Trying to work out electric usage for solar install

TractorFactor

Hi all

Just trying to work out my electric usage for a solar install.

Smart meter gives these readings per week for the last 5 weeks:

72.12
79.01
77.12
98.20
70.26

I guess this means the average per week is 79.34 or 11.33 a day, is that correct?

In that case, I'd need a battery that's 11kwh or bigger, correct?

Thanks

born_again

TractorFactor said:

Hi all

Just trying to work out my electric usage for a solar install.

Smart meter gives these readings per week for the last 5 weeks:

72.12
79.01
77.12
98.20
70.26

I guess this means the average per week is 79.34 or 11.33 a day, is that correct?

In that case, I'd need a battery that's 11kwh or bigger, correct?

Thanks

When you say Smart Meter (as they are odd readings) do you mean the In Home Device?

Best to look at you bill, as that will give exact usage.

QrizB

In that case, I'd need a battery that's 11kwh or bigger, correct?

That depends largely on what you are trying to achieve.

So, why are you fitting a battery, how do you intend to use it and what benefits are you expecting?

amanda1024

It might be worth looking at the whole year rather than just the last 5 weeks. And if you can access the half-hourly smart meter data (e.g. your electricity supplier might have an app) you'd be able to see the daily usage as well as how that's distributed. I'd assume if you're getting a solar install then the solar would provide some of your electricity consumption directly (although obviously a lot less at this time of year) - a battery might be useful to charge from the solar generation during the day (and/or from cheap overnight electricity), then discharge to power the house in the evening, in which case the relevant figure would be evening consumption rather than full day consumption.

Grandad2b

TractorFactor said:

Hi all

Just trying to work out my electric usage for a solar install.

Smart meter gives these readings per week for the last 5 weeks:

72.12
79.01
77.12
98.20
70.26

I guess this means the average per week is 79.34 or 11.33 a day, is that correct?

In that case, I'd need a battery that's 11kwh or bigger, correct?

Thanks

I've had solar for very nearly 2 years. I still haven't managed to work out if a battery makes sense for my circumstances even though I have daily electric consumption figures. 

Petriix

The ultimate aim with solar and battery is to minimise your import costs and maximise your export earnings. In practice that means trying to avoid self consumption where possible and transferring usage to the cheapest off-peak rate you can get. While you can export solar for 15p per kWh and import for 7p, it's better to send it to the grid than to use it yourself so you charge the battery overnight so you can earn more. 

Battery size is complicated. Each additional kWh of capacity represents a diminishing return in terms of self consumption; though there is an argument for exporting as much as possible but that may not be optimal in terms of longevity. The bigger you go, the more of it will sit redundant more of the time...

Ideally you'd have a battery big enough to cover all your peak usage every day. But remember you can do lots of things (like water heating and washing/drying etc.) in the off-peak period without needing the battery. And don't forget that batteries don't necessarily allow 100% discharge and will suffer losses on both charge and discharge. That means a 10kWh battery might actually cost you the price of 11kWh to charge but only give you 9kWh to use.

Before you can work it out, you need to find the cheapest tariff available. This varies by region with some requiring an EV for the lowest rates, though E7 can be competitive too. Then you need to make sure you have the right kind of meter - usually a SMETS2 smart meter will do. 

Going on a ballpark of 10kWh usage every 24 hours, 3kWh of that could easily be shifted to the off peak window leaving 7kWh of peak usage. An 8kWh battery would cover that even with solar contributing nothing (which it will on some winter days). This would allow you to move say 95% of your usage to the off peak price, maybe 7p per kWh, but for 110% of your usage.

Excluding the standing charge, that looks roughly like paying £320 for 4,000kWh of imports compared to paying more like £900 for 3650kWh at the standard rate. So the battery alone could save you £600 per year.

Then solar would be almost exclusively for export. A 4kWp array facing south would generate 4,000kWh. With 90% exported at 15p per kWh, you'll earn another £540.

If you can get that installed for £10k then the ROI is a little under 9 years. Ideally you'd pay more like £8k. But you can still earn 5% interest on cash or 10% on a global index tracker fund so you might be better off keeping the money. Obviously these things change and current electricity prices aren't guaranteed to still be available in the future.