A Weapon Power Mod is a craftable Device Mod. Applying it to a device will reduce the base Power cost of the device.
Note that a Device may only have one crafted mod applied at a time. Attempting to apply another crafted mod will overwrite the previous mod.
Weapon includes also off-hand devices and jetpacks. Those devices can also have this mod applied to reduce their power cost. Applying it to a off-hand is useless since off-hand devices do not require power.