How long does it take to heat up the thermal store?

The speed at which a water storage tank can be heated using a solar-electric heating element primarily depends on the available solar radiation—the more solar energy available, the faster the heating process. At the same time, factors such as water draw-off during heating or—when using a buffer or combination storage tank—the additional energy demand from space heating also play an important role. These factors extract heat from the tank and therefore extend the heating time.

Nevertheless, a theoretical estimate can be made under constant conditions. To heat 1 liter of water by 1 degree Celsius, an energy amount of 1.16 Wh is required. So, a water storage tank with a capacity of 300 liters that needs to be heated from 10 °C to 60 °C would store about 17.4 kWh of energy. With a constant heating power of 2 kW, it would take 8.5 hours to deliver this energy into the tank.

Where and when does a heating element make sense?

Heating elements run on electricity, and heating water using grid electricity is relatively expensive—something that has been known even before 2021. On average, a person in Austria or Germany (the figures are very similar) uses about 30 to 50 liters of hot water per day. To heat this amount of water, roughly 800 kWh of thermal energy per person per year is required.

From both an economic and ecological perspective, a heating element really makes sense when the required electricity does not come from the grid, but from your own photovoltaic system. my-PV’s solutions can convert photovoltaic energy into heat—continuously and without steps—so that the maximum possible share of solar electricity is used for heating.

There are two options for this: Either independent water heating with a heating element powered directly (i.e., without an inverter) from the photovoltaic modules via the DC Power Manager SOL•THOR, or surplus utilization: In this case, the excess photovoltaic power that is not used in the household and would otherwise be fed into the public grid is used for water heating—in a continuously modulating manner, as the technical term goes. This allows surplus PV power to be used intelligently and efficiently for self-consumption.

Devices such as the AC ELWA 2, AC•THOR, or AC•THOR 9s receive information about the power surplus from the my-PV WiFi Meter or another compatible source. This allows the water tank to be heated with self-generated PV electricity, optimizing self-consumption.

We have summarized here why, in general, it is sensible to heat a thermal storage tank (whether domestic hot water or buffer storage) using a solar-electric heating element, and which types are available.

To assess the system realistically throughout the entire year, we recommend using the my-PV Power-Coach. This free online tool helps analyze the interaction between PV yield, hot water demand, and device settings on an individual basis.