How to connect a photovoltaic to a heating element directly
As shown with load in source lead, Vgate must be driven to Vload + Vgson to turn FET on.
For > 20V supply FET will be destroyed.
SO Place load in Drain side of cct. PV+ - load - FET_drain ... .   

Floating gate may easily be driven to Vin by leakage.
FET will be destroyed.
SO Connect gates to ground with a resistor at all times (say 10k but anything smaller also OK).   

Consider connecting a say 12V zener gate to ground so gates can NEVER be driven above about 12V.  

5V gate drive is marginally low for IRF3205.
Probably OK but not as fully enhanced (on) as you may like.
Fig 1 &amp; 2 in the datasheet show Vdson vs  Vgs and Id.
At 8A with Vgs = 5V expect about 0.1 Ohm Vds 
FET dissipation = V x I = 0.1 x 8 = 0.8W - say about 1 Watt/FET.
You have not said what package you are using.
A modest amount of heatsinking will be advisable.







simulate this circuit &ndash; Schematic created using CircuitLab

http://www.irf.com/product-info/datasheets/data/irf3205.pdf