This might help:
These resistors are each rated at 300W, 2 ohms. All in parallel have a combined resistance of 0.5 ohms.
My system's voltage is 12V. When fully charged, the voltage can be up about 14V, more than that when it's cold. The max current is therefore:
15V / 0.5ohm = 30 Amps.
Power dissipated is 450 Watts; more than enough for my windmill which furls at 200W.
Your resistors must allow enough current to actually dissipate your 100W, at whatever battery system voltage you're using.
Make allowances for the resistors getting hot - a thing on my to-do list. My plywood wall is unprotected. Fine while it's winter. Must cover before summer.