How to assess solar quotes?

[Chris_in_the_UK]

Tbh i was surprised what my generation was with my small array, i have considered adding more because we have the room and adding a battery, think id be looking at c6k. There are some very good batteries coming through now vastly cheaper than a few years ago, plus you don't have the VAT on the batteries anymore which makes them cheaper. My issue is that we’re planning on downsizing somewhen between now and when i retire so around 6 years. I guess it also depends how long you intend to stay where you are Chris.

Given the amount of money we have spent on the extension/refurb I doubt we will ever move now = never say never, but unlikely.

I am going to get some new quotes with/without battery storage.

[sweets]

If you are on a tariff like Octopus agile, there is probably more to be gained by having just a battery (and little or no solar). Use cheap leccy to do the major tasks and top up the battery overnight, then sell the leccy back into the grid at peak times from the battery.

We have put solar on the roof of a property in Devon, but it has a large south facing roof (that needed replacing) and a higher seasonal load when the sun is shining, so it makes sense.

Our installers there said that most of their most recent work had been installing batteries, so that people can farm the difference between the cheap and expensive rates.

[noTAGlove]

If you are on a tariff like Octopus agile, there is probably more to be gained by having just a battery (and little or no solar). Use cheap leccy to do the major tasks and top up the battery overnight, then sell the leccy back into the grid at peak times from the battery.

We have put solar on the roof of a property in Devon, but it has a large south facing roof (that needed replacing) and a higher seasonal load when the sun is shining, so it makes sense.

Our installers there said that most of their most recent work had been installing batteries, so that people can farm the difference between the cheap and expensive rates.

Let’s say you get a 13.5 kW power wall installed for £9k and you set a minimum of an 8 year payback. Just battery and no solar.

8 x 365 days = 2,920 days each 13.5 kW, therefore 39,420 kW over 8 years. Assumes full charge and discharge once per day for overnight/daytime rate arbitrage.

13.5kW is highly optimistic given a battery’s useful output will not be the full amount quoted, just like an EV battery.

So you have spent 900,000p to get 39,420 kW, or each kW needs to be valued at a buy/sell spread of 22.8p.

The typical spread between buying cheaply overnight (~10p/kWh) and selling at a better rate in the day (~20p/kWh) is 10p/kW. Better to use all the electricity yourself in the daytime if you can.

So, no chance of breaking even after 8 years.

More like 8 x 22.8 / 10 = 18 year payback.

Battery system will be dead in 15 years.

I leave electricity trading to the big boys. And there is a reason they don’t use batteries. Energy density is very poor and capital cost is very expensive.

I’d also prefer the risk of running a washing machine overnight compared to a large volume of batteries constantly cycling in the house.

I have 69 kWh of paid for battery in my car. More sensible to somehow use and trade that, than adding domestic batteries in the house.

[gunner]

Let’s say you get a 13.5 kW power wall installed for £9k and you set a minimum of an 8 year payback. Just battery and no solar.

8 x 365 days = 2,920 days each 13.5 kW, therefore 39,420 kW over 8 years. Assumes full charge and discharge once per day for overnight/daytime rate arbitrage.

13.5kW is highly optimistic given a battery’s useful output will not be the full amount quoted, just like an EV battery.

So you have spent 900,000p to get 39,420 kW, or each kW needs to be valued at a buy/sell spread of 22.8p.

The typical spread between buying cheaply overnight (~10p/kWh) and selling at a better rate in the day (~20p/kWh) is 10p/kW. Better to use all the electricity yourself in the daytime if you can.

So, no chance of breaking even after 8 years.

More like 8 x 22.8 / 10 = 18 year payback.

Battery system will be dead in 15 years.

I leave electricity trading to the big boys. And there is a reason they don’t use batteries. Energy density is very poor and capital cost is very expensive.

I’d also prefer the risk of running a washing machine overnight compared to a large volume of batteries constantly cycling in the house.

I have 69 kWh of paid for battery in my car. More sensible to somehow use and trade that, than adding domestic batteries in the house.

We get that you don’t want to do it, but it still doesn’t mean it doesn’t work for anyone.

If you use the cheap overnight electricity at peak times the arbitrage is much bigger.

[noTAGlove]

We get that you don’t want to do it, but it still doesn’t mean it doesn’t work for anyone.

If you use the cheap overnight electricity at peak times the arbitrage is much bigger.

I agree. But when you stump up your £9k capital cost (or whatever it is), there is no contractual returns for the lifetime of the facility.

Buy/sell spread may be fine for the first few months, few years, or decade. Or the buy/sell spread may cave in just after you have laid out your investment. And it is now an investment.

As mentioned I have no ambitions to be a spot market electricity arbitrage trader.

[gunner]

Yes, it’s a fixed cost now against a variable return in the future.

My guess is that electricity costs will go up rather than down, which is why I’m happy using capital (while I’m earning) to install solar and batteries and reduce my costs in the future. It doesn’t have to be a buy/sell spread arbitrage.

Horses for courses.

[noTAGlove]

My guess is that electricity costs will go up rather than down

I think you are right. Cheap wind farms have been done, and what is left is more expensive to develop in deeper water.

Carbon capture and storage on the back of gas turbine ain’t cheap, and gas now has to be sourced as LNG in part. New nuclear is horrifically expensive.

Doesn’t mean the day/night spread is guaranteed to widen, but I agree with you we are going to be paying more for our electricity in the coming years.

[Franky Four Fingers]

I agree. But when you stump up your £9k capital cost (or whatever it is), there is no contractual returns for the lifetime of the facility.

Buy/sell spread may be fine for the first few months, few years, or decade. Or the buy/sell spread may cave in just after you have laid out your investment. And it is now an investment.

As mentioned I have no ambitions to be a spot market electricity arbitrage trader.

Solar opens up other tech that you wouldn't ordinarily access, it’s been mentioned on here before but if you have an immersion theres devices that will divert excess directly to your immersion heater. My iboost was installed in Feb 22 at a cost of £300, data directly from the unit tells me it’s diverted 1450kw which is roughly £435. Im £135 up in a little over 2 years. It’s there just doing its thing without any intervention from me.

[noTAGlove]

Solar opens up other tech that you wouldn't ordinarily access, it’s been mentioned on here before but if you have an immersion theres devices that will divert excess directly to your immersion heater. My iboost was installed in Feb 22 at a cost of £300, data directly from the unit tells me it’s diverted 1450kw which is roughly £435. Im £135 up in a little over 2 years. It’s there just doing its thing without any intervention from me.

As long as your immersion needs a boost during the day, because if not, it is only displacing cheap overnight electricity.

When I did my investigations, I found that you have to be very careful with the scenario and maths, but great if you have done your homework and it all works for you.

As previously mentioned those with large electricity demand in the daytime will benefit the most.

I was using 80% of my electricity in the daytime until recently and I am sure solar/battery may have benefited me.

But, now only use 20% of my electricity in the daytime and it just doesn’t work.

My recommendation is first shift what you can to overnight use, and then undertake the economics on that basis.