Solar diverter - flawed return on investment calculations

EcoScruples

You know what is nice though? Having hot water and not worrying about how you got it. 
Anyone who's already bought a hot water gizmo then just enjoy it, your money is gone. 

Niv

Screwdriva said:

Batteries still do make sense if you have a heat pump or drive alot in an EV but for the average household, Octopus Flux torpedoed any remaining financial rationale for a battery. 

The hot water diverter's best days are behind it I'm afraid, unless you're using alot of oil to heat hot water. 

My latest oil delivery saw oil priced at 60p / litre so ~ 6p per KW - even with oil heating a solar diverter is not worth it currently

EcoScruples

I've tried working out how much oil I use to heat my water tank from 21 degrees to 60 degrees and it's about 2 litres (1 litre gets the water to about 45 degrees). I don't run the boiler much over halfway on the internal thermostat as the manufacturer said it would shorten the life of the boiler.
I need about 5kw of solar to do the same job so not much in it price wise.
Environmentally better? Maybe.
Now I have the eddi I feel I need to use it.

markin

EcoScruples said:

You know what is nice though? Having hot water and not worrying about how you got it. 
Anyone who's already bought a hot water gizmo then just enjoy it, your money is gone. 

You can always sell it on.

Screwdriva

Niv said:

Screwdriva said:

Batteries still do make sense if you have a heat pump or drive alot in an EV but for the average household, Octopus Flux torpedoed any remaining financial rationale for a battery. 

The hot water diverter's best days are behind it I'm afraid, unless you're using alot of oil to heat hot water. 

My latest oil delivery saw oil priced at 60p / litre so ~ 6p per KW - even with oil heating a solar diverter is not worth it currently

True but factor in a heat pump with a SCOP of 3.5 powered by PV and the equation changes a bit.

Personally, I find the inconvenience of oil to be something I wouldn’t want to live with. 

JKenH

Screwdriva said:

Niv said:

Screwdriva said:

Batteries still do make sense if you have a heat pump or drive alot in an EV but for the average household, Octopus Flux torpedoed any remaining financial rationale for a battery. 

The hot water diverter's best days are behind it I'm afraid, unless you're using alot of oil to heat hot water. 

My latest oil delivery saw oil priced at 60p / litre so ~ 6p per KW - even with oil heating a solar diverter is not worth it currently

True but factor in a heat pump with a SCOP of 3.5 powered by PV and the equation changes a bit.

Personally, I find the inconvenience of oil to be something I wouldn’t want to live with. 

Some of us don’t have much choice. On Flux it now costs me 19p/kWh to heat water compared to about 7p using oil before taking into account losses in pipe work, and the boiler itself. The latter will be substantial though so most of the time, unless we have visitors or the central heating is on, we don’t heat the water tank at the kitchen end of the house, just using water from the insulated urn for washing up. 

Qyburn

EcoScruples said:

I've tried working out how much oil I use to heat my water tank from 21 degrees to 60 degrees and it's about 2 litres (1 litre gets the water to about 45 degrees). I don't run the boiler much over halfway on the internal thermostat as the manufacturer said it would shorten the life of the boiler.
I need about 5kw of solar to do the same job so not much in it price wise.

Two litres of oil is around 21.5kWh, is efficiency really only 25% when heating only DHW?

I really don't know how I would check ours as our oil meter only registers in 250l increments.

EcoScruples

It's a really rough calculation I did, taking daily drop tests on my oil tank I used about 1 litre per day last summer to heat my water to around 50 degrees which isn't enough to run a proper hot bath, mainly because I don't run the boiler stat at full to protect the boiler.
The eddi heats it to the full 60 degrees which is perfect.
Overnight my tank loses about 15 degrees.
Oil usage pre solar was about 200 litres per year for hot water so about 2,000kw.
Now I'm averaging just over 2kw per day with the eddi so if you said 180 days (the other 180 days I'll have the heating on anyway) that's only 360kw of solar.
It comes down to saving oil as it's a pain to try and buy when cheap and time so I'm not ordering in the middle of winter.

michaels

Back to the OP. Seem to be two issues
Firstly own use is 30% not 50%. 30% is a much more realistic figure than 50%, not sure why they are allowed to quote the higher number in the suggested savings calc.
Secondly entirely agree that diverted e port use should be valued at the cost of the alternative not the full electricity price.

I am sure solar is not the only thing being sold on unrealistic assumptions bit agree an industry body should be better bit not surprised. Those who can do their own maths are at an advantage as with most things in life.

Qyburn

iotum said:

My generation is 10% greater than predicted, I'm consuming 60% of what I generate and electricity prices are way higher than the assumed inflation rate. Yet in spite of all of that, I've saved 10% less than the provided estimate. 

Out of interest, what was the incorrect assumption in the estimate?