What Watt OTT Bulb do I need?

Looking at buying OTT Lights, and I am planning on getting 2 for photography. What watt bulb should I get? I have seen 13W and 18W. Thanks in advance.
-Paul
-Paul
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I am selling the other 2. If you are interested, please PM me.
These pics were shot w/ 2 OTT Lights.