Is switching to off-peak rate and charging my battery overnight always a good idea?

Starrfissh

Hi… Just had solar panels installed, and a battery which is charged by them. We're not on dual rate electricity at the moment: is it always a good idea to switch to dual rate and charge our battery at the cheaper nighttime rate? Or is there some kind of calculation I need to do? Thank you!

matt_drummer

Hi,

I am in a similar position to you.

It really depends on how much solar and battery you have.

Ideally you want to charge your battery from the solar power whenever possible, it's free.

Depending on the size of your battery(ies) you can decide whether to charge them on a cheaper rate depending on how you think the weather will be on the next day.

Obviously, there is no point in charging batteries at a normal tariff rate, you will lose due to inefficiencies of the charging process.

i use an economy 7 tariff as it works for me as I will never use any of the higher rate daytime tariff.

If you can make all your use solar and cheaper rate then it's a good idea, you'll never buy electricity at the `full' price.

If you tell us how much solar and battery you have, how much and when you use electricity and how you heat it will be easier to help you.

[Deleted User]

From March until October, my battery charges from solar so I have chosen a tariff that pays the highest export payments (Octopus Flux). Come September, when solar output falls away, I will choose a tariff that offers the lowest overnight unit price for battery charging.

The calculation that you need to do is based on three things: one, the size of your solar array and its predicted output; two, the size of your battery and, three, your home consumption.

FWiW, I have a 6.35kWp array which generates 6000kWh/year. I also have a 13.5kWh battery and my home consumption is 4500kWh/year (which includes EV charging). I import about 1000kWh a year over the Winter months and export about 2500kWh to the Grid.

Starrfissh

Output for the panels is estimated to be around 7100kwh, battery is 9.5kwh, our use is around 8100kwh pa, around 18 units each day and 4 units each night. Appreciate your help so far!

[Deleted User]

Starrfissh said:

Output for the panels is estimated to be around 7100kwh, battery is 9.5kwh, our use is around 8100kwh pa, around 18 units each day and 4 units each night. Appreciate your help so far!

On that basis, you are probably looking at an estimated solar output of 350kWh next month. As your home requirement is 660kWh for April, then you will need a tariff that tops up your battery for the least cost. Do you have an EV? I ask because your daily usage is very high.

Starrfissh

No ... but we do have an electric Aga!

matt_drummer

I would expect more like 600 Kwh in April of generation based on 7,100 Kwh per annum.

You are like likely to go to bed with a nearly full battery which will get you through the night.

If you need to import it will be during the day, and that will be best at the lowest possible cost, I would stick with a normal standard tariff.

At that level of generation I would have gone for more batteries.

If you have an ev then you could take advantage of cheaper time of use tariffs.

[Deleted User]

matt_drummer said:

I would expect more like 600 Kwh in April of generation based on 7,100 Kwh per annum.

Correct: old eyes and a spreadsheet on a small IPhone screen. I was looking at solar export rather than target solar which on my array in April 22 was 680kWh.

markin

Solar isn't free, You want to make back the install cost as fast as possible, if you can export it at 15p with Octopus and then charge up at night at 6-10p then you can lower the daily cost.

No Gas, Oil? 

Starrfissh

matt_drummer said:

I would expect more like 600 Kwh in April of generation based on 7,100 Kwh per annum.

You are like likely to go to bed with a nearly full battery which will get you through the night.

If you need to import it will be during the day, and that will be best at the lowest possible cost, I would stick with a normal standard tariff.

At that level of generation I would have gone for more batteries.

If you have an ev then you could take advantage of cheaper time of use tariffs.

We'll fully charge the battery each day in the summer, but not in the winter. Do you think we should go for a dual rate to charge it in the winter? Or stick to single (cheaper) rate because of daytime use? Thanks     : )

matt_drummer

markin said:

Solar isn't free, You want to make back the install cost as fast as possible, if you can export it at 15p with Octopus and then charge up at night at 6-10p then you can lower the daily cost.

No Gas, Oil? 

Can you charge at night for 6 to 10p?

Maybe you can on some tariffs but you won't be able to export with Octopus at 15p if you do.

Exactly for the reason you gave!