I have a 60W solar panel with a charge controller in my boat. I need a dump load resistor but I'm concerned about catching the boat on fire. The dump load resistor will dissipate the same amount of power as a 60W incandescent light bulb, right?
This is a common question we get and for a good reason. Dump load resistors can be confusing, especially with all of the bold sales claims out there. We'll try to explain in detail.
Typically with a 12 volt battery system with solar panels a dump load resistor isn't necessary. That's one of the good things about solar panels - when they're disconnected they stop producing power.
This is in contrast to a wind turbine. When the wind turbine is disconnected from a battery, it keeps producing power and dissipates that power as heat within the generator. Thus a dump load resistor is needed to prevent the generator from overheating.
If the resistor is connected to your solar panel, you are correct, it will try to dissipate the full 60W output of heat. You are also correct about the 60W bulb analogy (incandescent outputs about 90% heat / 10% light).
However, if there is a chance that the resistor is connected to a battery, it will try to dissipate as much energy as it can. Using the classical equations (V)oltage = current (I) x (R)esistance and (P)ower = (V)oltage x current (I), and assuming your battery's voltage is 14 volts,
V = IxR
14 Volts = (I) x 1 ohm
I = 14 amps
P = VxI
P = 14 Volts x 14 Amps
P = 196 W
Depending on the ventilation where the resistor is installed, the materials it is near, and how long it operates like this, there is a chance that it could cause a fire. If you have the resistor in one of our aluminum enclosures and mounted on a wall with good ventilation, that risk is very low.
However, if you're using a charge controller for this panel only, and don't have a wind turbine, you shouldn't need a dump load resistor. Solar panels are perfectly fine being left in the sun even when they're disconnected by the charge controller.
We hope this helps and please email us at Sales@CuttingEdgePower.com if you have any questions.