Thermostats in equipment.

[Estiban]

Hi I understand there’s more to it then just the kw rating of equipment and loses and the price of units but…
For example if a resistive load heater or immersion heater is rated at 3kw power and the kWh price is 0.50 p as an example the cost to run would be £1.50 per hour but roughly how much effect on price would the thermostat reduce this.
Thanks

 

[littlespark]

it depends on how well the tank is insulated and how much water is used in the time.

From cold, it will take the full 3kW for the time it takes to reach the temperature, then it will turn off because of the thermostat.
If any hot water is drawn off, or the heat is simply lost through the walls of the tank... then the temperature will drop, and the thermostat will turn on again.
Basically, if you can feel heat on the outside of an insulated tank, then "some" heat is escaping... although not very much.

 

[Avo Mk8]

For hot water storage you could work out various scenario's with this: Water Heating Calculator for Time, Energy, and Power - https://bloglocation.com/art/water-heating-calculator-for-time-energy-power

If you had a 125 litre tank with 3kW immersion set to 75 deg C, and you turned it down to 65 deg C, it would take about 1/2 hour less to heat up from scratch, so saving 1.5 kWh of electricity. The cost of maintaining the tank at 65 vs 75 would be a different value, dependent on heat loss through the insulation, and the usage of hot water, as per littlespark.
You wouldn't want to lower the temp much lower that 60 deg because of Legionella, though you could run it at 55 deg C and then heat the tank to/above 65 deg once a week.

For a room thermostat, you would need to know the heat loss of the room. There are heat loss calculators on line, so you could enter the details of one of your rooms (type of floor/ ceiling/ outside vs inside walls, their construction, number and size of windows, insulation, what's above and below you and whether those spaces are heated rooms etc. etc.) You could then see the (approximate) energy required to heat the room to the temperature you use, and do the sum again for a lower temperature, and see the difference in energy and hence work out the cost saving of running the room at lower temperature!

You might find the most reliable way of answering your question is to use the consumption shown on your electricity meter to measure the cost of running a room (or the house), and/or the hot water, at two different temperatures!