One of most cost-efficient and very reliable solar photovoltaic supplemental heating systems !

Disclaimer: This information is for educational purposes ONLY! Pease ask a local licensed electrician to check your setup.

In 2009, I installed three solar hydronic supplemental heating systems for my house. Learn more here. To go greener, in 2021 I decided to provide supplemental heating for three additional rooms.

This time, I chose solar photovoltaic panels.

My approach is straight forward: heating systems should be simple in design and as efficient as possible. To reach these goals, I bought six Renogy 320-Watt solar panels and divided them into two separate sections. In each section, I connected the panels in series.

I used 1-3/8" galvanized fence pipes and corners to make two strong rectangular frames. I then installed 1-1/2 in. steel angles from old bed frames.

These two sections generate about 2,500 kWh of electricity annually.

The maximum Open-Circuit Voltage of each section is about 117 Volts DC (Direct Current). I installed a 15-Amp Single-Pole circuit breaker for each section. Most household electric heating appliances use AC (Alternative Current). To make my system more reliable and more efficient, however, I did not use a DC-to-AC inverter to avoid any electricity loss.

Note: even some of the best inverters with the peak efficiency up to 99% may only operate in its peak efficiency range for a very small part of the day or not at all. This is why the CEC and Euro weighted efficiencies have been developed. They recognize that inverters don’t always operate in optimal conditions, and instead these measurements offer an indication of how an inverter might perform throughout the day. Inverter Efficiency

I used Jacobi's law to optimize the power transfer from the panels to the load. The law states that maximum power is transferred when the internal resistance of the source equals the resistance of the load. To calculate internal resistance of the solar panels (R) based on the Ohm's law (R= E/I), I used the E (voltage) and I (current) information from the panel sticker (see photo). In my case, total voltage for the three panels connected in series is 101.1V (33.7V X 3) and current is 9.50A. That means that the optimum load resistance for my setup should be close to 10.64 Ohms (101.1V / 9.5A = 10.64 Ohms).

Each section was connected directly to the heating elements of a 1500-Watt oil-filled radiator thanks to the 10/2 WG underground wires. The heating elements of each radiator connected in parallel to get a total resistance of radiator (load) as close as possible to the resistance, determined above.

Fortunately, the multimeter showed 9.4 Ohms, which was very close to the desired load resistance.

You can buy oil-filled radiators similar to mine for a reasonable price at a Home Depot, Walmart or Amazon.com

On November 24, 2021 at 9 a.m. radiator temperature was 125.1 °F. The outside temperature was around 33°F (1°C). The Operating voltage (Under Load) at that time was 73V.

At 10 a.m. the radiator temperature was 188.6 °F. The Operating voltage at that time was 89.4V.

At 10:28 a.m. the radiator temperature was 251 °F, therefore digital thermometer with a sensor switched to the HHH mode - over 200 °F. The outside temperature was around 35°F. The operating voltage at that time was 92.8V.

At 11 a.m. the radiator temperature reached 259°F (126.111°C). Formula: (259°F − 32) × 5/9 = 126.111°C . The Operating voltage at that time was 92.8V.

At 12:05 p.m. the radiator temperature was 250°F. The outside temperature was around 35°F. The Operating voltage at that time was 91.1V.

At 1:00p.m. the radiator temperature was 197.1 °F The Operating voltage at that time was 84V.

At 1:21 p.m. the radiator temperature was 185.4 °F. The Operating voltage at that time was 78.8V. And at 3 p.m. the radiator temperature was around 130 °F.

On January 11, 2022 at 11:45 p.m. radiator temperature reached 361°F (182.7°C)! Operating voltage at that time was 97.8V. Outside temperature was around 9.5°F (-12.5°C)! Note: The diathermic oil (used in the oil-filled radiators) has a high boiling temperature at atmospheric pressure conditions, between 750°F and 1000°F (400°C and 537°C). Therefore oil inside of radiators can store heat without getting hot enough to boil.