Home batteries - size?

[Stu999]

Out of curiosity, for those of you with home batteries, how did you decide what capacity to go with?

Do you size it for just your normal ‘device’ usage (lights, TV, computer, washing machine, fridge, etc.) or if you have, say, a heat pump did you size it partially for that too?

Not looking at it for myself (live in a flat!) but I like looking into the technology, I think it’s a really interesting area.

 

[arknor]

you can work out your electricity use over say a week, divide by 3 and you would have 2 days of battery.

 

[katie279]

Exactly right, I just worked out home much energy I used per day, that meant I could preload at 7.5p what I need overnight, and save paying 30p per kWh... Solar then becomes extra and I'm covered in winter months

 

[200sols]

I just sized it to cover a typical day's usage.

 

[b0rn2sk8]

Exactly right, I just worked out home much energy I used per day, that meant I could preload at 7.5p what I need overnight, and save paying 30p per kWh... Solar then becomes extra and I'm covered in winter months

This basically but I deducted a bit for the 6 hours of cheap time where I'll be using the grid and then added a bit as I knew I'd be getting a heat pump.

I then picked a product which closely met my wants/needs after considering the marginal/return cost of adding more. What I mean by that is that there is no point in adding another 9kwh battery at the cost of £3k to cover 1-2kwh/day shortfall.

 

[Nemiyen]

I keep thinking about this and ideally I'd want one big enough to last me until it can be fully charged by solar. In the UK though, that might need to be a months household usage.

 

[b0rn2sk8]

I keep thinking about this and ideally I'd want one big enough to last me until it can be fully charged by solar. In the UK though, that might need to be a months household usage.

You charge it from the grid when its cheap, that is its core value.

I can charge it overnight for 7.5p, I can sell my solar to the grid for 15p - that is there the value is.

 

[Stu999]

What brands did you guys use for batteries? Givenergy seem popular on YouTube.

 

[reef]

Its best to size it to about a day's usage in the heart of winter. I force discharge in the summer half of the year on Octopus Flux between 4-6pm down to 45%. If I didn't do this it would probably never drop much below 80% at this time of the year.

In winter, it's charged up cheap at 2-5am with the aim of it lasting until the next morning. We average about 10-11kWh per day, so the 12.25kWh battery we have covers us nicely.

I'd also bear in mind as your state-of-health drops over the years you'll lose capacity, so its better to have a little more.

 

[Mercenary Keyboard Warrior]

If you have solar when I modelled it I found the sweet spot was around 60-75% of daily usage.
You will fall a little short probably in winter most days, but spring through to autumn you should be able to avoid any peak units the vast majority of the time.
The current position with being able to import units and export later really depends on how hard you want to work your batteries for a few p per unit.

When you look at the marginal benefits of more storage it clearly falls off quickly at some point.
My batteries are around 60% of winter usage, when I ran the numbers with another (I did end up with diff spec in the end but I could have had more originally) the ROI of the third battery was almost exactly 10 years.
Prices have fallen since then, but so have elec unit prices so I doubt its moved that much.

Export prices have been going up. As such the benefit from batteries is lowered. I now rarely charge from the grid (only when agile is super cheap, or a powerup is active). I actually use energy from the grid when its below the 15p export price.

The balances and equations are constantly changing with battery prices and tariffs available.

 

[alphaomega16]

When it came to battery I wanted to plan for winter, not summer.

 

[tamzzy]

i've got 2x 5.12 kwh batteries (90% dod) so total usable capacity = 9.6kwh
this will last me 1.5 days during heavy use and 2 days normal use without accounting for any solar generated

on eco 7, so 8p/kwh charging the battery overnight, daytime is run on solar + battery power
electricity bill = £20/mth, of which £15 is the bleeding standing charge grumble grumble expensive standing charges
selling the excess solar during the day time, on average over the year, will cover the leccy bill and more, so basically, free leccy

my calculated net leccy + gas bill after getting paid for solar export = £10/mth on average

my current setup cost £8335 for 4.4kwp solar panels, 3.6kw inverter, 2x 5.12 batteries - my ROI for this is approx 9 years
if i didn't get the second battery it would've been £6885, and the ROI for this would've been 8 years, but this would've been less flexible due to the smaller batteries and might have cost more if my energy requirements increased

 

[Troop]

15kw house battery, as has been said, Winter charge on the cheap rate, summer garden array covers everything.
Plan to add another 15kwh of batteries this winter.(after the Fit payment pays out)