Now let's compare this with what the calculation looks like if we use the surplus for hot water. Here we assume that the entire PV surplus is sufficient to cover 80 per cent (i.e. 1,000 kWh) of the electricity requirements of the heating element.
Costs for PV surplus: 1,000 kWh x 0 cent per kWh = 0 euro per year
Costs for mains electricity: 250 kWh x 36 cents per kWh = 90 euros per year
Effective costs: 90 euros per year
In our example, using the PV surplus for hot water saves around 260 euros per year - and this can be achieved automatically with a clever energy management system like Shelly's.