Looks like the party is over....

[Dillsue]

8 hours ago, ProDave said:

And if you have PV on your roof, you can pretty much guarantee when you need to run your cooling system, there will be enough power from the PV to power it, so no demand on the grid.

If youve got AC isn't one of the prime times to run it in the evening and overnight to help with sleeping?

[Oz07]

33 minutes ago, Dillsue said:

If youve got AC isn't one of the prime times to run it in the evening and overnight to help with sleeping?

Assume with well insulated house you bring temp down during day when sun shining?

[JohnMo]

1 hour ago, Dillsue said:

If youve got AC isn't one of the prime times to run it in the evening and overnight to help with sleeping?

That isn't how you run a heat pump so why do that with A2A (which is a heat pump)? You keep on top of the temps and run low and slow. So when sun is out, the house starts to heat up and cooling starts then. You can always add a battery then it's still powered by solar overnight as well.

[Beelbeebub]

Batteries. 

[JohnMo]

24 minutes ago, Beelbeebub said:

Batteries. 

if you want

[sharpener]

3 hours ago, Beelbeebub said:

Batteries. 

 

It's still not a costless option. If you can sell all you generate at 15p/unit then that is what it is costing you for domestic consumption of every kind.

[Beelbeebub]

20 minutes ago, sharpener said:

 

It's still not a costless option. If you can sell all you generate at 15p/unit then that is what it is costing you for domestic consumption of every kind.

But batteries allow you power your air con in the evening/night with solar from thr day atow cost (4p/kwh assuming 4.5k cost for 14kwh usable over 8k cycles)

 

And crucially at zero extra load on the grid. 

[sharpener]

18 minutes ago, Beelbeebub said:

But batteries allow you power your air con in the evening/night with solar from thr day atow cost (4p/kwh assuming 4.5k cost for 14kwh usable over 8k cycles)

 

Yes I get the depreciation calc which is often forgotten, but to arrive at the true cost of night-time usage you still have to add the cost of exports foregone which (with Octopus) is 15p/unit, and also divide that by the charge/discharge efficiency which might be as low as 0.8.

[Beelbeebub]

But your import rate is likely to be higher - at least outside of night rate. 

 

So exporting at 15p and importing at 25p makes the financial efficency 60%. 

 

Say you produce 10kwh during day and over the same 24h use 10kwh 

 

During the generation period you use 5kwh and export 5kwh (earning 75p) 

 

Then after suunset you use 5kwh, buying it all from the grid at a cost of 125p

 

Net loss of 50p

 

If you stored the 5kwh you would forego 75p but not spend 125p.  And lose 20p in depreciation. 

 

Overall a difference of 5p - chicken feed really, though it does widen if your power requirement is higher or if the imoort/export gap widens (eg the 15p rate is cut or price of day rate elec Rises) 

 

*but* the battery option has zero impact on the grid - there is no import or export 

 

 

[sharpener]

3 hours ago, Beelbeebub said:

If you stored the 5kwh you would forego 75p but not spend 125p.  And lose 20p in depreciation. 

 

Overall a difference of 5p

 

Don't quite understand where your 5p comes from, 125 - 75 - 20 = 30 not 5 which is an appreciable saving.

 

But in the real world you might need to sacrifice more like 6 or 7 kWh in exports to get 5 kWh out of the battery again which is starting to make it a very close run thing.

[JamesPa]

15 minutes ago, sharpener said:

 

Don't quite understand where your 5p comes from, 125 - 75 - 20 = 30 not 5 which is an appreciable saving.

 

But in the real world you might need to sacrifice more like 6 or 7 kWh in exports to get 5 kWh out of the battery again which is starting to make it a very close run thing.

I run the numbers for batteries at least every 3 months and can never quite get it to work out after accounting for cost of capital (even if I allow for tax on savings).  I want them to and its close, but they don't for me.  A decent nighttime tarrif, doing washing and charging ev overnight, and running the heat pump 24x7 without setback still works out the best bet.  Drop the price by 20% and it changes, but the installers don't drop the prices.  Even factoring in arbitrage doesn't tip the balance.

Edited 10 hours ago by JamesPa

[sharpener]

26 minutes ago, JamesPa said:

A decent nighttime tarrif, doing washing and charging ev overnight, and running the heat pump 24x7 without setback still works out the best bet. 

 

Battery is now a sunk cost and depreciation not a consideration as it will probably outlast me! 

 

I cannot be doing with chopping and changing e.g. to milk Octopus Agile (even though I have an EV). So am on Cosy and charge battery to 100% every cheap period. This means I do not have to guess what the next day's PV will bring in order to leave enough room for it. (Havenwise might do a better job of this but is not compatible.) Also means we can do laundry/dishwasher at any time without penalty.

 

With a buy price now 14.xx p and sell price at 15.00 I am still probably losing a bit bc of the round trip inefficiency but it is totally set and forget.

 

 

Edited 10 hours ago by sharpener

[Gus Potter]

Hi all.

 

The party is over indeed. I'm going to lay this on thick as it's time.

 

I've picked UFH first but will turn to PV in another post.  But to wet your aptetite! 

 

I've argued on this forum for a long time that underfloor heating is the bees knees. I've done this and worn the tee shirt. Long before most of you were in short pants. So if any of you can tell me how you installed and designed an UFH 22 years ago on a self build then I'm all ears! I would love to hear from you how we all flew by the seat of our pants at that time. I do hope that there may be one person. Many think they have invented the UFH wheel.. well you have not! The Scandinavians were at long time before the Brits..

 

I've made the case for simplicity and the need for UFH (the pipes in the slabs) to last the lifetime of the building, the cost of maintenance, reacting to how technology will evolve in terms of boilers etc. BUT what is in the concrete floor is a FIXED ASSET! Just like the walls and the roof!

 

I also happen to be an SE who has actually been designing these slabs, insulated rafts to work with UFH for decades. So If there are any on BH that want to have a serious structural discussion with me then I would welcome that.

 

You can ponce about with your weather compensation as much as you like, the flow rates.. your controls and buffering.. but see.. your heating has to work for 50 years so until you can make that arguement fdor life time performance, then I'll write down the value of your house if the pipes in the floor are not designed to adapt. SO GET A GRIP!

 

I can and estate agents will also take a dim viw on a house value!

 

Now some may just want to have a house that is a hobby, that is fine and I'm ok with that. But actually that may also make your house unsellable! Seriously.. you need to WAKE UP and smell the coffee, surveyors et al are clocking this so your hobby might come at an unexpected expense! Don't trust me. maybe ask a valuer?

 

What I'm not OK with-  if your house gets a loss of value as you have played with (and it is playing) home automation / complex UFH controls then that is a price you pay.  But if you try and sell your house to some unexpecting young couple.. you can f...  off., and I'm going to go hard on you! You are  not  going to sell your house to my kids!

 

Advance Foundation technolgy have grasped some of it, Tanners in Ireland are competant SE's but no one has actually put all the bits together!

 

The thing for me is that I want to design and build stuff that lasts, that delivers profit for self builders and that encourages young folk to enter the market,  that benefits society. It's not a big ask? 

 

But as a designer with lots of real life experience I want to help young folk see the wood for the trees. I want them to get on the self build ladder on the self build ladder, I did it on a shoe string and that is why I keep pushing the simple stupid... as that means you can get a wider coice of Contractors.. which drives cost down.

 

That is why I often push against the UFH complex thing and other stuff. It's also basically bonkers, but a lot of tecky folk on BH can'r see beyond 5 years.. 

 

So in summary I would like to see a lot of experienced folk on BH saying.. this works and if you are young folk embarking on a self build, on a limited budget this can work for you to.

 

OK this is not going to go down well. But the life and future of BH is in the young folk, that are strapped for cash..

 

This for me and is why I'm still here.. I want to help and this is my hobby. To enthuse young folk, we need to solve problems, use our experience to say this can be not as hard as you think, if you take our advice, but at the same time give a bit of tough love if need be.

 

PV is following much the same cycle.. 

 

 

 

 

 

 

 

 

Edited 9 hours ago by Gus Potter

[Gus Potter]

I wish BH had a spell checker. I just can't be arsed checking my dyslexia bit and spelling when I'm writing about stuff as a hobby. 

 

For all, my grammer and spelling may be a bit off when I'm off duty.. but neglect what I'm saying at your peril.

Edited 9 hours ago by Gus Potter

[SteamyTea]

4 hours ago, Gus Potter said:

I wish BH had a spell checker

Most web browsers have them built in

 

[Dillsue]

14 hours ago, Beelbeebub said:

And crucially at zero extra load on the grid. 

If you've already got PV then in most instances any surplus goes to the grid and powers your neighbours. If you then start using the surplus or put it into batteries to power AC then your neighbours are deprived of your surplus so have to increase their draw on the grid.

 

Obviously it's up to the PV owner what they do with the PV they generate, but I don't think that adding AC has no effect on the grid.

[JohnMo]

8 hours ago, sharpener said:

cannot be doing with chopping and changing e.g. to milk Octopus Agile (even though I have an EV). So am on Cosy and charge battery to 100% every cheap period. This means I do not have to guess what the next day's PV will bring in order to leave enough room for it. (Havenwise might do a better job of this but is not compatible.) Also means we can do laundry/dishwasher at any time without penalty

I followed your lead on this, exact same, fill battery over each cheap cosy slot, if there is excess PV it just gets exported. Doing a predictive charge may save a few pence here and there, but life's too short. The batteries own control does all the work.

[Roger440]

On 18/11/2025 at 21:35, Michael_S said:

Logically I think you are right but in reality people seem to be very short-sighted and only look at the up front cost - se for example EVs.

 

I dont think they are short sighted. The vast majority of the population simply cant afford to drop a 5 figure sum on heating, an EV etc, no matter how good it looks finanicially over the long term

 

If electricity was free for heat pump use, thay still cant fund the heat pump in the first place. Running costs are acedemic if you dont have the capital.

 

You have demonstrated, once again, that BH members are not representative of society at large.