Solved

Solar storage - How can I work out what sized batteries I need?


Userlevel 2
Badge +1

We have a 12 panel array which is feeding into our  Pylontech US3000 battery. 
On a good day like today our battery is 90% full the panels are producing 2.97kwh. As our usage is low at this time, 2pm, the energy produced by the panels is being fed into the grid. 
we are just about to go onto a new tariff paying 44p/kwh   
A new Pylontech US5000 battery will cost £1400. 
Is an additional battery worth investing in? Not sure how to work it out. We are currently producing between 1/3 and ½ of our daily usage   This obviously goes down considerably in winter months between 1-3kWh depending on the weather.
All our hot water at this time of year is coming from the panels via a solar iboost to an immersion heater 
Summary, half our production on a sunny day is being fed to the grid. What can I do? How many batteries would I need. 

icon

Best answer by Sean T 23 March 2022, 23:07

View original

26 replies

Userlevel 7

Good question @philipH,

 

I know some other forum members have mentioned making similar calculations and they may be willing to give their thoughts:

 

@Mark C @Graham110011 @chrisjw37 @Llamedospete and @Sean T are the first that come to mind… I think @timciren has 2 storage batteries… also @EverythingNeedsAUserName @hambrook and @f0rtune may be willing to step in here.

Userlevel 2
Badge +1

Thanks Tim. Look forward to some good insight into this

P

Userlevel 2
Badge +2

Hi philipH

If you don’t have sporadic and excessive power demands, such as EV charging, then you should be able to use an app to calculate how much you’re sending to the grid on your best days (potential for storage) and how much you’re consuming from the grid and when. If you’re sending to the grid all day and draining your battery before the sun starts charging it again then it may be worth considering an additional battery.

Userlevel 2
Badge +2

I’m guessing 2.97kW is how much the panels are producing now? Better to look at the total amount produced in a day and how much is sent to the grid. Our 16 panels generate anything between 15 and 30+kWh per day in the summer (295w panels on different faces of a roof). 

We’ll have to defer to some maths here…. it’s excel time…

If there’s more than 5kWh being sent to the grid on a regular basis you could probably make use of an additional 5kWh battery. 

Payback time is going to depend on the number of days you send back 5kWh to the grid (i.e. when would a full 8kWh battery pack be charged up). At £1,400 and 44p/kWh I’m guessing the payback period wouldn’t be very long at all. 

EVM on YouTube has some (painfully dull, but quite informative) videos on home battery storage that might be worth a watch. 

Userlevel 7
Badge +3

Why are you paying 44p kWh? Isn't the variable tariff a lot less than this (approx 29p)?

Userlevel 7
Badge +1

Did you lock in at 44p on a fixed rate tariff recently? That does sound particularly high. Is that including VAT? 

Userlevel 2
Badge +1

We were with green energy @11p/kwh that went up to 22 Shell took over when Green went bust and we have just been informed that their new variable rate will be 44p/kWh 

Userlevel 7
Badge +1

We were with green energy @11p/kwh that went up to 22 Shell took over when Green went bust and we have just been informed that their new variable rate will be 44p/kWh 

The price cap does vary by region, how you pay etc. so although it varies i would expect it to be around 28p if you are on a single price capped variable tariff

44p doesn't sound right unless they have put you on a fixed rate. Might be worth going back and checking with them?

Userlevel 2
Badge +1

Thanks @Jeffus  I shall look into it

Userlevel 2
Badge +1

@f0rtune 

today our panels produced 14kwh

We consumed 17kwh excluding this evenings use. Our battery was full by midday but the bulk of our use was before the batteries had charged. 
our feed in today has been 7kwh because the battery was full and our usage at the time was low. So at that rate we are sending half of our production to the grid. We are not on any feed in tariff

thanks

p

Userlevel 2
Badge +2

@f0rtune

today our panels produced 14kwh

We consumed 17kwh excluding this evenings use. Our battery was full by midday but the bulk of our use was before the batteries had charged. 
our feed in today has been 7kwh because the battery was full and our usage at the time was low. So at that rate we are sending half of our production to the grid. We are not on any feed in tariff

thanks

p

I’m surprised your hot water tank isn’t using more of that!

 

but fair enough, if you’ve got 7kWh spare on a sunny spring day that probably means 3-5kWh spare on a cloudy day, you could look back over a year or two of annual data if you have access to that and see how much you send to the grid each day, then you could approximate how long it’ll take to pay off the extra battery.

Userlevel 2
Badge +1

@f0rtune  we have lashings of hot water so some of that production is certainly being used. 
I’ll see what I can find with past data. 
thank you

P

Userlevel 3
Badge +2

Hi all, my installation begins tomorrow in time for the end of the RHI payments. I have opted for 12.5kW of solar PV, 2 Tesla batteries and a Vaillant Arotherm plus ASHP. I currently use 40 kWh/day, which is expected to rise to 60kWh/day when the ASHP becomes active. My calculations are based upon the following - I am on a variable tariff of 28p/kWh due to an inoperable smart meter (that’s another story for another thread!). I currently have an oil boiler which has cost me £1,850 for 2600L over the past 12 months for CH and hot water. However oil costs are currently £1,500/1000L so if that remains for the foreseeable future my projected oil costs will be £3,900/year. Based on my current 40kWh/day at 28p/kWh and the oil costs my annual energy bill would be £4,088 + £3,900 = £7,988. Switching to the ASHP and buying from the grid at an average of 60kWh/day would result in an energy cost of £6,132/year, so simply moving to the ASHP will save me £1,856/year (provided it does what it says on tin and electricity costs and oil costs remain as they are now). The 12.5kW of solar PV is projected to produce 10,299kWh/year. Now I appreciate that this is not linear over the year and this is where it begins to become guess work until my 1st year of experimentation has been completed. However, my purchase from the grid will be reduced whatever happens and even if this results in me being able to use 80% of what I generate due to the 2 Tesla batteries then my energy costs will be 60kWh/day x 365 days = 21,900kWh/year - 8,239kWh/year (utilised 80% of generated solar PV) = 13,661 x 28p = £3,825. So on current prices my new system will save me £4,163/year over my current system. The entire new system is costing me £38,000 with a RHI payment of £11,600, so an actual outlay of £26,400, which will take me approx 6.5 years to repay, then I’m saving £4k/year in energy costs. In the future I may install further solar PV and another Tesla battery if that takes up some of the £3,825 from the grid, or alternatively a wind turbine to produce more of the winter requirement and level the annual curve. I will let you know how I get on but my entire system should be installed and active within the next 2 weeks (the solar PV install is now delayed until after 1/4/22 so i get 0% VAT after today's budget!). 

Userlevel 7

A really good example of why households who adopt technology are often best placed to guide others by sharing their own experiences. Hopefully you get the answer you need, @philipH.

 

There was another solar PV post yesterday, and perhaps one of you can recall how the panels are connected to the property and its consumer unit. I know @mattwillhill will be grateful if anyone can steer him in the right direction:

 

 

Incredibly, the import smart meter has the same consumption rate as before the solar panels were fitted, how is this possible and is it the wiring? @f0rtune @Sean T @M.isterW @Graham110011?

Userlevel 7
Badge +1

I enjoyed reading your experience @Sean T

It was interesting to see the environment benefits and costings, particularly the good payback time, although the upfront costs are quite eye watering for me when i read them, especially given the RHI scheme is being replaced. 

If upfront money had been tight, what might you have done differently and how much might it have reduced the cost? I appreciate there are obviously lots of alternatives. 

Would you have done everything if the RHI wasn't available and you had the replacement £5k upfront payment? 

 

 

Userlevel 7

I second what @Jeffus said - Great to hear the calculations you made when investing in a new ASHP and energy storage, @Sean T .

 

Any closer to working out the best option for you, @philipH? 🌞

Userlevel 3
Badge +2

I will be able to give further calculations and updates soon as my 12kw solar pv and 2 Tesla batteries are being installed today and tomorrow. I am currently using 54kwh/day of electricity (was 40kwh/day before the ASHP) so it will be interesting to see what happens to my grid consumption once these other components start to contribute to my reduced draw from the grid. 

Userlevel 3
Badge +2

Hi all, I am now at the end of my first full day of solar pv (12Kw) and storage battery use (2 x powerwalls). Its been a sunny interval day at my home address and I have generated 68kwh of electricity. I’ve not drawn from the grid since 7am this morning and I am currently operating on my battery power (self powered mode) as the sun has gone down. At 20:25 hours I have 77% of my storage left so i am going to see if I can make it around to a full 24 hours without pulling from the grid (sorry OVO!). My calculations say I am likely to fall short by about 2 hours due to my ASHP heating hot water at 02:30 - 04:30 and then the central heating kicking in at 05:00. We will see. However, my first day of solar pv and storage batteries has saved me approximately £14 at £0.28p/kwh. even if I can average a yearly saving of £7/day, my payback for the system will be 6 years, based on a grid cost of £0.28p/kwh. Since grid electricity is likely to increase further over the coming months/years, then 6 year payback should be worse case scenario (hopefully). 

Userlevel 6
Badge +2

Only just found this thread but we’ve been using solar PV for around 13 years and installed battery storage towards the end of last year. 
My own experience was to monitor typical use so that maximum draw on supply can be noted together with potential charge in a typical day. 
Many of the systems available have maximum outputs which many not supply peak draw (electric shower or similar). You also need to factor in the effect over 10-15 years as you should expect the tech to deliver over that period. Many installers will provide their own calculations to show the benefit compared to cost but I would suggest to calculate against a riding energy cost trend as well. 
I’m quite happy with our own 8kW storage system but will at some point look to update the controller to deliver more than the 3kW max output. It is a balance as during winter months, the system is poorly utilised in our ‘sunny’ UK. 
 

LuxPower Tech control system with PylonTech battery banks and Effinity 3.75kW solar array

Userlevel 7
Badge +3

Dropping in to ask a similar question.

 

We're getting a 16 panel east/west solar PV installation (7 east, 9 west) and I'm trying to work out if it's worth getting a battery. The company have offered a battery at a really good price (£3200 for 9.9kWh, £1800 for 3.3kWh, 6.5kWh somewhere between those two) that I'm unlikely to be able to match if I buy one later. So do I take a chance and get one now or do I wait and see what our usage and export is?

 

We're a 4 person household using a lot of electricity because we have a heat pump, although our usage is a lot higher in the winter when there is less solar generation. We plan to use a diverter to heat our hot water and possibly a Zappi to charge our car (currently a phev but the next one will be an ev).

Userlevel 6
Badge +2

Dropping in to ask a similar question.

 

We're getting a 16 panel east/west solar PV installation (7 east, 9 west) and I'm trying to work out if it's worth getting a battery. The company have offered a battery at a really good price (£3200 for 9.9kWh, £1800 for 3.3kWh, 6.5kWh somewhere between those two) that I'm unlikely to be able to match if I buy one later. So do I take a chance and get one now or do I wait and see what our usage and export is?

 

We're a 4 person household using a lot of electricity because we have a heat pump, although our usage is a lot higher in the winter when there is less solar generation. We plan to use a diverter to heat our hot water and possibly a Zappi to charge our car (currently a phev but the next one will be an ev).

Having had solar PV for sometime and more recently seen the daily use vs feed in to grid I would say that it’s definitely worth installing battery storage. It will take a similar time for your investment return (it was 9 years on our solar array) but with energy costs increasing as they are you should certainly benefit by using more of your generated solar energy. 
If you’re still unsure, why not monitor your daily use and compare to predicted solar production. 

Userlevel 7

Great question and advice here, @M.isterW and @BPLightlog.

 

Given the increased interest wonder whether we might need to cover solar storage options in more depth on our Solar PV guide:
 

 

Do let us know which option you decide to go for, @M.isterW.

Userlevel 7

We're getting a 16 panel east/west solar PV installation (7 east, 9 west) and I'm trying to work out if it's worth getting a battery.

 

@M.isterW have you decided on your need for energy storage alongside your solar panels? 

Userlevel 7
Badge +3

After much deliberation we're going for a fairly small battery (3.3kWh). The rationale is.…

- It's cheaper to fit a battery at the same time as the solar panels. You save on vat (0% if fitted with solar panels, 20% if fitted afterwards) and you can use a hybrid inverter to manage both panels and battery. If you fit a battery later you may need a separate inverter, or swap out an existing solar inverter for a hybrid.

- We were offered the batteries at a really good price.

- The payback period on a small battery will be shorter than a large battery. I'm quite confident that we will make full use of a 3.3kWh battery. I don't know how much more benefit we'd get from a bigger one, particularly as we have other options for using spare solar PV.

- The system we're installing allows for additional batteries to be plugged in. That means we can fit more at a later date without paying for someone to install them.

Userlevel 7

I’m looking forward to hearing about how this smaller storage battery performs in your home, @M.isterW - that rationale makes a lot of sense!

 

When is the storage and solar going in?

Reply