watt per meter
One watt delivered along each metre of length. Wave power is quoted this way — per metre of wave crest — and so is heat loss along a pipe run.
Watch out: Wave power per metre describes the resource arriving, not the power a device produces. Capture width and conversion efficiency both intervene, and a converter typically extracts a modest fraction of what crosses it.
| 1 | 1 |
Power divided by length. It earns its own name in marine energy because a wave front has no meaningful width of its own: the resource is however much crest you choose to intercept, so the honest statement of it is per metre.
About the watt per meter
Some quantities have no natural extent of their own, and stating them per unit length is the only honest form. Ocean wave power is the clearest case. A wave front stretches to the horizon, so asking how much power "a wave" carries is meaningless — what you can answer is how much crosses each metre of crest.
The relation for deep water is , and the numbers are large: a moderate Atlantic sea with a two-metre significant height and an eight-second period runs about 30 kW per metre of crest. A hundred-metre array intercepts three megawatts of resource, of which a real converter takes a fraction.
The same dimension appears in thermal work as heat loss per metre of pipe, which is how insulation is specified and how a heat-trace circuit is sized. There the working units are W/m in SI and BTU per hour per foot in North American practice.
Welding uses the same dimension for heat input, though by convention it is written as energy per length rather than power per length — kilojoules per millimetre — because what matters to the metallurgy is the total energy the weld received, not the rate while the arc was moving.
One watt per meter in every power per length unit
| 1 | watt per meter (W/m)this unit | |
| 0.001 | kilowatt per meter (kW/m) | |
| 0.001 | watt per millimeter (W/mm) | |
| 0.3048 | watt per foot (W/ft) | |
| 1.04002 | BTU per hour per foot (BTU/(h·ft)) |