# Resistance for water heating using 10 W solar panel

I am trying to do an experiment and see if I can use PV panel heating for a water tank. The problem is the PV panel is only 10 W. What would be the maximum resistance that I can attach to it to see some changes in water temperature, and what kind of resistance?

I was thinking of an aquarium resistance but I don't know if I can find something rated under 10 W.

Parameters of my panel are:

Pmax: 10 W; Maximum Power Current: 0.57 A; Maximum Power Voltage: 17.49 V; Short Circuit Current: 0.61 A; Open Circuit Voltage: 21.67 V; Maximum System Voltage: 1000 V.

• Include parameters of your solar panel, or datasheet. – Chupacabras Apr 14 '17 at 5:36
• 10W in the sun, might be more helpful to paint the water tank black. – Wesley Lee Apr 14 '17 at 6:24
• There is absolutely no reason to use a 10W solar panel to heat an aquarium. Just cut out a piece of black plastic the same size as the panel and put it inside the aquarium, with the aquarium in the sun. This will heat up the water much more than a resistor connected to the panel. – mkeith Apr 14 '17 at 7:18
• The fundamental problem is that solar panels convert maybe 10 or 20 percent of the incident solar power to electricity. But a piece of black plastic will convert 90% of the incident solar power to heat. Solar heating of water is best done by direct heating, not by generating electricity from a photo-voltaic and converting it to heat in a resistor. – mkeith Apr 14 '17 at 7:24
• Not an electrical engineering approach, but dedicated solar thermal collectors (especially if evacuated tubes are used) will outperform any electrical solution. – Sanchises Apr 14 '17 at 10:21

So, your maximum power voltage is 17.49V and maximum power current 0.57A.
It means you need load with resistance 30.68 ohms.
You can use heating wire (nickel chrome) with this resistance. Of course you have to cut wire to length you calculate from parameters of that wire.

• The question is not about the size of the water tank,because i can find water tanks in all sizes, but about what kind of resistance would i need so small 5w that can be inserted in water?Can you tell me more about this solution Nickel Chrome? – adrian.bercovici Apr 14 '17 at 12:18
• en.wikipedia.org/wiki/Nichrome – Dampmaskin Apr 14 '17 at 14:18
• Use insulated nichrome. Or, maybe, use 30 feet of 40 AWG magnet wire (this is copper wire). You can wrap it around a PVC form if desired. It will be around 30 Ohms, and is insulated so that it will not add impurities to the water. I am pretty sure bare nichrome with 17V across it will undergo reactions with the water and cause lots of problems. I also want to emphasize that this is a poor idea from an engineering perspective. You can get much more heat in the same area using direct solar absorption. – mkeith Apr 15 '17 at 18:32

10 watts of solar power can only give you 10 watts of heating power, at best. In a real-world situation, this is probably closer to 5 watts.

Unless your water tank is really tiny (like, one liter or less), this won't even be enough to keep it warm. You'd need a lot more power to do that.

• Is it picky to criticize "watts of heat"? "heat" is energy, "watt" is a unit of power, which is energy per time unit. Contentwise I fully agree. Given that water has the "exceptionally high", largest specific heat capacity (4.18 kJ/kg/K) of all liquids, which means it takes much more energy to heat it up compared to others. – GeroldBroser reinstates Monica Apr 14 '17 at 7:00
• If heat is energy, whats the proper word for the associated power? – Christian Apr 14 '17 at 7:02
• @Christian The proper word for power is, well, "power". ;) What do you exactly mean by "associated"? – GeroldBroser reinstates Monica Apr 14 '17 at 7:03
• Yes. It is slightly picky. – mkeith Apr 14 '17 at 7:21
• @mkeith Well, if it's only "slightly"... The next time I visit Sir Newton's gravesite I'll tell him that the world is getting him only "slightly" wrong. :) – GeroldBroser reinstates Monica Apr 14 '17 at 7:25

Mount the solar panel to a metal plate, and attach cooling water pipe to the back of the plate. Use some of the 10W electrical power to pump water from the aquarium through the cooling pipe. You'll get considerably more than 10W of heating that way.

Use a larger plate and paint the rest matt black for more heating, a glass cover and insulation beneath it will help too.