How many solar panels do I need?
Key points
- 1 kW solar immersion heater: 1,500–2,500 Wp, usually four panels.
- 550 W solar immersion heater: 800–1,600 Wp, maximum 2,000 Wp with up to five panels.
- The MPP voltage (Vmpp) determines the power, ideally 38–43 V. The open-circuit voltage (Voc) must not exceed 50 V. Always connect panels in parallel.
- Cable run no longer than 25 metres, 6 mm² solar cable.
Most standard solar panels work with fothermo heaters. To make sure your tank heats up reliably, three things matter: the right number of panels, the right voltage and a short cable run.
[grafik:rechner]
Why more panel power than heating power?
Four panels of 430 Wp each add up on paper to more than the 1,000 watts the 1 kW solar immersion heater uses. That's intentional. Panels only deliver their rated power in ideal sunshine. On cloudy days, in the morning and in the evening, the extra area makes sure enough energy still reaches the tank.
It all comes down to voltage
Two voltages on your panels' datasheet matter:
- MPP voltage (Vmpp) – the power. The higher it is, the more heating power the heater reaches. For the 1 kW solar immersion heater we recommend 38 to 43 V.
- Open-circuit voltage (Voc) – the limit. It must not exceed 50 V. Panels with an open-circuit voltage above 50 V must not be connected, as they would damage the heater.
Ideally, the open-circuit voltage is just below 50 V – then the MPP voltage is as high as possible too. The dial in our MPP voltage article shows how much power you get with your panels.
Always in parallel, never in series
All panels are connected in parallel. This has a pleasant side effect: if one panel is in the shade, the others continue to deliver full power. A series connection is not permitted.
Power decides how quickly your water heats up. Voltage decides whether it works at all.
Orientation and tilt
- South gives the highest yield.
- East-west spreads the output more evenly over the day.
- A tilt of around 30 to 35 degrees is ideal in Central Europe.
- Leaves, dust and bird droppings cost power – occasional cleaning pays off.
Cables: short and thick
There should be no more than 25 metres between the panels and the heater – that's 50 metres of cable there and back. Beyond that, losses rise sharply. Use high-quality solar cable with a 6 mm² cross-section.
Not sure whether your panels fit? Send us the datasheet – we'll be happy to check it for you.
