Electricity Basics

Language of Electricity

A few terms are used regularly to describe the storage and flow of electricity through wires and in and out of components. The terms interrelate to each other so some simple math is needed.

Voltage (Volts)

Voltage is a measure of the potential energy between two points in the electrical circuit. It is measured in volts using a voltmeter and abbreviated with an uppercase V (E is also used). Also called electromotive force, voltage is analogous to water pressure in a garden hose. For a battery, the voltage is defined by the battery design. Many small batteries measure about 1.5 volts. Typical car batteries are ~12 volts. The voltage of a battery pack can be increased by connecting more than one battery in series (meaning positive to negative). A simple example of a series connection is a flashlight with 2 AA batteries stacked in the handle. Assuming the batteries are pointing the same way, the voltage measured from the negative end of the first battery to the positive end of the second one is about 3.0 volts. Similarly, two 12 volt batteries can be connected by wiring the positive of one battery to the negative of the other. The voltage between the free terminals will now be 24 volts. Four batteries connected in series would create a 48 volt battery pack. Packs of many battery cells connected in series for hybrid cars can be over 200 volts. 

Current (Amps)

Current is the flow of electricity through a conductor such as a copper wire. It is measured in amps using an ammeter and abbreviated with an uppercase A (I is also used in formulas). For the garden hose analogy, current is the flow of water. Current, voltage and resistance (R) of a circuit are related by the formula A = V/R. Current is directly related to voltage meaning that increasing the voltage will increase the current if the resistance is unchanged. Current is inversely related to resistance meaning that current will increase if resistance drops and voltage is unchanged. 

Current Type (DC & AC)

Direct Current (DC) is continuous one-directional flow which comes from batteries and solar panels. A basic DC circuit connects the positive side of a battery to a switch, then to a light and then back to the negative side of the battery. When the switch is on, current flows out of the battery, through the light turning it on and back to the battery.  The amount of  current will be based on the voltage of the battery and the resistance in the wires and light. 

Alternating current (AC) is the form of current created by rotating turbines and generators and the flow alternates back and forth 60 times every second in the US. On our human time scale, this 1/60th of a second may see fast, but electricity is so quick, it can travel about 3,100 miles in that time. AC is used to power homes and so many appliances are designed to use 120 VAC. 

Because it is convenient to use household appliances in a mobile vehicle, inverters are used to change the DC voltage of the battery bank into the AC current needed to run the appliances. In reverse, a battery charger takes the 120 VAC current from a home or campsite and changes it to the appropriate DC voltage to charge a battery bank. 

Resistance (Ohms)

Resistance is included here because it is commonly referenced in connection with voltage and current. It is measured in Ohms using an ohmmeter and is abbreviated with an uppercase R. In designing mobile electrical systems, we will not use or measure resistance very often. Resistance comes into play when selecting the size of wire for a circuit. Thinner wires have more resistance than thicker ones and that resistance creates heat and can create a voltage drop along the wire. Therefore, when picking wires, there is a balance between the lower resistance and the higher cost of larger wires. 

Power (Watts)

Power is a measure of the electrical energy transferred by an electrical circuit. It is also a measure of the electrical work performed by the circuit to do things such as pump water or cook dinner. Power is calculated by multiplying voltage and current and is abbreviated with the uppercase W (so W = V x A). As an example, if a 12V light that draws 0.5A of current is using 6W of power. 

Power is a particularly useful way to plan and keep track of energy usage because it is independent of the voltage. For example, a DC powered refrigerator might accept either 12V or 24V and require 72 Watts to run. This 72W number stays consistent so if you power the fridge with a 12V battery it will take 6A of current (72W/12V = 6A); if you use a 24V battery, the current will be just 3A (72W/24V = 3A). 

Energy (Watt-hour)

Energy is a measure of the total amount of power available over time. It is useful when measuring battery banks and estimating how long different appliances could be run. It is measured in Watt-hours and abbreviated as "Wh". It is also sometimes simpler to talk about Kilowatt-hours which are abbreviated as "kWh" and are simply 1,000 Wh. 

Battery examples are useful because batteries are rated in Amp-hours and so multiplying this by the battery voltage gives the Wh of the battery bank. For example, a 12V battery rated at 200 Ah has a total energy of 2,400 Wh or 2.4 kWh. On the solar panel end, if a panel is in full sun for 3 hours and is producing its expected 250 Watts, it will have produced 750 Wh after the 3 hours are over.