Solar & Battery Install - Where do you start?

How much solar or battery do I need?

When looking at what size of solar and battery system I need, everyone I've asked gave me the same answer “get as much as you can afford”. However I wanted to know what's the minimum I need and then I can could use that as a starting point. Here's how I approached it:

1. What's my base load

I took an electric meter reading after everyone went to bed and another when I got up in the morning. The difference divided by the number of hours is my base load. Did a few readings just to get a good average and in my case my base load is around 150 watts (0.15kwh). 

Tip: If you charge an electric vehicle overnight, try and do your measurements on a night when the cars not charging. The reasoning is that if you have an EV you'll likely be charging it using an off-peak tariff and not solar or battery so your sizing needs to exclude it.

2. What's my daily consumption

I took a daily reading before I went to bed for a few nights, I also took a full weeks reading and divided by 7. Again I took a few readings to get an average and my daily consumption is about 8kwh

Tip: Again if you charge an electric vehicle, try and do your measurements without a charge being included. 

3. What's my peak load

I don't use a lot of high demand items, but I looked at the big appliances (kettle, cooker, immersion heater, hair dryer, electric shower, electric radiator etc) and thought about any that were used at the same time for extended periods (ie greater than an hour). My peak load seems to be when I have my morning shower as I use an electric shower. The shower peak was just over 9kw however that was only for 15 minutes. My next peak after that was around 4kw when I use a cooker and a kettle at the same time.

Tip: If you're unsure and your daily load is under 7.2kwh then it's likely a 3kw peak will be sufficient.

4. So how much solar/battery do I need? 

Solar panels - I'm sizing my panels based upon a typical Scottish day (mostly cloudy and not many hours of sun 🤣). From what I've read even on mostly cloudy days solar panels produce energy, however it's as low as 10% of their capacity. So I sized my panels by multiplying my base load by 10 giving a minimum solar panel capacity required of 1.5kw. In other words, if I have a 1.5kw array and I get 10% generation on cloudy days, that will be enough to cover my base load of 150 watts.

Battery - I need a battery that's going to provide good chunk of my daily load. So for me 8kwh battery is the starting point. However as the solar panels will cover the base load (0.15kwh) on most days during daylight hours, I can reduce the battery size by that amount. In my case I worked on 4 hours of sun during winter and that would mean my battery could be 0.6kwh less (ie solar would provide 4 hours at 150 watts). Also I plan to add any extra charge to the battery using an off peak tariff like Octopus Go, so as the battery will be charging during the off peak period it won’t be able to give power during that time, so if my off peak period is 4 hours that’s another 0.6kw less.

So minimum battery size would be around 6.8kwh of usable capacity. During winter it would get most of its charge from an off peak energy tariff, during summer it would fully charge in 4-5 hours of full sun from the 1.5kw solar panels.

Lastly I need an inverter & battery that will allow enough to meet peak load. Whilst I could look at one to meet my peak peak (ie 9kw) that's going to be much more expensive than my next peak at 4kw. So max discharge rate of 4kw will do and I'll accept that occasionally I may exceed it's peak and have to draw any excess from the grid.

5. My Conclusion

The minimum I need is 1.2kwp of solar panels (I will likely go for more, but this is the minimum I could get away with),  a battery that has a usable capacity of 6.8kwh, and an inverter that can give a max load of 4kw. I can add more if I can afford it, but this setup will meet a large chunk of my needs.

Next Step: Selecting your supplier 

One final note

If this blog has helped and you go on to installing a solar or battery solution, please let me know by leaving a comment and sharing this page as I really like to know I've been able to help. 

If you're considering a move to Octopus to charge your batteries in the darker months, please consider using my referral link below (click on the Octopus) as it will give each of us £50 off our Octopus bill.