Let's look at the cost of heating a 100 litre tank from 18ºC to 50ºC.

The specific heat capacity of water is 4200 J per kg per degree Kelvin. This is the energy required to raise the temperature of 1 kg of water by 1 degree Kelvin (or Celcius).

The temperature change is 50ºC – 20ºC = 30ºC

100 litres of water is 100 kg

The energy E, in joules required to heat the water is given by E = mass x specific heat capacity x change in temperate,

So , E = 100 kg x 4200 Joules x 30ºC => E = 12600000J (Joules)

1 J/sec = 1 Watt (W)

Hence 12600000J /(3600s) 3500 Wh or 3.5kWh

The actual time it takes for me to electrically heat the tank is 2.5 hours.

I have a 1000 Watt heater.

Therefore 1000 x (2.5h ) = 2500Wh

This is equivalent to 2.5 kWh (kiloWatts)

It would appear I use 1kWh of electricity less that estimated. This maybe be due to the fact the water sinside the tank remains at a higher temperature that the estimated 20ºC

when I start heating electrically.

I get my electricity from the energy co-operative, Coopérnico .

This is their 2019 tariff for bi-horário.

So heating the water during off peak hours (vazio) costs 0.0987 cents per kiloWatt hour (kWh)

So 0.0987€ x 2.5kWh = 0.24675€

(7 days x 4) x 0.24675€ = €6.909

With IVA (VAT) at 23% = €8.4981 is what I can save a month.

I have a hot bath by boiling a pot of 5 litres of water on my wood burner. Doing this 2-3 times a week during winter is worth my while. Besides boiling bath water I can also do a little cooking on top of the wood burner to make further savings.