Watts per foot times # of feet. (don't forget, there is a max run allowed for a few reasons, don't exceed it)
ie: 5 feet of 11" Flexwatt (20W/ft)
5 x 20= 100W
All the power is only needed during startups, etc. With a pulse proportional thermostat its like a trickle of energy to the heat tape instead of full blast, shut off, full blast, shut off.
Dave
the helix 1007 is a single 1500W unit (that's 1500W in once source allowed instead of plug in modules at 500W max each. The 1007 allow allows 2 additional 500W plug ins. (maybe 3?).