Minimum Fixed resistance load for the average 235w, 24v panel?

[LETitROLL]

Just playing with a extra panel I have and wanted to hook it direct to a 12v, 300w element, the element data says you can exceed the rated voltage as long as you don't exceed the wattage. The element has a fixed resistance around .5 ohm and I dont know if the panel would have a problem or eventually be damaged by that low of a load?
thanks in advance

[sensij]

0.5 ohm is pretty much a short circuit as far as the panel is concerned. In midday direct sunlight, you will get something close to the Isc of the panel flowing through the resistor, probably approaching 7.8 A. The power dissipated by the element would be P = R * I^2 = 0.5 * (7.8^2) = ~30 W. In less direct sun, the power will be less. It won't hurt the panel at all.

[Sunking]

Will not do any harm at all, nor much good as the resistance is way to low. A panel is a current source, so forget about voltage. The resistance would dhave to be around 2 to 3 ohms to get any meaningful power out of the panel. What is the Imp and Isc?

[LETitROLL]

The Imp is 7.81, and the Isc is 8.31

[Sunking]

The Imp is 7.81, and the Isc is 8.31

OK call it 8 amps...

At best a few minutes around noon 8 x 8 x .5 = 32 watts. You need a 250 watt 3.8 Ohm Resistor. That would give you for a few minutes around noon of 7.8 x 7.8 x 3.8 = 230 watts.

[LETitROLL]

Thanks, what happens at the lower resistance, (high current flow but at a low voltage?).

[Sunking]

Thanks, what happens at the lower resistance, (high current flow but at a low voltage?).

You panel is a Current Source, not voltage source operating between Vmp and Isc. Lower resistance means lower wattage and volts. It will source current up to 8 amps with 30 volts pushing it. Power = Current x Current x Resistance. Example 8 amps x 8 amps x .5 Ohms = 32 watts. Another example 8 amps x 8 amps x 3.8 Ohms = 230 watts.

You you go above 3.8 Ohms it becomes a Voltage Source of 30 volts. Power = [Voltage x Voltage] / R. Example at 4 ohms around solar noon 900 volts \ 4 Ohms = 225 watts. Another example 900 volts / 8 Ohms = 112 watts.

At .5 Ohms is almost a dead short circuit and with 8 amps you will never see more than 4 volts and 32 watts.

So here is the deal with a current source and your panel. You have a 8 amps current source with a 30 volt battery pushing it. That means it will deliver 8 amps of current into a resistance of 0 to 3.8 Ohms. Simple Ohm's Law.

[LETitROLL]

Thanks so much, I use to know most of that, but it has been 30 years ago. So In a less than optimum sun time, lets say the panel would only be able to produce 3 amps, you would have 3 x 3 = 9 * 4(r) = 36w, but if you changed your element to 10 ohm you would get 3 * 3 = 9 * 10= 90 (which cross checks with the 30 volt * 3 amp = same 90). If that is correct, then that is what a properly working MPPT control would adjust for and keep you in the optimum zone? If that is all correct now I see why a fixed load (like said heat element) is close to a waste of time to try and run directly off of a source that varies so much. Basically if the source varies greatly (and solar panels do), then the resistor (load) also needs to vary if you want to keep producing any significant power (wattage).?

[Sunking]

Thanks so much, I use to know most of that, but it has been 30 years ago. So In a less than optimum sun time, lets say the panel would only be able to produce 3 amps, you would have 3 x 3 = 9 * 4(r) = 36w, but if you changed your element to 10 ohm you would get 3 * 3 = 9 * 10= 90 (which cross checks with the 30 volt * 3 amp = same 90).

Correct.

If that is correct, then that is what a properly working MPPT control would adjust for and keep you in the optimum zone? If that is all correct now I see why a fixed load (like said heat element) is close to a waste of time to try and run directly off of a source that varies so much. Basically if the source varies greatly (and solar panels do), then the resistor (load) also needs to vary if you want to keep producing any significant power (wattage).?

Correct again. You got it.