Skip to main content
Answer

Excess solar to heat water or to sell - which is best?

  • May 6, 2025
  • 3 replies
  • 121 views
F
Carbon Cutter***
Forum|alt.badge.img

Hi.

I have a 14 panel array with a battery and generate more energy than I use.  I added an Eddi unit to divert excess energy to heat the water tank, however……

The price of gas is 6p kWh.

My export rate is 15p kWh. 

Assuming the gas is less efficient than the eclectic at heating the tank, say 85%, then the cost to heat the water is 7p kWh.

Assume it takes 1 hour to heat the tank:

Using gas cost 7p and I can sell the excess at 15p so I am 8p better off.

Using excess means I save 7p but I dont sell at 15p so am 8p down.

Have I got this right, or am I going mad? 

Best answer by BPLightlog

I would say there are several factors in play ​@felsteaduk .

Your hot water loses heat at a rate which also needs to be taken into account. Your batteries also lose power compared to the charge level.

It is a time served issue which several people solve in different ways. 
It will depend on your situation and energy usage over each day/week etc and so I don’t believe there to be a precise answer 

M

3 replies

BPLightlog
Plan Zero Hero
Forum|alt.badge.img+5
  • BPLightlogPlan Zero Hero
  • Plan Zero Hero
  • Answer
  • May 6, 2025

I would say there are several factors in play ​@felsteaduk .

Your hot water loses heat at a rate which also needs to be taken into account. Your batteries also lose power compared to the charge level.

It is a time served issue which several people solve in different ways. 
It will depend on your situation and energy usage over each day/week etc and so I don’t believe there to be a precise answer 

Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP}
Abby_OVO
F
Carbon Cutter***
Forum|alt.badge.img
  • felsteaduk
  • Author
  • Carbon Cutter***
  • May 6, 2025

I have left the setting on the Eddi unit to top up the hot water tank throughout the day with any excess, this will in turn reduce the gas use associated with heating the tank.

As I always have the battery full by 16:00 with solar and then by 03:00 due to cheap nightly tariffs, I was thinking of using the stored electric to heat the tank in the morning and night, rather than the gas.  But then I realised that that would result in me loosing out the chance to sell that electric at a better price than it cost to use the gas.

Then I got confused and didn't believe in my maths abilities.  

BPLightlog
Plan Zero Hero
Forum|alt.badge.img+5
  • BPLightlogPlan Zero Hero
  • Plan Zero Hero
  • May 6, 2025

Your maths look fine .. I actually have an off-peak rate at 3 time slots in the day and as this is at a rate less than my export rate, I choose to prevent battery usage and import from the grid.

I don’t have a diverter and so simply store excess in my batteries. I do regularly add additional battery storage to my system.

My hot water is heated by an ASHP and once heated overnight tends to last the day.

Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP}
Abby_OVO
Chris_OVO
Terms & ConditionsAccessibility statement