Do I have to have a power plant to use energy weapons? I assume that's what power requirement means.
Yup. Bear in mind you only need enough power to satisfy all capacitors for a 5-second increment. (And actually, you don't even need that, but as someone said they're so tiny you might as well).
Example:
1.You have six lasers (or mesons or railguns or microwaves...they all take power) with a 5-second ROF and a power requirement of 3 each. This means you need 6x3=
18 power available to keep firing them non-stop.
2. You have six reduced size lasers with the "75% size/4x recharge rate" option. This means that while each laser still has a power requirement of 3 to fire, let's say you only have capacitor recharge 3 technology. So each laser can only charge by 0.75 each 5-second increment and has a 20-second ROF. Now, you only need 6x0.75=
4 power available to keep fully charging your guns at all times.
This actually has some serious use when you get to talking about BIG guns. I recently prototyped a Swarm-killer design with 10 35mm lasers, with the "50% size/20x recharge" option. The smaller laser size and smaller reactor meant I could pack a shitload of guns onto a fast frame and wind up with a cruiser that could pull 10000 km/s and still had range and firepower to blast several Swarm drones to bits every few minutes while staying out of meson range. Cost-effective way to clear out really big packs of the Swarm.