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Any views on how much capacity I need? I probably use c.7kwh per weekday 10kwh each day at the weekend, excluding the car which I’ll charge mainly with octopus go at night, plus a zappi to top up in the day. Most of my weekday usage will be in the evening / morning, but with some background in the day. 3.6kwh seems the right sort of region, but wondered what people think!
 

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I have in excess of 140kWh of Li storage, at times even that isnt enough - especially when its windy.

Whilst usage should be one of the deciding factors, your ability to recharge xyz kWh within a reasonable timeframe should also be considered .....
 

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Discussion Starter #5
I have in excess of 140kWh of Li storage, at times even that isnt enough - especially when its windy.

Whilst usage should be one of the deciding factors, your ability to recharge xyz kWh within a reasonable timeframe should also be considered .....
Does that include your car? I’m working on the basis that I’ll charge my car (95kwh) and heat my hot water (?), but that I’d also like to be able to not use the grid before I go to bed. I can’t see how I need more than 7kwh storage to achieve that. Am I missing something? Also, because of lower cost energy 0030-0430 than the price I’ll get selling back solar, I’m not fussed about the car and scheduling things for the cheap period.

I’m only looking at getting a 4.6kwp array...
 

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Does that include your car? I’m working on the basis that I’ll charge my car (95kwh) and heat my hot water (?), but that I’d also like to be able to not use the grid before I go to bed. I can’t see how I need more than 7kwh storage to achieve that. Am I missing something? Also, because of lower cost energy 0030-0430 than the price I’ll get selling back solar, I’m not fussed about the car and scheduling things for the cheap period.

I’m only looking at getting a 4.6kwp array...
I have a 4kWp array and bought a small battery setup last winter. We are with Octopus Go so we charge the car between 0:30 and 4:30 like you. That’s also when I charge the battery. At the time I could not decide between two batteries (4.8 kW) or three, but took the view that I’d start small and add another one later if necessary. So far, the battery has been enough for us. The aim is to get through the day with enough charge that I rarely have to import from the grid except for combinations of high power such as cooking a d boiling a kettle at the same time. I am also trying to vary how much charge I put in at night to minimise the export, although I don’t really need to do this as I am paid for export at a similar rate to the Octopus cheap rate.
The downside of my approach is that it thrashes the batteries by cycling them between 100% and 10% quite regularly. My plan seems to be working OK so far, but I don’t know how well it will work long term. If I had had enough cash I would have got a Powerwall 2 because it’s the cheapest form of storage per kWh, but it’s a lot to pay for what is still a bit of an experiment/hobby for me.
 

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I have a 5.7kWh array, 2 x 3.5kWh PylonTech batteries, Zappi And Eddi.

My plan was similar to yours, after 5 weeks I am happy with the functionality but winter is still an unknown, Planned with spreadsheets and data but theory at this point.

A few points though, your consumption numbers, I would try and get them a bit more accurate, what you are interested in is the worst case scenario of how much you will use between sunset and sunrise on the shortest day of the year. The reality is you will likely find that number is probably 50% more than on the shortest day and you then need to figure out if you want to pay extra for more batteries or accept for a more or two you may not be majority on batteries (notice I don’t use the word of grid, more on that below).

You are thinking the same with Octopus as me so in affect you actually only need the time between sunset and 12.30am or between 4.30am and sunrise (whichever is the most usage, for us it is cooking dinner which throws in the next point someone Above mentioned, the PylonTech batteries with 2 can draw a maximum of 50a BUT this is at 48v which equates to around 2.5kWh maximum discharge, in the day this is not an issue because my solar generates at least 1kWh even on cloudy rainy days for the majority of the time and none of my appliances on their own use more then 3.5kWh, this is why I said not off grid, just accept that you will need to still be on grid, we find we use on average 1kWH per day from the grid plus standing charge (when we are not topping up the batteries)

Also don’t over estimate consumption, it is so variable best days in the last month 39kWH, worst 9 all within the 2 best months of the year, it comes down to how “committed” you are, even with 9 I was able to still come in at 1kWh or grid by not putting the dishwasher and tumble dryer on until the next day, winter will be different but will use grid top up at off peak times.

So don’t plan on generating enough to charge a car battery of that size regularly, if your car is always at home even with 4.6 I would say your excess in an average week during the summer is likely to only be enough for one charge if you are lucky l best bet IMO is use what you can if at home but if not don’t bother spending more on a zappi and just sell back at 5.5p and charger between 12.30 and 4.30 (if that is enough based on your mileage per day)

As for hot water, I posted yesterday about I need to do some testing, I think if you can get a good deal it is worth it (~200) but if you have gas CH financially I am not 100% sure yet Until I test.

Hope that helps, any questions let me know.

Oh one final thing, when you know what inverter, battery inverter (if not the same) and charger you are getting make sure there is a way to stop the charger draining the batteries, I have a Solis Hybrid and the Zappi will empty the batteries before using the grid, there is a way around it by exporting limiting the batteries but a bit of a pain making the manual change on the days I need to charge.
 

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Does that include your car?
No, it's a DIY grid tied non FIT system comprised of 5x entire Leaf packs feeding a Victron ESS - mainly fed from wind along with 16kW of PV (a mix of AC and DC coupled) - it provides a number of functions to the household - the ability to charge a number of EVs overnight using anytime generated surplus (zero export), whole house backup, diversion to whole house resistive heating and thermal storage etc etc .... along with the usual zero import feature.

Even with that size of storage there have been a very few times when it's been almost entirely depleted (to 20%) due to lack of wind/irradiance - so you have to consider the economics and practicality of not having enough daily surplus (avalable energy after your baseload) to fully charge whatever size you opt for.

With a 4kW array you'll likely have circa 20kWh of surplus available during the summer, and probably next to no surplus during many weeks during the winter.
 

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Discussion Starter #9
Thanks all - super helpful posts. Since I have no worries about remaining connected to the grid I think 3.6kwh is probably enough for me - but will keep doing the maths!
 

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If your aim is to lower CO2, it is nearly always best to have no batteries and to export to the grid whatever power you don't use, as no battery system is 100% efficient, and they all result in CO2 emissions from manufacturering and installing.
 

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If your aim is to lower CO2, it is nearly always best to have no batteries and to export to the grid whatever power you don't use, as no battery system is 100% efficient, and they all result in CO2 emissions from manufacturering and installing.
Fair point. That also weighs in in favour of smaller batteries as compromise between cost saving and CO2
 

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I can never see batteries giving a great long term cost saving in the UK, as it will always be cheaper for the generation companies to install batteries then for us. Hence the difference between peak and none peak prices will reduce.
 

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What is the yearly cost of this going to be? Is it significant benefit, for me I have solar panels giving good summer car / dhw/ household tasks and the surplus to grid.

The yearly power costs are better and I know that the panels produce more power kWh than i use in my car, so car use is over provided for by solar PV. Anything more is just fun.
 
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