Solar battery capacity - how to work out what's needed

Good day. Personally I wouldn’t go for less than 5-6 kWh battery. 

 [Disclaimer: I don’t have any PV panels and no experience of them or of battery storage 🙂]

I would have thought that the maximum draw would also come into the equation. Your daily consumption (7-12 kWh) won’t be neatly spread over the 24 hours; to take an extreme example, much of it might be concentrated into a couple of half-hours as residents take electric showers drawing perhaps 10 kW for five or ten minutes. If the sun isn’t shining at the time, you might end up with flat batteries until it does.

I suspect that @BPLightlog will be along shortly to say ‘work out what you think you need, then double it.’

I’m also hoping BP will give some advice.

Assuming your meter gives half hourly readings you can study your usage charts to see your actual pattern. In my case with a heat pump I’m most interested in the amount used during the winter months. 

I don’t have PV yet (getting very close now) so I’ll be paying for PV and battery together, so I don’t want to overspend on one bit of tech and not be able to afford the other. I also wondered if the size of the inverter is just related to the PV size or is it influenced by the battery size ?

Great feedback on this. Suggest higher than 9.6kw. 
 

with regards to 10kw shower that will “gulp” power immensely. 😆 that is something that I never use now. 😝 

😁

Your inverter will be your ‘limiter’ in this instance @juliamc . Its specification will determine the maximum that can be delivered (same for charging rate) at a time. Obviously the batteries will soak up the excess generation and deliver their power over time but the inverter is the restriction to how much, so check what its rating is alongside your DNO approval level

So I need to know what size battery I’m getting before I commit to the inverter.

Not necessarily. The inverter will usually cope with more battery storage than its rating. 
For example a 5kw inverter can have more storage than 5kw in batteries. It just won’t allow more than 5kw through at once.

Calculate your system ‘size’ as I mentioned earlier and then add batteries to cover a day or so in a separate calculation. Just make sure that your inverter can have more battery storage added later should you want/need

Thanks  all who have commented on this issue.   Lots of different suggestions. 

I have checked the output on my inverter and it indicates 2 kW.

We dont have sudden huge surges such as 10Kw shower

maximum daily peak over the summer and winter is 6 kW and that for a very short time...I’d estimate the average is no more than 4 kW

Bearing in mind all these points I think a 6Kw should suffice…..

You might like to refresh your elementary physics knowledge, because you may be confusing energy and power. The three figures you quote here are amounts of  energy, measured in kilowatt hours (kWh). Power is the rate of consuming or generating energy, measured in kilowatts.

You multiply power P by time t to find the amount of energy E; you divide an amount of energy by time to get the power: 

   P x t = E

   E ÷ t = P

In round, nominal figures only, subject to losses and other factors, it would take a 2kW inverter 3 hours to charge a 6kWh battery.  

My household uses 85% of its daily electricity consumption between 7 AM and midnight. If the same applied to yours, your average power draw during the day would be about 0.75 kW, so your fully-charged battery would cover you for 8 of the daytime hours (8 h x 0.75 kW = 6 kWh).

Sorry if I’m trying to teach grandmother to suck eggs 😉

I definitely need help as a fog descends every time I approach this subject. 

Rather than looking at my daily usage to work out what battery and inverter I need, I thought I might try to calculate how long a full charge would last on a given day, with various sizes of kit. Does this make sense ?

My heaviest electricity usage day this winter this was 18th Jan. As an experiment I ran the heat pump all through the night, and with other usual stuff too I used 48 kWh, and the outside temperature went from -7 degC to +3 degC

This graph is from my energy bill - I’m on Octopus Go so have 4 hours at a cheap rate (red line):

I’m not intending to buy battery storage to get me through this amount of use of course !!! and this would have been a very low solar day too.

The 16th Feb was an unusually mild day, outside temps between +9 degC and +14 degC:

I’d hope a battery would get me through this kind of day…

Yes, that’s a decent view to have a look at

juliamc wrote:

My heaviest electricity usage day this winter this was 18th Jan. As an experiment I ran the heat pump all through the night, and with other usual stuff too I used 48 kWh, and the outside temperature went from -7 degC to +3 degC

This graph is from my energy bill - I’m on Octopus Go so have 4 hours at a cheap rate (red line):

 

I’m not intending to buy battery storage to get me through this amount of use of course !!! and this would have been a very low solar day too.

The 16th Feb was an unusually mild day, outside temps between +9 degC and +14 degC:

I’d hope a battery would get me through this kind of day…

Again, yes, you might assume so. 
Unfortunately all of the calculations are a little on the guesstimate side as you never know exactly what solar activity will give, nor exactly what draw will be required. 
Given the OE Go pricing you could also recharge batteries at that time of course and it would be a huge system to cover every possible eventuality. Most don’t look to fully cover the worst day so perhaps also look at an average during say October. 
But as long as you have your system (particularly the inverter) which allows more batteries to be added later (not all do) then I’m sure it will be a significant benefit. 
Don’t forget that any good installer will also look to plan best coverage depending on location, shading etc