How Well Does Solar PV Water Heating Actually Work?
The debate is over. While evacuated tubes are technically more "efficient" at heat transfer, PV heating has won the war on reliability, simplicity, and longevity. But the question remains: How much solar power do you actually need to get a hot shower?
The Golden Rule of Thumb
We often get asked: "How many panels do I need for my 200L geyser?"
To answer this, we look at the physics of heating water. The table below shows exactly how many degrees of temperature rise you get for every 1 kWh of solar energy generated.
Key Takeaway: A smaller tank heats up faster. If you have a large tank, you simply need more panels to achieve the same temperature rise in the same amount of time.
| Tank Size | Temp Rise per kWh | Panels Required* |
|---|---|---|
| 150 L | 5.7 °C | 3 Panels |
| 200 L | 4.3 °C | 4 Panels |
| 300 L | 2.9 °C | 4 - 6 Panels |
*Based on standard 455W - 550W High-Current panels.
Let's Do The Maths: The 150L Example
The Goal: Heat a 150L tank from 20°C (cold) to 55°C (hot).
Temperature Rise Needed: 35°C.
Energy Required: 35°C ÷ 5.7°C/kWh = 6.14 kWh.
The Solar Setup: 3 x 460W Panels = 1.38 kW peak output.
Time to Heat: In good sun, these panels generate ~1.2 kWh per hour.
Result: Your tank is fully hot in just over 5 hours.
(In summer, it happens much faster. In winter, the grid may need to top up the last 10°C, but you still save 80% of the cost).
Does it Pay for Itself?
Standard Electric Geyser
R950
Average Monthly Cost
(Based on R3.50/kWh)
With PV Solar Kit
R150 - R200
Average Monthly Cost
(Grid backup only)
Annual Saving: ± R 9,000.00
At current kit prices, the system pays for itself in roughly 2 to 2.5 years. After that, your hot water is essentially free.