Hello from the West Midlands

[cojmh]

You also neeed to bear in mind that the PV approach will contribute whenever there is sufficient light to produce power, the solar thermal has to get to a minimum temperature before it heats the tank. What happens when the tank is upto temperature, with PV it stops heating the tank but could heat something else, with ST you need to have a way to dump the heat.

Thank you,

All valid points ....

I guess it would be interesting to understand at what points each systems cuts in and out at to get a more accurate picture of the best approach. Anecdotally this would possibly mean in the shoulder months that ST makes more of a difference - but I would need to do more digging.

I worked out that to increase a 300 litre tank of water from 20 to 90 (close to the dump temperature) it would require circa 25kwh .... which is about what my whole PV array achieves on a pretty sunny day. Given that it is 15 panels and I would probably only get 2 or 3 on the extension roof (as there will also be a skylight) I would at best hope to achieve 5kwh from 3 panels (so it would be more like 20 to 34 degrees) and I think that is the upper limit

When the tank is up to temperature I guess there are two questions here. Firstly how often does this happen. If it is once in a blue moon then it might not be a factor worth worrying about. Secondly can this be diverted a bit like the solar diverter (in my case this would be the hot tub again!). I would be interested to know from people who already have the system installed how often they experience a dump of the hot water. I know it has happened with my dad's system but it is not something he tracks. Not sure if others can or do?

Thanks for the advice.

[AE-NMidlands]

That all makes sense. Do you mind me asking the following:

1. How big is your solar water tank and how many people would it cover for average use?
2. How much of the year do you think the solar thermal works?

I think it does most of our water-heating between the equinoxes, 2 of us and neither extravagant. We have baths about weekly. From June onwards we probably wouldn't use any gas, except that we cook with it. As you get into the summer the tank gets hotter by the day, so you use less (diluting the water from the hot taps with cold) which means a head-start on the next day - for it to get hotter still.
We have a standard domestic cylinder about 45cm by 110 cm high over its original spray-applied foam. A back-up twin-length immersion heater is fitted, but I don't think we have ever used it. However I might (shall) start using it as a winter dump load if my planned pv system batteries ever get full...

[Sunrisemike]

I did get a very good deal on 6 panels about 12 years ago. pretty standard size vacuum tubes, copper pipe up and down, made in China. It heats a 150 litre tank. From Mid Feb to November, it pretty much does all of the hot water, providing there's about 4 hrs of sun a day. By mid march, it start pumping excess heat round the radiators. Absolutely brilliant. It's ground mounted about 10 meters from the house. Wood gasification boiler heats it and the house in the winter.

Mike

[cojmh]

I did get a very good deal on 6 panels about 12 years ago. pretty standard size vacuum tubes, copper pipe up and down, made in China. It heats a 150 litre tank. From Mid Feb to November, it pretty much does all of the hot water, providing there's about 4 hrs of sun a day. By mid march, it start pumping excess heat round the radiators. Absolutely brilliant. It's ground mounted about 10 meters from the house. Wood gasification boiler heats it and the house in the winter.

Mike

Thank you for your thoughts ..... seems to be quite a few in both camps.

I keep swinging between the two!

In the depths of winter, do you know how much the solar collectors heat the water by? I have read that they can still achieve 25% of the hot water needs of a property - but I was wondering if this is the case in reality - if you know?

Thanks,

[Stinsy]

Take any winter predictions with a pinch of salt. Sure when it is cold-but-sunny you can get a bit of solar from PV or tubes. However, how many of those days do we really get?