Diverting power from the solar panel to a load

[kevtherev]

Why not just connect a 100w heater to the battery, the solar cell should keep up delivering this power just as your alternator would so your battery is still charged.
as for switch on id think thats the difficult bit maybe just a timer if your battery use is consistant?

Good point regarding the existing batt.

[kevtherev]

Or you could paint a jerry can black, fill it with water and stand it on the roof.

Which is how I already raise the temp of the water.

Panel I have put out maximum recorded figures of 7.3 amps (23 volts), that was a sunny day without any tilt.