Am thinking of not having a battery (definite) or solar pv (probable)

[offthepiste]

Given imminent Enerphitish project have just been noodling over back of fag packet estimates for space heating (SH), direct hot water (DHW) and direct electricity (DE), e.g. sockets & lights, given assumed space heating requirements of 50kWh/m2 for 300m2 and various options from keeping existing Viessmann 222-f though ASHP for SH and possibly DHW as well as Solar PV @3,680 kWh projected output p.a. and battery such as Powerwall 2. 

 

Am a bit surprised by my conclusions … but working out electricity needs after estimating Solar PV generated and SH needs by calendar month it looks like Powerwall would take about 20 years to pay back and Solar PV about 7 years - assuming Octopus Go Faster tariffs and ignoring trivial opportunity to export back to grid, i.e. Tesla tariff.

 

On the assumption we are heading back to the good old days of sustained inflation I could perhaps argue that capital investment now would reap benefits but then again there may be new innovations and I avoid extra complications by more things to go wrong. And willing to risk blackouts..

 

So am thinking of not having a battery (definite) or solar pv (probable). 

 

Any thoughts? Cheers

[joe90]

You only buy insulation once so why not make it a lot and build right/tight. I am having second thoughts about PV now I can diy instal like @ProDave at a much lower cost and time high loads to the middle of the day. All energy costs are going to rise so let’s see how little we can consume (save the planet/wallet) .

[offthepiste]

1 minute ago, joe90 said:

You only buy insulation once so why not make it a lot and build right/tight. I am having second thoughts about PV now I can diy instal like @ProDave at a much lower cost and time high loads to the middle of the day. All energy costs are going to rise so let’s see how little we can consume (save the planet/wallet) .

Hi @joe90 yes likewise on insulation and air tightness, working on 50kWh/m2 as Airtightness and having to have IWI will stop us going to full Enerphit without spending huge amount. 

 

Will probably such it and see how much electricity we are consuming after first year - hope to finish end 2022 - and then look at Solar PV again. Batteries look quite expensive.

[joe90]

1 minute ago, offthepiste said:

Batteries look quite expensive.

Yes and have a limited life, not like insulation ?

[joth]

13 minutes ago, offthepiste said:

Powerwall would take about 20 years to pay back and Solar PV about 7 years

 

Sounds quite plausible  / in line with the general findings. I think ~10yr payback on PV is what most find ballpark, and batteries twice that. Aside from the cost of energy increasing (and inflation pressure), PV and batteries in particular are expected to drop in price, so there's a good case for leaving it 5 years and seeing.

The only case for PV now is if you have the scaff up and doing a lot of roof work anyway. Maybe you can save on tiles by doing in inline inroof system (which also look nicer and lower maintenance).

But if you're not tackling the roof now, leave the PV until such time as you are?

 

As joe (and all buildhub) says, fabric first. Fancy tech later ? 

 

[offthepiste]

Just now, joth said:

 

Sounds quite plausible  / in line with the general findings. I think ~10yr payback on PV is what most find ballpark, and batteries twice that. Aside from the cost of energy increasing (and inflation pressure), PV and batteries in particular are expected to drop in price, so there's a good case for leaving it 5 years and seeing.

The only case for PV now is if you have the scaff up and doing a lot of roof work anyway. Maybe you can save on tiles by doing in inline inroof system (which also look nicer and lower maintenance).

But if you're not tackling the roof now, leave the PV until such time as you are?

 

As joe (and all buildhub) says, fabric first. Fancy tech later ? 

 

Hi @joth will be tackling the roof - complete rework and new dormers etc. basically ripping the insides and outsides out. Plan to pre position cabling or at least ducting & possibly thin ply fixings for Solar Pv and possibly battery cabling - the latter would go in the garden as I believe there is a yet to surface risk of domestic battery fires and want to keep it well away from house if ever I get around to one.

[pdf27]

Slightly random thoughts:

• Are you allowing for the avoided cost of tiles/slates from in-roof PV? For this and assuming a reasonable export price (5p/kWh is available from Octopus, and Agile export is averaging about 20p/kWh at the moment) I got a payback of 5-6 years.
• A 7 year payback is actually really good - where else are you going to get a 10% return on an investment? Note that this is reliant on the cash being available however - PV prices are only going down, so it isn't something you really need to do now.
• Battery prices are coming down really fast: if you have the opportunity then leaving a space allocation for a battery makes a lot of sense, but the economics of buying one at the moment are really questionable unless you treat it as a toy with any cost savings being a bonus.

[BenP]

Fabric first baby! 

 

Our extension is all about pretty decent levels of insulation but still balancing cost and impact on space (we had limited footprint to play with due to specific planning policies). Super airtight, decent MVHR. UFH designed to run at 35 degrees with close pipe spacing. Solar PV going in now as it saved us nearly 1000 tiles (so nearly £1000). Essentially making the PV system cost just over £2k as a DIY install. 

 

Existing cottage will be next - upgrading a 9" solid wall structure. 

 

Existing combi boiler will be kept until it pops! Probably going to go for a clever unvented cylinder that takes excess solar energy, soonish. 

 

Following that, we'll then look at clever tech...

[offthepiste]

@pdf27 & @BenP Hi, keen to retain our existing and relatively (25 year) new clay tiles (Victorian house).

 

New dormer is partially for energy improvements but also a platform for potential Solar PV panels. If I was to go Solar PV I had been looking at the Sunpower Maxeon 3 400W which seem to be the most efficient on the market - however am going to hold off.

 

Agree re MVHR - it will be a pleasure to have better air as well and also UFH at 35 deg. Highly likely will get an ASHP for UFH and possibly use for DWH at some time - using overnight cheap rates. Am working on assumption - having read various posts - that will run UFH during the day say for 15 hours rather than 24/7.

[ProDave]

If you are basing your expected electricity use (and hence PV and battery use) just on your heating needs, you are missing the mark by a country mile.

 

Our heating requirement is a little lower than yours.  BUT we find we use THREE TIMES as much electricity on "non heating" uses in this house.  That's everything, washing machine, dishwasher, tumble dryer, fridge, freezer, televisions, computers and the list goes on. Not forgetting the low power things like mvhr and treatment plant on all the time.  In a low energy house, heating is a small part of your total energy useage.

 

When you properly add up your usage, it becomes extremely easy so self use almost all that a 3.68kW system will generate, particularly so if you use a diverter to send any ecxess to an immersion heater.

 

I am not convinced a battery is viable for a small PV array given we find it so easy to self use it all, so all a battery would allow us to do is store it for a "better" use than just sending surplus to the immersion heater,.

[joe90]

2 minutes ago, ProDave said:

am not convinced a battery is viable for a small PV array given we find it so easy to self use it all, so all a battery would allow us to do is store it for a "better" use than just sending surplus to the immersion heater,.

+1, but sending it to the immersion heater is another form of battery, one that stores heat not volts, so it will save you volts/gas as the DHW will need less of either ? (and you already have the DHW cylinder)

[offthepiste]

7 minutes ago, ProDave said:

If you are basing your expected electricity use (and hence PV and battery use) just on your heating needs, you are missing the mark by a country mile.

 

Our heating requirement is a little lower than yours.  BUT we find we use THREE TIMES as much electricity on "non heating" uses in this house.  That's everything, washing machine, dishwasher, tumble dryer, fridge, freezer, televisions, computers and the list goes on. Not forgetting the low power things like mvhr and treatment plant on all the time.  In a low energy house, heating is a small part of your total energy useage.

 

When you properly add up your usage, it becomes extremely easy so self use almost all that a 3.68kW system will generate, particularly so if you use a diverter to send any ecxess to an immersion heater.

 

I am not convinced a battery is viable for a small PV array given we find it so easy to self use it all, so all a battery would allow us to do is store it for a "better" use than just sending surplus to the immersion heater,.

Hi @ProDave hah.... yes, I am a bit anal, and have scoured various white goods and other electrical devices specs as well as statistics and common sense to estimate both DWH and DE usage even spitting kWh required to heat water (estimate 12 degrees) to 55 or so degrees for dishwasher and washing machine and then estimating energy used for pumps etc, e.g. we have 2 Macbook pros always on charge, 2 iPad pros, as well as about 5 iPhones (!!!) and large screen etc which I have estimated at 357 kWh for 1 hour each a day 365 days. 

Have estimated direct electricity is 2,560 kWh / year and DWH is 6,028 kWh / year

Edited December 17, 2021 by offthepiste

[ProDave]

Our "non heating" electricity usage runs at about 90kWh per week in winter, dropping to about 70kWh per week in summer.  Even when we go on holiday so no televisions, washing machines etc, it only drops to about 45kWh per week.  I keep looking for ways to reduce that, and the only one I could come up with is throw the tumble dryer away, but that is "required" to make things fluffy and only used for those things.

[offthepiste]

2 minutes ago, ProDave said:

Our "non heating" electricity usage runs at about 90kWh per week in winter, dropping to about 70kWh per week in summer.  Even when we go on holiday so no televisions, washing machines etc, it only drops to about 45kWh per week.  I keep looking for ways to reduce that, and the only one I could come up with is throw the tumble dryer away, but that is "required" to make things fluffy and only used for those things.

As we will need a new tumble drier have been looking at ones with heat pumps such as Miele TCR780WP rated at an average 174kWh per year

[ProDave]

3 minutes ago, offthepiste said:

As we will need a new tumble drier have been looking at ones with heat pumps such as Miele TCR780WP rated at an average 174kWh per year

A meaningless figure, about 3.4kWh per week.

 

We only use ours (condensing dryer so at least the heat stays in the house and for half the year is useful) about half an hour, 3 times a week, so if it has a 2kW heating element that will be about 3kWh.  The key to minimising use is only put things that need to be fluffy in it (everything else goes straight onto the airer or outside) and then it does not need to run long enough for it to come out bone dry, just a short time in the machine, then onto the airer makes it sufficiently fluffy.

 

Used like that I would expect a heat pump dryer to be half what we use so perhaps 1.5kWh per week or 78kWh per year.

[Iceverge]

A larger hot water cylinder is a pretty cheap battery.

 

2 adults 2 kids here. DHW 3650kWh/year. 

 

Space heating looks like (its our first winter)  about 2000kWh/year ,185m2 passive class house, direct electric heating. 

 

Everything else 2920kWh/year. We plan on solar PV at some stage and should be able to use everything a 3kw array produces on DHW , about a 7 year payback. 

 

With a bit of careful timing of appliances 4kw should be easily consumed. 

[joe90]

A tumble drier is not allowed in our build ?, use too much lecky when there is ample windy stuff outside.

[SteamyTea]

3 hours ago, offthepiste said:

As we will need a new tumble drier have been looking at ones with heat pumps such as Miele TCR780WP rated at an average 174kWh per year

Get a washing line, costs 2 quid (with pegs from Poundland) and uses 0 kWh.year^-1.

 

If you are doing the roof with Dormers in it, will these cause any shading issues?

[offthepiste]

38 minutes ago, SteamyTea said:

Get a washing line, costs 2 quid (with pegs from Poundland) and uses 0 kWh.year^-1.

 

If you are doing the roof with Dormers in it, will these cause any shading issues?

Hi @SteamyTea From what I can work out no shading issue, panels will be at about 155 degrees, and I overlaid a sun map https://www.gaisma.com over google map and also used https://findmyshadow.com where you can specify 3d shapes ... I was originally worried but looks like not an issue.

[offthepiste]

An update, have bitten bullet and decided on Solar Pv - Sunpower Maxeon 6 - hopefully about 5.2kw and Powerwall 2.

 

And having done the sums, the total energy saving gadget package of ASHP, MVHR, Solar PV, and Powerwall will have payback of about 11 years assuming some sort of E7 currently capped tariff. And payback 10 years with export such as the Tesla tariff. 

 

Assuming energy costs go up, e.g. no cap, then payback will obviously be better. Plus, I am guessing given inflation & related demand panels and battery may be more expensive.

 

Also @SteamyTea have decided on a small laundry room with UFH and MVHR extract - so I will follow your advice and hang things up to dry

[SteamyTea]

27 minutes ago, offthepiste said:

have decided on a small laundry room with UFH and MVHR extract - so I will follow your advice and hang things up to dry

Should work really quick.

May be worth looking into an enthalpy MVHR, grad some of the latent heat of condensation back, and stop the house getting too dry, not that it really a problem here.

[Radian]

4 hours ago, offthepiste said:

An update, have bitten bullet and decided on Solar Pv

Self install or contractor? 

[Dillsue]

8 hours ago, Radian said:

Self install or contractor? 

Can you get a powerwall for self install?? I thought they had to be professionally installed???

[S2D2]

20 hours ago, offthepiste said:

An update, have bitten bullet and decided on Solar Pv - Sunpower Maxeon 6 - hopefully about 5.2kw and Powerwall 2.

 

And having done the sums, the total energy saving gadget package of ASHP, MVHR, Solar PV, and Powerwall will have payback of about 11 years assuming some sort of E7 currently capped tariff. And payback 10 years with export such as the Tesla tariff. 

Do you have any existing consumption data (or a DCC connected smart meter) that could be used to model PV/Battery performance in the finished build? I could never make the powerwall add up when I looked, have you considered other brands? I was also surprised how significantly battery clipping (fully charged causing export) decreased ROI when modelling with half-hourly data, it really does drive the choice of battery to avoid an excessively long ROI: 

 

[offthepiste]

Hi @Radian contractor install I'm afraid. Was a bit worried that I had dithered too long .. should have commited back in May. Understanding is that the Solar PV will be installed soon and Powerwall next September! Just going my optimistic view on number of panels, i.e. 11, will be realised. Panels should be Sunpower Maxeon 6 AC - ideally 440w, possibly 420w, each with an enphase microinverter.

 

@SteamyTea yup, haver the Zehnder Comfoair Q600 with enthalpy. 

 

Cheers