Yes they will take electricity. Although these can be used to supplement at the start or end of the day rather than all day if you want to limit the power used.
Nothing stopping you from only using it every other day if the plant is in daylight.
Think of it like this - 400W = 400W/hour, 0.4KWh * price per KWh * hours you use it = cost.
So for my 400W HPS on for 10-12hours/day then a month = £32.
If you're supplementing then a CFL 250W would help. If you're really serious and have a lot of plants to cover then a 400W+ HPS (dual spectrum bulb) but these will need space and will kick off a lot of heat compared to the CFL.
Have a read about CFLs and HPS lights here.
You will require a reflector either way and the bulbs will not fit normal bulb sockets.
Nothing stopping you from only using it every other day if the plant is in daylight.
Think of it like this - 400W = 400W/hour, 0.4KWh * price per KWh * hours you use it = cost.
So for my 400W HPS on for 10-12hours/day then a month = £32.
If you're supplementing then a CFL 250W would help. If you're really serious and have a lot of plants to cover then a 400W+ HPS (dual spectrum bulb) but these will need space and will kick off a lot of heat compared to the CFL.
Have a read about CFLs and HPS lights here.
You will require a reflector either way and the bulbs will not fit normal bulb sockets.