Solar - Where to start?

[Simon R]

10 hours ago, SteamyTea said:

Storing the PV energy in a cylinder as hot water is the cheapest type of storage, way cheaper than batteries.

Very true but storing hot water is a losing game in energy/CO2 terms, you may get a lot of hot water, but how useful is it in practice.

Given there is already a gas boiler heating water on demand is far better in energy/CO2 terms.

With two in our household we're using 22kWh of gas a month , pretty much all on showers and the occasional bath. The washing machine and dishwasher both heat there own water. I don't get paid for the surplus electricity exported but it does reduce the total CO2 production, every little helps....All the energy from the heated water just wicks away over time.

 

[Marvin]

Hi @DazRave

 

In my humble opinion I would install as many panels as possible as long as the mains doesn't need to be upgraded.  You will produce too much energy in the summer and not enough in the winter. We also have battery back up. both are DIY installed - with DNO permission.

 

We have AIM and APE,

 

That is Airtightness, Insulation, Mechanical Ventilation with Heat Recovery, and an Air Source Heat Pump, Photovoltaics and an Electric Vehicle.

 

We installed our own PV system (not including batteries) and it cost about £4k for a 5.12kW system ( that is to say all things being perfect on the best sunny point of the best day the system would produce 5.12kW an hour - never seen it by the way) Inverters usually allow you to instal up to 20% more panels than its limit. Our inverter is a 6kW one but allows up to 7.8kW of panels.

 

We judge our PV system (not including batteries) would last about 7 years so the daily cost is about £1.60.

 

Batteries are a funny thing, and unless you are paying more for your day time electricity and less during the night on some scheme I would recommend thinking long and hard about your decisions.

 

In the winter you would have almost nothing spare from the PV to charge your batteries

In the summer you would have far too much to charge your batteries.

In the other times you would have power to charge your batteries.

 

If you charge your batteries using cheap rate night mains you waste the spare power from the PV for about 5 months of the year OR you charge the batteries using the cheap rate night mains and waste the PV power.

 

When you do all the maths, if you have PV, the most logical system I see is one that supplies enough over night power to run your night time low demand load. This system can support say about 3 days worth of night time energy, but go beyond that and the figures don't really work.

 

If you want to go through the iterations leading to the same conclusion PM me. This goes for anyone else as I would love to know of the exceptions. 

 

 

 

Good luck

 

Marvin

 

PS. Don't forget the significant losses converting AC to DC and back again.

 

 

 

 

 

 

 

 

 

 

[SteamyTea]

There seems to be some confusion about my comment on storing energy as hot water.

It was never meant to mean store all your energy as hot water. That would be a bit silly.

It is also silly to heat and store more water than you actually need.  So while the volume of a cylinder may be fixed i.e. 200, 220, 250, 300 litres, the mean stored temperature and be varied, within limits i.e. not too hot, not too cold.

There is, as @pocster and @ProDave illustrate with their set ups, a zealous need to export nothing and overstore.  That is a bit silly as well.

 

When the original question was asked by @DazRave, apart from spending £10k on a system, which says nothing about the size of the system, I was also unaware that he had gas already.

But let us look at the cost of gas.  I don't have it, but there is a standing charge, is that similar to electricity i.e. 60p/day?

How much is an annual service £150?

Assuming a combi boiler, how efficient are they when heating water for a few minutes shower, 80 to 85%?  Never seen anyone work that one out.

 

There is also the added benefit that roof integrated PV can decrease solar energy entering a loft/roof void by about 18 to 20%.  This may help, depending on house design, reduce the need for summertime cooling.  Cooling should not really be necessary though, it is a building design failure.

[joe90]

19 minutes ago, SteamyTea said:

Assuming a combi boiler, how efficient are they when heating water for a few minutes shower, 80 to 85%?  Never seen anyone work that one out.

Interesting as I recently moved to a house with a combi gas boiler having previously having  a large DHW tank that gave hot water to any tap in seconds. My dilemma now is, is it worth running the combi for several minutes to give me a sink of hot water to wash up a few dishes or boil the kettle? I know gas is a third of the price of leky but the boiler runs three times as long 🤔 ( I can’t have a shower by boiling the kettle 🤣).

[Simon R]

16 minutes ago, SteamyTea said:

But let us look at the cost of gas.  I don't have it, but there is a standing charge, is that similar to electricity i.e. 60p/day?

How much is an annual service £150?

Assuming a combi boiler, how efficient are they when heating water for a few minutes shower, 80 to 85%?  Never seen anyone work that one out.

You're absolutely right about efficiency, but again if you're not using much you're not wasting much...back to the mantra of always minimise your inputs. Utilita have a gas options that has no standing charge, just a higher rate of 23.7p a kWh making it a very good option for users who don't use much gas.

Regarding annual service, essential for a rental property but optional for your home. If the boiler is undisturbed and working properly it should not be an appreciable safety risk provided you have good mains connected CO alarms fitted. Rightly or wrongly I work on the principal of if it's not broke then don't fix it.

 

I didn't plan solar into our build, but having added it I can't imagine trying to run without a battery, even given how expensive the battery was.

[SteamyTea]

21 minutes ago, joe90 said:

My dilemma now is, is it worth running the combi for several minutes to give me a sink of hot water to wash up a few dishes or boil the kettle? I know gas is a third of the price of leky but the boiler runs three times as long

Not just the length of time, it is the power of each heating source.

If your kettle is about 2.8 kW and your gas boiler is 20 kW, then 

5 minutes for the kettle is 0.23 kWh, boiler 5 kWh.

So get some proper timings, the power ratings and we can see.

18 minutes ago, Simon R said:

back to the mantra of always minimise your inputs

And outputs.

18 minutes ago, Simon R said:

didn't plan solar into our build, but having added it I can't imagine trying to run without a battery, even given how expensive the battery was.

What size is your battery system and what is the utilisation like?

Finding the Goldilocks zone is the hard part.

 

There are 3 parts to having a battery system (4 if you include cost).

The actual usable storage, the kWh.

The charge rate, the input kW.

The discharge rate, the output kW.  It is this last one that is important as some system need a 200W+ load to start delivering and may top out at a 2 kW load, though I would hope most would be capable of 3 kW at least (enough for a kettle).

[Pocster]

2 hours ago, SteamyTea said:

There is, as @pocster and @ProDave illustrate with their set ups, a zealous need to export nothing and overstore.  That is a bit silly as well.

Eh ? . I export loads and don’t over store . What you on ?

[SteamyTea]

4 minutes ago, pocster said:

Eh ? . I export loads and don’t over store . What you on ?

Stop mucking about, you would have all the storage in the nation if you knew the difference between Kw and kW/h. 

[Pocster]

2 minutes ago, SteamyTea said:

Stop mucking about, you would have all the storage in the nation if you knew the difference between Kw and kW/h. 

You said “ export nothing and overstore “ ; please explain .

[Pocster]

@DazRave 

You need to decide your “ reasoning “ as you’ll get multiple views .

 

I personally think of all this like a car . If I want flash , fast then I’ll buy what I want and ROI is meaningless. If I want efficient , low cost then I’d choose a different car .

Are you simply trying to lower bills ?

Do you want your annual lecky bill to be zero ? ; would you like to ‘earn’ from excess PV etc ?

People fixate on ROI - which is fine if that’s the ONLY thing that matters .

Even then what’s a ‘good’ ROi ? 5 yrs ? , 10yrs ? ; compare it to a savings account .

Your reasoning for having pv/battery determines what’s important to you and (expletive deleted) everyone else 😎

[Marvin]

1 hour ago, SteamyTea said:

There seems to be some confusion about my comment on storing energy as hot water.

It was never meant to mean store all your energy as hot water. That would be a bit silly.

It is also silly to heat and store more water than you actually need.  So while the volume of a cylinder may be fixed i.e. 200, 220, 250, 300 litres, the mean stored temperature and be varied, within limits i.e. not too hot, not too cold.

There is, as @pocster and @ProDave illustrate with their set ups, a zealous need to export nothing and overstore.  That is a bit silly as well.

 

When the original question was asked by @DazRave, apart from spending £10k on a system, which says nothing about the size of the system, I was also unaware that he had gas already.

But let us look at the cost of gas.  I don't have it, but there is a standing charge, is that similar to electricity i.e. 60p/day?

How much is an annual service £150?

Assuming a combi boiler, how efficient are they when heating water for a few minutes shower, 80 to 85%?  Never seen anyone work that one out.

 

There is also the added benefit that roof integrated PV can decrease solar energy entering a loft/roof void by about 18 to 20%.  This may help, depending on house design, reduce the need for summertime cooling.  Cooling should not really be necessary though, it is a building design failure.

Hi @SteamyTea. All good points.

 

Storing hot water: From our experiences we find that using a PV diverter into a very well insulated 205litre tank to a temp of 75degC will allow hot water for 2 shower a day for 3 days before it runs out. This means if we have odd sunny days, like today, PV will heat the water, and if tomorrow its cloudy we still have hot water. During June 2023 we purchased 87.6 kWh of electricity. There is no mains gas here. We also fully charged our EV during this time. Bear in mind we live on the sunny Isle of Wight!

 

Storing Energy. When doing the calculations I have found that 3 nights of energy storage in batteries is about the best option.  We have about 6kWh of storage. If you try to accommodate all your daily use in batteries, the batteries, power inverter, cabling and conversion losses become inhibitive cost wise in my opinion.

 

The best set up for using PV is to have items that will use electricity. So gas and PV is an interesting one....

[Marvin]

5 minutes ago, pocster said:

You need to decide your “ reasoning “ as you’ll get multiple views .

Well at least this part I totally agree with.

[Pocster]

4 minutes ago, Marvin said:

Well at least this part I totally agree with.

The problem with the excess pv into water is ; it is the most efficient route ; but how much hot water do you need in summer ? . My general view is stored electric I.e battery allows for *any* use . 

[SteamyTea]

1 hour ago, pocster said:

You said “ export nothing and overstore “ ; please explain .

Shall I point you to your topics about battery storage and adding extra?

You use a combination of PV and one of the many, confusing, Octopus tariffs to charge your batteries.

Now I admit, that you are selling some back to Octopus, but that is a very short term benefit, come winter, it will cost you I suspect.

But as you said about the car, if you want something, then you get it (mind you there are many think I want, but have no hope of getting).

 

Regarding the 'how much hot water do you need', that has been answered nicely by @Marvin.

Enough and only a little more.

As for having batteries charged up 'to use with anything', it is an expensive way to heat water when you need it.  A 12 kW shower will soon gobble though your battery storage, and that assumes they can reliable deliver that sort of power.

Do you know the cut in power your SE/PW systems needs, will it run 20W of light bulbs?  Or 10W for a laptop?  Many systems won't.

Was it @Radian that showed a chart of his inverter getting confused when the induction hob was on?  I can't remember.

1 hour ago, pocster said:

You need to decide your “ reasoning “ as you’ll get multiple views .

That bit is true, but seems to have been answered, finacial.

  

19 hours ago, DazRave said:

We've been with Octopus energy for quite some time and so certainly want to capitalise on cheaper evening electricity

 

 

[Pocster]

16 minutes ago, SteamyTea said:

but that is a very short term benefit, come winter, it will cost you I suspect.

No

Unless you are aware of future tariffs !

cost me ? How in winter ? . Worst case charge off peak and self consume 

[SteamyTea]

1 minute ago, pocster said:

Worst case charge off peak and self consume 

But you will have spent quite a few thousand quid on a system to mitigate against price variation.

 

I use about 1.3 kWh/day, consistently, the rest of my usage is at night (remember I ma on a basic E7 tariff).

If I charged up at night, say 2 kWh to cover losses and variation, then that would currently cost me 30p a day.

Say I could get a battery system installed that can store that amount for £2000.

£2000 / £0.30 = 6 years 7 months

How much would I save at current prices.

Day rate is currently £0.486/kWh, so on average, and my day usage is pretty consistent, £0.63/day, an increase of £0.33/day.

£2000 / £0.33 = 6 years.

 

There are some big assumption there, fitting a system for £2000, highly unlikely, and energy prices stay at todays relatively high amounts.

To be realistic, the system would have to cost around £500/kWh.  And even then it is marginal and relies on a £0.15 to £0.2 price different between day and night rate.

15 minutes ago, pocster said:

Unless you are aware of future tariffs

I am not, but we can look at historic 'half hour' rates and adjust to today's prices.  You are the programmer, so write something that will link to this https://www2.bmreports.com/bmrs/?q=eds/main

[Pocster]

5 minutes ago, SteamyTea said:

But you will have spent quite a few thousand quid on a system to mitigate against price variation.

 

I use about 1.3 kWh/day, consistently, the rest of my usage is at night (remember I ma on a basic E7 tariff).

If I charged up at night, say 2 kWh to cover losses and variation, then that would currently cost me 30p a day.

Say I could get a battery system installed that can store that amount for £2000.

£2000 / £0.30 = 6 years 7 months

How much would I save at current prices.

Day rate is currently £0.486/kWh, so on average, and my day usage is pretty consistent, £0.63/day, an increase of £0.33/day.

£2000 / £0.33 = 6 years.

 

There are some big assumption there, fitting a system for £2000, highly unlikely, and energy prices stay at todays relatively high amounts.

To be realistic, the system would have to cost around £500/kWh.  And even then it is marginal and relies on a £0.15 to £0.2 price different between day and night rate.

I am not, but we can look at historic 'half hour' rates and adjust to today's prices.  You are the programmer, so write something that will link to this https://www2.bmreports.com/bmrs/?q=eds/main

Price variation . Yes . The variation could be large ! - I don’t know . Equally in winter it may be possible to export excess presumably at a significantly higher rate .

I don’t know . What I do know is my bill is zero and I ‘earn’ approx £1000 a year - based on past rates / pv etc etc .

Your consumption figures are low . Therefore any pv / battery is pointless for you unless you just want to earn a green badge . But your figures are not typical .

 

As I posted before my current ROI is around 6/7 years .

I can’t see electricity getting significantly cheaper at any point in the near term future ( personal opinion ) . Battery costs have dropped a bit whilst installer costs have risen significantly.

My original install in 2020 of 4kw pv , Tesla pw , inverter etc and install was under 12k ( from memory ) . A similar setup today would be a fair chunk more . So whilst I don’t consider myself a particularly early adopter - a cost saving at getting in at that point must be factored in .

[Marvin]

6 minutes ago, pocster said:

The problem with the excess pv into water is ; it is the most efficient route ; but how much hot water do you need in summer ? . My general view is stored electric I.e battery allows for *any* use . 

Yes power into hot water is only good for the next 3 days really even with our equivalent 200mm PIR insulated storage. The biggest energy demand for a typical home is temperature stabilisation. So really, the first on the list for anyone should be thermal resistance and then energy storage.

 

 

[SteamyTea]

3 minutes ago, pocster said:

Your consumption figures are low .

It can be scaled though by multiplying until it gets to your usage.  There may be marginal saving to be made on the system install prices.

My savings are made by not importing a lot of the time, last week was 49%, made up of 43% over the night 7 hours and 52% over the day 17 hours.

That saving has probably cost me less than £100 for a few timers and the monitoring kit.  Been making those saving for the last 15+ years now.

 

I suspect that having PV and battery storage makes people a bit more prolific with their usage as they think it is cheaper, so a lot of double counting goes on.

 

[Marvin]

Hi @SteamyTea and @pocster

 

I see no mention of the losses converting AC to DC and back again. Allow 10% for conversions and losses via cables? The whole thing is a great idea but don't be drawn in to thinking it's simple to understand savings. That's just sales talk.

 

 

 

 

[Pocster]

1 minute ago, Marvin said:

Hi @SteamyTea and @pocster

 

I see no mention of the losses converting AC to DC and back again. Allow 10% for conversions and losses via cables? The whole thing is a great idea but don't be drawn in to thinking it's simple to understand savings. That's just sales talk.

 

 

 

 

Indeed there is that 10% loss factor . Of course you could minimise that by having a dc battery .

[Marvin]

3 minutes ago, pocster said:

Indeed there is that 10% loss factor . Of course you could minimise that by having a dc battery .

I assume you mean charging the DC batteries directly from the PV? : True. but don't expect much of that during the winter...

[SteamyTea]

5 minutes ago, Marvin said:

I see no mention of the losses converting AC to DC and back again.

No, PVGIS assumes 14% losses on the PV side, probably similar on a DC side.

But I was making an Anti-Sales pitch.

Batteries are all the rage, and tomorrow there will be a 'game changer' i.e. V2G, Hydrogen, novel chemistries....

[Pocster]

1 minute ago, Marvin said:

I assume you mean charging the DC batteries directly from the PV? : True. but don't expect much of that during the winter...

Yes . And yes 

[Marvin]

Hi @pocster

 

Impressed with your outlay to yearly repayment! How do you heat your home? We can't manage that!