Its about electrical power. Ampere x Voltage = Watt.
If you leave ur carputer running from a battery with the engine turned OFF, you will be draining the battery.
You need a certain amount of AmpereHours left in your battery to crank your car. If your carputer drained too much from the battery, you won't be able to crank your car.
So you see.. it has nothing to do with the HorsePOWER of your engine. In fact, I don't think connecting electrical devices will take any HP from your engine, since the energy comes from the battery. The alternator charges the battery, but its a fixed amount, so it won't be more if you connect more devices.
Watch out to use more batterypower then the alternator can recharge, you will be draining the battery even if the car is running, but you won't be draining HP.