Every electrical job starts with four numbers: volts, amps, watts and ohms. Once you know what each one measures, labels, manuals and meter readings start to make sense.
This lesson explains them in plain language, shows how they connect, and works through a few everyday examples.
1. Volts: the push
Voltage (V) is the electrical pressure that pushes current around a circuit. A higher voltage can push more current through the same resistance.
Common examples in Australia:
- A single AA cell: about 1.5V.
- A vehicle or caravan battery system: nominally 12V (sometimes 24V).
- Cordless tool batteries: commonly 18V or 36V nominal, depending on the brand.
- Household power points: 240V AC.
A useful way to picture it is water in a hose. Voltage is like the water pressure.
2. Amps: the flow
Current, measured in amps (A), is the rate at which electric charge flows through the circuit. In the hose picture, current is how much water is flowing.
Current is what heats cables and connections. That is why cable size and fuses are chosen mainly around current, not voltage.
3. Ohms: the resistance
Resistance, measured in ohms (Ω), is how much a material or component opposes the flow of current. In the hose picture, it's like a narrow section or a kink.
- Good cables and tight connections have very low resistance.
- Heater elements and globes are designed to have resistance.
- A corroded terminal or loose crimp adds unwanted resistance, which turns into heat.
4. Ohm's law: how they connect
For a simple resistive circuit, volts, amps and ohms are linked by Ohm's law:
Volts = Amps × Ohms
Rearranged:
Amps = Volts ÷ Ohms · Ohms = Volts ÷ Amps
For example, a 6Ω resistive load across 12V draws about 2A:
12V ÷ 6Ω = 2A
Motors, LED drivers, chargers and electronics don't behave like a simple resistor, so treat Ohm's law as a way to understand a circuit, not a replacement for the equipment's rated current.
5. Watts: the work being done
Power, measured in watts (W), is the rate at which energy is used or delivered.
Watts = Volts × Amps
Two examples show why the same wattage means very different currents at different voltages:
- A 2,400W kettle on 240V AC draws about 10A (2,400 ÷ 240).
- A 120W 12V device draws about 10A (120 ÷ 12).
Same current, very different power. At 12V, even modest loads mean high current, which is why low-voltage systems need heavier cable than many people expect.
6. Watt-hours and amp-hours: energy over time
Watts tell you how fast energy is used. Watt-hours (Wh) tell you how much energy is used over time.
Watt-hours = Watts × Hours
Batteries are often rated in amp-hours (Ah). To compare batteries of different voltages, convert to watt-hours:
Watt-hours ≈ Amp-hours × Nominal volts
For example, a 100Ah battery at a nominal 12.8V stores roughly 1,280Wh. How much of that you can actually use depends on the battery chemistry and the manufacturer's recommendations.
7. Reading a label
Most equipment labels show at least two of these numbers. If a 12V fridge label says 60W, you can estimate its running current:
60W ÷ 12V = 5A
Remember that fridges, compressors and motors cycle on and off and may draw a higher current at startup. Use the manufacturer's figures when sizing cable, fuses and batteries.
8. Common beginner mistakes
- Mixing up amps and amp-hours.
- Comparing batteries by amp-hours alone when their voltages differ.
- Assuming a 12V item is low-risk because the voltage is low. High current can still melt cable and start fires.
- Using Ohm's law on motors or electronics and ignoring the rated current.
9. Safety boundaries
Low-voltage DC systems can still deliver very high current. A short across a battery can cause burns, fire or a battery explosion.
240V AC mains can kill. In Australia, work on household mains wiring must be carried out by a licensed electrician.
When in doubt, stop and get advice from a suitably qualified professional.
10. The main lesson
Volts push. Amps flow. Ohms resist. Watts do the work. Watt-hours measure the energy over time.
Get comfortable with those five ideas and the rest of the Revival Academy 101 lessons will be much easier to follow.
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Revival Academy 101 shares practical tutorials to help you understand electrical systems and make informed decisions.
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Technical note: This is general educational information, not a substitute for a design calculation, manufacturer instructions or applicable Australian standards.