The ZEB can take in electricity at whatever time there is available capacity on the local 11kV grid. For the moment please ignore how a ZEB would know that to be the case. I have solutions for that, but it would take this Topic off at a tangent.
I would appreciate if @wully could push the above diagrams and explanations in front of his colleagues, and then give us a considered response.
When anyone else posts here, please remember that DESNZ and NESO may be reading it.
I don't think Heat Pump control electronics would attempt the recovery in the same time as the switch-off.
My understanding of HP control systems tells me that it will increase the Flow temperature and rate. That lowers the COP... ... which means that a higher proportion of the recovery heat will be coming from electricity and less 'from the air'.
No they don't although some people have built automations to do so after setback.
Many will do almost nothing in response, other than to output slightly more energy at the same FT because the oat hasn't changed but the house is slightly cooler so the emitters will emit a bit more even though the ft hasn't changed. It will take half a day or more fully to recover from a longish setback. Households may react by turning the heating up, if the event occurs frequently they will turn it up permanently thus increasing electricity bills and electricity consumption. I think a lot will depend on the frequency and duration of the interventions and without a view of that it's difficult to say exactly what will happen...which brings us back to...what problem are we trying to solve!
The other factor is that the recovery after a 4-7 peak will largely be at a lower oat than the 4-7 period, so a lower cop, perhaps adding 10% or so.
I will give longer thought to the rest of your post and respond further, possibly tomorrow, but without knowing the problem, or the likely frequency and duration of the outages, giving intelligent comment is nigh on impossible.
This post was modified 4 weeks ago 2 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Fortuitously, there is already enough spare capacity on the 11kV cable network to support electrification of heat and transport to meet Net Zero.
... but only if we time-slice its use!
Can you clarify that comment please. What do you mean by 'time slicong'? Are you saying that, over a period of 24 hrs in the height of winter the existing 11kV network can still transport sufficient energy to meet our future electricity needs over that 24hours. If so thats hugely significant and hugely encouraging because the problem is then only one of load levelling. It's made even more significant by the fact that the principal effect of electrification is, so far as I can see, an increase in baseload not in dynamic range.
If this is what you mean then a corollary is that the whole problem, or at least most of it, is soluble at individual house level with the right technology. Just enforce max 60A (or whatever it takes) and householders have to either have a battery or devices to turn down the ev charger or heat pump when the kettle is on. EV chargers will already do this! I'm not saying this is the way to do it but if we already know that the 11kV can transport sufficient energy that's highly significant however you look at it.
This post was modified 4 weeks ago 6 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
I think a lot will depend on the frequency and duration of the interventions and without a view of that it's difficult to say exactly what will happen...which brings us back to...what problem are we trying to solve!
Unless DESNZ can add clarity, I'm afraid that the definition of "the problem" will continue to elude us.
They've obviously been listening to representations from the commercial energy sector, and believe that we're all aware of a concept called Peak Demand.
The Smart Systems and Flexibility Plan 3 sets out how government will support achieving the UK’s net zero goals through facilitating the transition to a smart and flexible energy system. A smart and flexible energy system will reduce consumer energy bills by reducing the amount of generation and network assets that need to be built to meet peak demand. It will give consumers greater control over their energy bills through access to smart technologies and services, such as enabling participation in Demand Side Response (DSR).
In my submission to that 2024 Consultation I raised a problem with Government naming their strategy Demand Side Response.
DSR had already been defined in the SMETS2 Specification documents, ratified by Parliament in 2014. That was then announced to the general public in publications issued by HMG in 2016. The two approaches are not the same, and would thus create confusion.
That's why the SSES Team have renamed their new strategy Consumer-Led Flexibility (CLF).
As for the duration of Peak Demand, the consultations only provide illustrative explanations.
Here's one which preceded Q. 24 in the Electricity Smart Appliances Section:
Well, I've seen thousands of hours of LV Supply & Demand timelines, and I don't think the DESNZ time-frame adequately reflects what's actually happening on the ground.
There are some geographical areas which show hardly and evidence of a Demand Peak. This is from the Bulk Supply Point transformers at Twelveheads in Cornwall, close to the Spring Equinox
Why have I chosen Twelveheads?
Because it's where Kensa Heat Pumps are based! Their definition of peak demand isn't going to reflect what DESNZ themselves experience in Westminster.
I think the entire SSES strategy is based on national statistics... ... which are then being stretched beyond breaking point and forced to apply to the LV (240/440v) level of the grid.
This post was modified 4 weeks ago 3 times by Transparent
What do you mean by 'time slicong'? Are you saying that, over a period of 24 hrs in the height of winter the existing 11kV network can still transport sufficient energy to meet our future electricity needs over that 24hours. If so thats hugely significant and hugely encouraging because the problem is then only one of load levelling.
Yes, that's precisely what I mean.
And it's load-leveling which I've been looking into since 2018 when I first starting monitoring substations.
I'm deliberately avoiding the term Time of Use Tariff (ToU) because that implies that the householder's decisions are to be dictated solely by "price which can change every half-hour".
There are better innovative approaches which would also be more grid-friendly.
These solutions would also include elements of
reducing phase-imbalance at the local substation, hence not throwing away 10% of electricity as heat!
slow ramp-up/down of demand to avoid surges or sags at tariff changeover intervals
a bias slightly in favour of social housing tenants, and/or those households with pre-payment meters
storing electricity which would otherwise be discarded by Active Network Management (which therefore requires priority on the 11kV level)
We can create a single strategy, which hits all those ideals in one go. This is technically achievable.
That's what I would call a genuinely Smart approach... (unlike a 'Smart' EV controller!)
... if this is what you mean then a corollary is that the whole problem, or at least most of it, is soluble at individual house level with the right technology.
Precisely.
We need a 'controller' that's no more complex than our existing IHDs, and operates autonomously from within the home.
It can retrieve the data it requires from across the internet, or from the home Smart-Meter, but it cannot be sent commands from a third-party.
I have a generic name for such an autonomous controller: Forward Acting Demand Regulator, abbreviated to FADeR
So the bulk of time which DESNZ and the NCSC has spent on encryption and cyber security for CLF over the past 2 years wouldn't be required. 😘 We'd end up with an electricity supply system which has greater resilience and better at coping with a hostile cyber attack.
