Solar panel quote comparison

greenbee

Sometimes we want to heat the hot water in the morning before I charge the battery (if someone has had a bath the night before). Sometimes we only want to heat the water if we know the battery is full and will remain so until the evening. It gives us more flexibility than filling the battery and then using that to heat the water (we've got an Eddi, not an iBoost, and prior to this the immersion was controlled by switch on the wall with no timer, so something was needed anyway). It also means that any use during the day is topped up before the sun goes off the panels. We also use it to take advantage of low pricing on Agile to reduce oil usage. 

I guess it depends how the OP wants to use their solar/heat their water, and how much effort they want to put into managing it. This works for us - minimal effort now it is set up, and we have hot water when we need it. But we're not focussed on maximising the ROI on the solar - otherwise we'd be doing a lot more tactical import/export - as we earn more money working than playing with the solar. Maybe when we're retired - if microgeneration is still paid for (if it isn't, we'll just have to install a swimming pool and use the excess generation to heat that 🤣).

Martyn1981

greenbee said:

Sometimes we want to heat the hot water in the morning before I charge the battery (if someone has had a bath the night before). Sometimes we only want to heat the water if we know the battery is full and will remain so until the evening. It gives us more flexibility than filling the battery and then using that to heat the water (we've got an Eddi, not an iBoost, and prior to this the immersion was controlled by switch on the wall with no timer, so something was needed anyway). It also means that any use during the day is topped up before the sun goes off the panels. We also use it to take advantage of low pricing on Agile to reduce oil usage. 

I guess it depends how the OP wants to use their solar/heat their water, and how much effort they want to put into managing it. This works for us - minimal effort now it is set up, and we have hot water when we need it. But we're not focussed on maximising the ROI on the solar - otherwise we'd be doing a lot more tactical import/export - as we earn more money working than playing with the solar. Maybe when we're retired - if microgeneration is still paid for (if it isn't, we'll just have to install a swimming pool and use the excess generation to heat that 🤣).

Sorry, I must be missing something then. I don't understand how you can heat the water sooner. Perhaps start to heat the water sooner with the excess, but the amount of leccy going into the water must be dependant on the excess PV generation.

So it's all the same total isn't it, be it 3kWh over 2hrs (for example with a diverter) or 3kWh in the 2nd hour using the stored leccy from hour 1 plus the excess in hour 2, buffered by the battery.

You don't have to fill teh battery first, as it's simply acting as a buffer to allow you heat at a higher rate (if you want) before importing once the batt is running low.

Also, the battery will allow you to time shift some cheap rate leccy to later in the day to top up the water heat (if it's run down too much during the day) which a diverter can't do. [Edit - I meant when PV gen isn't good, like the winter.]

I'm not in any way knocking the idea of diverters, especially when they first appeared before export rates, but in this example it was suggested not to bother these days, and for me, it seems like a cost duplication if you are already getting a battery.

For comparison, I don't have a diverter for teh EV (like a Zappi), but so long as there's a few kWh's in the battery, then I can granny charge the EV when generation is around 2kW, but fluctuating (say 1kW to 3kW) due to clouds. The battery is buffering, v's a diverter fluctuating. Same result.

greenbee

The diverter (at least ours) can be programmed to use cheap electricity - so if there's a short window at a price below the price of oil on Agile, we can top up the battery and heat the hot water at the same time. The way the system is set up we can choose to prioritise hot water or battery for the excess PV, and to heat the hot water using the battery if we want to - but generally we don't. We try not to run the battery down too much until after working hours (these vary depending on which timezone I'm working with), as the primary reason for having the battery is to ensure continuous power for work. We haven't had a power cut since the battery was installed, so clearly it's working  

TBH it's working for us so far. Everyone else will need to look at their own requirements, lifestyle, budget etc and make their own decision. As we had no controls of any kind on the immersion at the time we were installing this, we decided that we'd integrate it with the solar rather than adding something separate. 

Qyburn

greenbee said:

Qyburn said:

It's worth mentioning that nowadays it's almost certainly not worth using solar power for water heating. 

The OP has no gas, just oil. 

Oil's currently a bit less than 6p per kWh. So the oil fired system would only need to be around 40% efficient to make it cheaper than the export value. 

Martyn1981

greenbee said:

TBH it's working for us so far. Everyone else will need to look at their own requirements, lifestyle, budget etc and make their own decision. As we had no controls of any kind on the immersion at the time we were installing this, we decided that we'd integrate it with the solar rather than adding something separate. 

Thanks, yes I can see that bit, effectively using the diverter as a smart switch, so you don't have to think about heating it from the PV/battery.

Still seems to me, that it would be better to heat the tank on cheap rate, and export the PV excess, rather than divert it. And avoid the cost duplication. But of course products and situations are still in flux. The diverters for hot water and EV's, certainly made good sense (to me) in the past. And may again, if export rates fall as RE, especially excess periods, increase.

greenbee

We definitely heat the water if it's under 6p/kWh, but otherwise don't bother - when the weather is nice we tend to ignore the whole system as we know we're importing 1kWh/day and exporting between 20 and 60. During the winter we think about it, but want to avoid becoming too obsessed! That could change in retirement.

 My biggest issue is that we are trying to heat 2500 square feet with a 1000l oil tank. Using the solar diverter means the oil tanker doesn't need to visit quite as often. A heatpump is very much on the shopping list, purely to reduce the admin overhead and serious irritation factor of having such a small tank and a 500l minimum order quantity. And no, there is nowhere to site a larger tank - apparently the old 2500l tank was taken out when the extension was built (and an electric Aga put in, which we told the vendors to take with them when they asked for £6k for it - it came with a £300/month electricity bill!).

Martyn1981

I'd definitely recommend looking into the HP idea, it may not be viable, but worth considering.

As a big fan of A2A units (A/C units that can heat or cool), I'm hoping that some support will be thrown their way by the Gov review of the warm homes scheme. If so, then that's a way to go 'HP light' with a hybrid system. Just get one installed (hopefully ~£1,500 at the moment) heating into a large common area/room, and let the heat spread wider via leaving doors open. That may (hope I don't mislead you) provide enough heat in the spring and autumn months when heating requirement is lower. Also at these times you will have a bit of spare PV gen, and the warmer air outside (compared to mid winter) will improve the COP.

If you're on a cheap rate leccy deal, then running the A2A through the night in the winter, works well to reduce the load on the oil CH system too.

Best of luck.