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Battery sizing - How low can you go!

bontwoody · 2024-10-10T14:56:33Z

When I was selecting my new solar array and house battery, I had to cut my cloth according to my budget, and in the end settled for a 5 kWh Huawei Luna battery module. I wasn’t sure if this was large enough at the time, but figured that I could add additional 5 kWh modules at a later date if not. There is something intrinsically appealing about having enough battery capacity to see you through a whole day and it certainly gives you flexibility about when to charge and if to sell some electricity to the grid. Now that I have a whole year of data, I am in a position to determine whether increasing my battery capacity would make sense financially. I am currently on the Octopus Intelligent Go Tariff, which gives a tariff of about 7p per unit overnight and 27p during the day. The first graph below shows my heat pump usage throughout the year. Obviously the house uses more electrical energy that just that, but during the day this is my biggest use during the colder months. Domestic Hot Water heating and EV charging are done overnight on a cheap tariff as is washing dishes and clothes (mostly). For large parts of the year, charging my 5 kWh battery at night is sufficient so that I don’t need to draw any significant power from the grid during the peak rate period. It is clear from the graph that all days when I use less than 5 kWh hours fall into this bracket. The second graph shows my solar output for the same period. Even in the depths of January to February, it is rare not to produce at least around 5 kWh, so in fact I am probably not importing from the grid when I use less than 10 kWh of electricity with my heat pump. Looking at the first graph again, there are very few days (less than 30, I would estimate) when this happens. If any of these days coincide with a good solar output day then this will be reduced again. If we assume I was to purchase a second battery module then this could hold another 5 kWh. The difference in unit price between peak rate and off-peak rate is about 20 pence a unit, so let us assume I could save about £1 for every day when I import 5 of more kWh’s of electricity. From the previous estimate, let’s be generous and say 30 days. So I could save £30 per year. Given the cost of an additional battery module is about £2000, the payback period would be about 67 years (ignoring future price changes). Now I know that this is a bit of a “back of a fag packet” calculation and that there may be opportunities to make money by selling back to the grid. Also no one knows what will happen to electricity pricing in the future and other makes of battery may be cheaper. However even given all these variables, I would suggest that the financial argument doesn’t look good for me to upgrade. Also I would suggest that with a little thought and planning, a heat pump user like myself can get most of the advantages of a home battery without purchasing a large battery pack. I’m anticipating a healthy debate on this conclusion. :-)

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s_gatorator

(@s_gatorator)

Trusted Member Member

198 kWhs

Joined: 6 months ago

Posts: 20

09/01/2025 4:03 pm

Posted by: @transparent

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h, we're both right about how the ASHP would be supported from a Sunsynk inverter, but we've made different initial assumptions

Hi @transparent , that's all good then. Yes, I'm not changing wiring or adding a backup power distribution board (having recently done a major refurbishment) and we're in a city, all power distribution is underground and power cuts rare and short.

This is the diagram approved by the DNO (no surprises where I got the original, thank you 🙂) - I was stymied by not being able to PM you for the OpenDraw file without first posting.

End 1980's terrace in Southern England
NIBE 12kW air source heat pump
2 solar water panels
1 x Ohme ePod
1 x VW ID Buzz

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Transparent

(@transparent)

Illustrious Member Moderator

13528 kWhs

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Posts: 2356

09/01/2025 4:41 pm

I would happily send you ODT files of my diagrams for you to dissect and amend.

The more graphics we have on this forum, the easier it is for others to understand.

You will note that I keep a graphics library of 200+ PNG/JPEG images from which I can quickly build an appropriate diagram.
It's not as slick as I'd like, but at least the software is free...
... and therefore anyone else can replicate the style.

You will occasionally find my graphics on other 'official' sites and in government leaflets.
Provided that no one is trying to gain an income by their use, then that's fine by me.

Save energy... recycle electrons!

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bontwoody

(@bontwoody)

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5444 kWhs

Joined: 3 years ago

Posts: 795

Topic starter 09/01/2025 5:15 pm

Posted by: @s_gatorator

↑

@bontwoody , thank you. I'm looking to do a similar off-peak storage (ASHP, but no Solar PV) and was interested to compare your figures with my calculations. Could I just check? - your December off-peak usage is 550kWh, presumably this supports about 150kWh of charging your 5kWh battery once every day (assuming the PV is not charging it) - does that mean your normal off-peak December consumption is 400kWh (i.e. 400/756 = 52%)?

Yes thats right, although I dont totally use all my battery capacity and its closer to 4.5 kWh charging rather than 5kWh per night

I think what Im seeing is that depending on the cost of the battery and usage, its doesnt aways make financial sense to try and cover 100% of your daily usage for all days of the year. That said extending my current battery pack is quite expensive at £2500 for an extra 5kWh and the calculation will alter with something like the Fogstar 15.5 kWh battery at a similar price.

hope thats helpful

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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bontwoody

(@bontwoody)

Noble Member Contributor

5444 kWhs

Joined: 3 years ago

Posts: 795

Topic starter 03/04/2025 2:47 pm

So just for completeness here is may last update on this topic regarding the cost effectiveness of adding another 5kWh to my battery module.

Between September and March the additional 5kWh battery would have saved about £105. Given I was quoted £2500 to install one it would not make financial sense to do so.

Switching from Octopus Intelligent Go to Cosy would only have saved a small amount in December and January, the rest of the time it would have been more expensive.

Overall throughout the period I paid an average of 11p per kWh.