Warpgates draw power in two ways, idle and jump
When a gate is idling, aka no ship is jumping thru, it will draw 100 e/s per warp gate block.
When a ship tries to use the gate, it will try to draw an amount relative to the ships mass, ship mass x 100.
So lets say a 5000 mass ship tries to use the gate, upon trying to warp the ship, the gate will try to draw 500000 (5000x100) energy, should the gates power capacitors not hold at least that amount of power, the warp will fail, otherwise the energy gets drawn and the ship jumps.