Then I decided to use the MyEnergi eddi device to control the hot water tank.

PV feeding an immersion made financial sense when exports were "deemed" (e.g. 50% of generated under the old FIT arrangement), but now I think many people are on an smart export guarantee tariff, so every unit not exported means a reduction in income - it effect it costs about 15p/kWh to heat water that way (compared with maybe 6p ish to do it by gas (depending on boiler efficiency etc)). Environmentally it's swings and roundabouts of course, but quite likely your kWh might be better spend displacing a kWh of gas fired electricity generation (at perhaps 60% efficiency)  than gas fired local hot water generation (at perhaps 90% efficiency). Depends on your gas boiler of course - if yours still has a pilot light, I guess it's not in the efficient condensing category...

Another approach (which I've got) - is to have solar thermal panels in tandem with PVs (if you've got the roof space of course) - simple thermal panels get many more kW out of a sq m of roof than PVs, even if it's only "low grade" energy as warm water, it's perfectly useful for heating (even pre-heating) a stored hot water system. In my case it feeds a thermal store that provides space heating as well as domestic hot water - so the thermal panels make a contribution to the underfloor heating in winter too.

   - Andy.

Before I had the hot water tank fixed we were using gas heating to heat the hot water from 5am and again at 3pm. This enabled us to monitor what was going on. The records indicate that a months worth of gas heating for the hot water tank cost £25. Albeit with the old water tank.

I don't yet have the same records for the eddi but one week uses 23.9kWh. This uses the battery or excess export, this cost/price of this is about 15p per kWh, which is about £3.60, which is about £15 for the whole month.

So using the eddi is both environmentally sound and probably saves money.

We run our gas boiler at 60 degrees centigrade. We also heat the water up to 60 degrees.

Although we have a traditional gas boiler is still has about 20 years of life left and while it maybe less efficient than a condensing boiler I believe that it is more environmentally sound than a condensing boiler. The technicians inform me than a condensing boiler lasts on average 7 years, while a traditional boiler last about 40 years. So the environmental assessment needs to include the disposal of the previous condensing boiler and its replacement with one which had gone through design, development, delivery and installation.

I think that condensing boilers need engineering input to extend their lives either by improving the design or make it based on standard components so they can be replaced when they fail.

I agree with you on standard component solutions. Although, It seems that many products are intentionally designed with a limited lifespan, which benefits manufacturers in terms of increased sales and profits.

I agree with you on standard component solutions. Although, It seems that many products are intentionally designed with a limited lifespan, which benefits manufacturers in terms of increased sales and profits.