Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other advanced electronic system of your choice.
The electrical system consist of a battery and an alternator (you will find a generator on older type aircraft) to power the system and recharge the battery. You will also find fuses and switches and lights for indication purposes. A volt and/or ammeter is used for monitoring.
All of this is connected through several meters (kilometers in large aircraft) of wire and connectors, attached to the airframe with insulation material as cushion clamps, ty-wraps and what not. Even for the private pilot it pays to have some basic knowledge of the electrical systems of his or her aircraft, which could be a life saver in case of an emergency.
Volts are not Amps. You can compare it to water flow, where volts is the water pressure and amps is the amount of water that is flowing per second when the faucet is opened. You can then think of the faucet as a variable resistor or a transistor/ fet controlled by another circuit.
The characteristics of electrical measurement are expressed in volts (force) and amps (current). To have a flow (current) of electrons, we need a force (volts) to push them around the circuit. The higher this force, the higher the number of electrons at the negative terminal and the higher the voltage or electrical pressure.
The amount of electrons flowing through the circuit is called amperage (amps) and is directly proportional with the voltage and inversely proportional with the resistance in that circuit.
All conductors have an internal resistance, this depends on the material they consist of (copper, alu) and the thickness, size and length of that conductor. The relation between volt, current and resistance is known as Ohms Law. Which states that one volt applied over a resistance of one ohm results in a current of one amp, in formula: U = I × R. Where U is the voltage (V), I is the current (A) and R is the resistance in Ohm (Ω).
This formula can be transformed to obtain the current if voltage and resistance are known: I = U / R.
Of course, when a current of 1 Amp flows through a resistor of 1 Ohm the voltage is 1 V. But what about power? The formula to calculate power is P (W) = U (V) × I (A). Thus in our 1 Ohm resistor with 1 V applied and a current of 1 A the power generated is: 1 watt (W). So with a 12 V battery connected to a 1 Ohm resistor the current is 12 amps and the power dissipated in that resistor is: 12 V times 12 A = 144 W.
Energy is power delivered during a certain time period, usually a second or an hour. The amount of electrical energy is expressed in Joule or Watt/second and 1 kWh is equal to 3.6 MJ. In our above resistor example that would mean an energy consumption of 1 J or 1 W/s.
Batteries create a direct current/voltage, the current can only flow in one direction. It will not change direction without us changing the wires, which would kill the connected device... The alternator in an aircraft generates an alternating voltage internally which is then rectified by solid state devices (diodes) creating direct current flow and voltage between its terminals.
This DC voltage is then used to recharge the battery and to supply power to the rest of the electrical system. Aircraft with a FADEC controlled engine must have a fail safe electrical system by either using a standby battery or a second alternator on the engine.
These have a built-in generator capable of 250 W and 13,8 V at 5500 engine RPM. Maximum current is then 18,5 amps. Cruise RPM is normally around 5000 RPM so expect less power during cruise. If you have an aircraft with a full EFIS panel with autopilot, strobes, navigation and landing lights then the electrical requirements will to too much for this engine. Voltage will drop and the battery will drain until its empty.
You must try to limit current usage by not switching everything on. Use only the systems essential to the flight. Installing LED strobes (conventional strobes take some 7 amps) or full LED navigation lights in stead of the old school bulbs will reduce power consumption and if possible install the optional alternator which can deliver 40 A at 13,8 V (540 W). Its weight is about 3 kg / 6 lbs so be aware that your empty weight will go up too, a little.
Airplanes with more than one engine will have an electrical system wired in such a way that it allows for a complete failure of any alternator, so the aircraft systems can run on the other alternator/battery driven by the operational engine. In such a system you will find multiple alternators and one or more batteries and failover is or should be automatic. Usually devices will be connected to several buses according to their importance.