Storage heating - basic maths?

Polfers

I almost dare not start this thread, given the amount of threads on this forum about the subject already, but to be honest no one seems to fully answer this particular question, instead just refering to cheaper night time rates etc. I think I've figured it out, but I'm not sure if I'm right, as I'm not an expert here. Please could someone who is, either confirm if I'm broadly correct, or not as the case may be please? Many thanks! :0)

Keeping it simple, and exclusively to use of Modern Storage heaters, is this about right?:

Night Storage Heaters (NSH), are apparently designed to use around 3 times the amount of electricity to heat up per hour during the night than they are designed to release per hour during the day.

For example, I understand (from their website) that a top of the range night storage heater from a well known brand recognised as about the best there is (and made in 2023), uses 3.3 KW per hour to charge up at night and releases a maximum 1.5KW per hour during the day/when heat is needed outside of the economy 7 charging hours. 

Its storage capacity is 23.10 KWh, so once it has reached this capacity (which would be, not co-incidentally, 7 hours (23.10 / 3.3 = 7), hence E7 tarif), it will stop drawing electricity. 

On this basis, a fully "empty" radiator heating up from scratch at current E7 night rates (Feb 2024) would cost around £3.50 per day at 15.49 pence per KWh. If I  had, say, 3 radiators of this size, that's £10.50 per night, which is £73.50 per week in the colder months, which is £316 per month.

However, this would likely average out over the year to maybe £175 per month or less (see below), but that's before I use electricity for anything else, like a couple of panel heaters in bedrooms for occasional night use, any hot water, and of course, the day-time use of appliances/lighting etc. as may be applicable.

That being so, it is reasonable to assume (with reasonable levels of insulation), that the radiators won't be running at full-bore all day every day, especially so if you are out during the day and only draw a low level until the late afternoon/evening after work (modedern NSH can do this very well I understand). Therefore the radiators may well not be "empty" at the end of each day, and may require less input of electricity to top up again overnight as a result. So depending on your circumstances of usage outside E7 hours, in this example, that might mean the cost would range between, say, £110 to £150 per month on average (for room heating only).

Have I understood this principle correctly? 

Reed_Richards

Your house will lose heat at certain rate.  If the entire house was at the same temperature then your rate of heat loss would depend linearly on the difference between the house temperature and the outside temperature.  So if you keep the house at 20 C and the outside temperature is 0 C the rate of heat loss could be Q Watts.  If the outside temperature were 10 C then the rate of heat loss would be half that, Q/2 Watts.  If you only heat some rooms then the rate of heat loss from that room gets more complicated and depends on how well-insulated your internal walls are.  But the principle is the same.

Your storage heaters can supply a room with (you say) 1.5 kW maximum.  A big room with a large length of outside wall might require more than this to keep it warm in cold weather.  A small interior room might require a lot less.  The storage heater will be thermostatically controlled so will only supply the amount of heat required.  So when you are trying to work out running costs you must factor in how much heat you actually need.    

GibbsRule_No3.

Re the price for 3 radiators being £10.50. I have 3 Quantum Dimplex radiators (not sure if they were the ones you were hinting at?) Like you say they heat up overnight and I have two on for 6 hours a day the other for two hours, to take chill off bedroom, morning and night. My IHD shows around £4 overnight, that includes hot water cylinder. So I think the £10.50 a day for three sound expensive. They are smart and on colder days the overnight IHD does show a bit more used but never near £10.

EssexHebridean

I used to find that with my Quantum the only time in the year it ever heated fully from stone cold was the first time it was turned on each winter. 

Scot_39

Firstly - you may well find the other spaces do not actually need the same high powered 3.3kW input rated devices.

The basic maths is sound - the presumptions behind it at the worst case scanrio - are however propbably pessimisitic.  But putting an actual figure on it much more difficult.

And you clearly understnad that heaters ratings are not sized for a typical winters day - but a typical coldest winter day.

 

So go to for example the Dimplex rating (note it's a sizing guideline - so covers the max potential required rating not the expected average daily use) calculator - where you can enter your room and window sizes etc and home layouts  - you will see they do those calcs based on -3C outside in England and recommend a change to -5C in Scotland - and for living room spaces default to 21C room temperatures and bedrooms 18C.

Any warmer outside or if you set a cooler inside requirement - they should not need the rated power - derived from such rating / sizing calcs.  But other factors matter too - many will find need less heating on a sunny still day compared to a dull windy one - even at the same nominal air temperature.

How often for instance is it an average of -3C / -5C all day where you live ?

For instance - according to met office the average daily max min temp for last c30 years around here Dec through Feb - at the closest weather station to me - were c1.5% min, the average daily max over 7C (so maybe +3-4C daily average) - well above -3C.

But getting back to your core question

Working from worst case full rated power winter and extrapolating to months of typical - whilst I might suggest could be very pessimistic - if you simply want an upper bounds on your running costs - then it's a marker in the sand.

But I would suggest not to get too obsessed by it.

In Dec and Jan for instance - in a 2 bed mid terrace house with 1 large (c3.5xkW input) and 2 medium (c1.7xkW input) storage heaters.  I have gone from all off to all 3 on at upto half input - pretty much as high as I ever go - to back to 2,1,0 in use repeatedly. And use those 2 lower rated heaters in halls to also heat kitchen, bathroom, bedrooms as well as hall space.   And had to manually adjust old heaters settings about once every 2-3 days given weather swings.

As such my heating energy costs exc HW - have ranged from literally 0 -> c30kWh per day.

Whereas - doing your worst case utilise full rating scenario - and bearing in mind I have E10 not E7 (and suspect the heating was probably rated accordingly as next door neighbour had their one LR model replaced by 2 mid sized quantums after old devices struggled on E7)

That E10 charges my heating 3x per day for potentially 10 hours in total - with 5 hr 3 hr and 2 hr charge slots - seperated by gaps of 8 hours and 4 hours - so time to cool a bit after previous top ups - that would cost me in theory upto c70kWh max. 

If I used that 70kWh number - I probably would be more than a little afraid of my heating bills.

Now how close I would be if I ran whole house to 21C / 18C etc as Dimplex rating calcs etc - not sure. (But I just did a run on Dimplex rating tool and my guess at window sizes and build etc - changing from 18 to 21C suggested rating could need to rated 40% higher using -3C)  

So could based on that see maybe worst case bills going into the 40-45kWh daily range at 21C/-3C - but still interspersed with much lower typical needs day to day - week to week - even in winter months.

Polfers

Dear all who have very kindly responded: thank you - it is greatly appreciated :0) 

Apologies for not getting back to you all sooner (long story!)

Anyhow: my post was indeed statimg a basic "worst case scenario" .

To explain further:

My house is a tad unusual, in that it is a "topsey-turvey" house. It is an inboard terraced house of 6, built on a hilside - i.e. downstairs has an entrance hall at the front, a double bedroom, and a small on-suite shower room/loo, and a utility room (not needeing any heat). There is also an in-built 1.5 car sized garage, which is (obviously), not heated, but carries insulation issues for the house (built in 1985), whch  I am in the process of addressing, as it undersits the master bedroom.

A stairwell goes uptairs to a landing, which leads on to a kitchen diner, and thence, via an archway, into an open plan living room. Also off the upstairs landing, is access to a single bedroom come- office, a double bedroom, and a family bathroom.

The company looking at installing the storage heating (we have no natural gas, and after assessments, we've dismissed ASHP's, Oil. LPG gas etc.).

Given heat ruises and having looked at what can go where NSH output wise & in terms of physical size possibilities, the downstairs hall could entertain a 0.7 KW radiator (and yes, they would be Dimplex Quantum), a 1.0 KW output on the uptsairs (in daytime living space) landing, a 1.5 KW in the kitchen diner, and  a 0.7KW in the lounge area itself, so, 3.9KW all-told in terms of output

We also have a small (up to 4KW) high efficiancy wood-birning stove in the lounge as a top-up heating source as required.

We have water heasting sorted.

Does this sound sensible, given that whsslt we cannot insulate as well as we'd like to (prohibitive cost), we have double glazing (all be it 1985 standard), good lofyt insulation, and we will be insulating underflorr for any upstairs rooms?

To recap: NSH solution, Solar & storage battery to provide daytime electricity.

Many thanks to all!

Polfers

...P.S. - sorry for typo's! 

Polfers

... and we would use simple electric programable/thermostatically controlled panel heaters for bedrooms only.

Scot_39

From the Quantum manuals - that sounds like you are sitting on quote for 

about 40kWh maximum NSH energy storage capacity upstairs - which can be supplemented by c4kW log burner for several hours I guess )

and 

c11 kWh maximum downstairs 

(using Dimplex's pseudo 7 hr single charge ratings - e.g. on 150 7x3.3kW input power - 23.1 kWh storage max - at 240V, add c15kWh and 2 c11kWh to match the3.9kW Pnoms from tables. And note - you cannot get 1.5kW for 24 hours out of a 23.1 kWh standard E7 charge cycle input - 23.1kWh/24 = 0.96 kW max on average)

Does that seem reasonable given the respective floor / wall areas etc and likely times of use / required room temperatures etc.

How often is the downstairs bedroom with ensuite used - and how hot do you like to keep it and for how long - would be my only real question area.

50kWh is a non trivial level of energy but by no means an impossible one to reach - even in a typical family home - there will be many running hotter home temps than me - who do exceed that on the coldest of days I suspect. But you also have the wood burner.

( Even I can hit 30kWh purely for heating in cold snaps in a small 2 bed mid terrace - but the Dimplex rating tool suggests I could push that 50%+ maybe 45 kWh overall - if enter data for living room (21 vs 17 currently at their default -3C outside coldest day assumption).

Do you have any idea of how much electrical heating energy in kWh you have been using in coldest periods of year over last couple of years ?

How does the cost of wood burner compare to the various potential tariffs you are considering ?

BellaBlondykeTheThird

The good news may be that there are some below 10p night fixes on their way in the next few months for economy 7 storage heaters users (guessing this from the available predictions)

If we had storage heaters we would consider a fix after the April price cap has come in to go through winter into next year knowing the costs.

Reed_Richards

Polfers said:

The company looking at installing the storage heating (we have no natural gas, and after assessments, we've dismissed ASHP's, Oil. LPG gas etc.).

That seems highly dismissive.  I presume that if you had natural gas then you would also have dismissed it on the basis of whatever assessments you made to dismiss the other options. 

I'm not really sure why you need the solar + battery if you are using Night Storage Heaters.