Four quantities, two equations
Ohm's law states that voltage equals current multiplied by resistance: V = I × R. Power is voltage multiplied by current: P = V × I. Between them these two relationships describe almost everything you need for a direct-current circuit, and every other form is algebra applied to them.
The water analogy holds well enough to be useful. Voltage is pressure, current is flow rate, resistance restricts the flow, and power is the rate at which energy is delivered. A narrow pipe at high pressure and a wide pipe at low pressure can carry the same flow, which is exactly why the same power can be delivered at different voltages.
That last point has real consequences. Transmitting power at high voltage means lower current for the same power, and because heating losses in a conductor go with the square of current, high-voltage transmission wastes far less energy. It is why the grid runs at hundreds of thousands of volts and your appliances do not.
Mains voltage is not universal
Australia, New Zealand, the United Kingdom and most of Europe use nominal 230 to 240 volt single-phase supply at 50 hertz. North America uses 120 volts at 60 hertz for general outlets, with 240 volts available for large appliances.
This changes every practical number. The same 2,400 watt kettle draws about 10 amps at 240 volts and about 20 amps at 120, which is why North American kitchen circuits are sized differently and why an appliance moved between regions without a suitable supply either underperforms or fails.
Any calculator, table or rule of thumb for electrical work is therefore region-specific. Applying a North American circuit table to an Australian installation produces answers that are not merely imprecise but wrong.
Load calculations and why they are not arithmetic alone
Adding up the wattage of everything on a circuit and dividing by the voltage gives the current, which is where a load calculation starts. It is not where it ends.
Real design accounts for diversity, not everything runs at once, and for inrush current, since motors and compressors briefly draw several times their running current at startup. It accounts for voltage drop over the cable run, which on a long run can require a larger conductor than the current alone suggests. It accounts for how the cable is installed, because cables bunched together or buried in insulation cannot shed heat and must be derated.
Then the protective device has to be selected so that it trips before the cable overheats but not during normal operation, and earthing and fault-loop impedance have to be verified so a fault actually trips the protection quickly.
Where the licensing line sits
In Australia, fixed electrical wiring work is licensed. Installing, altering or repairing wiring, switchboards, socket outlets or hardwired appliances is work for a licensed electrician, and the installation must comply with AS/NZS 3000, the wiring rules.
This is not bureaucratic. The failure modes are electrocution and house fires, both of which occur after the mistake rather than during it, and neither of which is visible on inspection by an unqualified person. Insurance is also routinely voided by unlicensed electrical work, which turns a small saving into an uninsured total loss.
What you can legitimately do yourself is replace a plug top on a flexible cord in some jurisdictions, change a lamp, and use plug-in devices. Everything beyond that is for a licensed electrician who issues a certificate of compliance.
Using the numbers for the things they are good for
Ohm's law calculations are genuinely useful for understanding what a device will draw, sizing a battery bank or solar array, checking whether a circuit is plausibly overloaded before calling an electrician, and working out running costs.
Running cost is the most practical everyday use. Power in watts, divided by a thousand, multiplied by hours of use, gives kilowatt-hours; multiplied by your tariff, gives dollars. A 2,000 watt heater run six hours a day at 30 cents per kilowatt-hour costs $3.60 a day. The kind of number that changes behaviour once it is visible.
For solar and battery sizing the same arithmetic tells you how long a load can run from a given storage capacity, though real systems lose ten to twenty per cent to inverter and charging inefficiency, and batteries should not routinely be discharged fully.
Questions people ask
- What is Ohm's law in simple terms?
- Voltage equals current times resistance, written V = I × R. Increase the voltage across a fixed resistance and more current flows; increase the resistance at a fixed voltage and less current flows. Combined with power equals voltage times current, it covers most practical direct-current calculations.
- How many amps does an appliance draw?
- Divide its power rating in watts by the supply voltage. A 2,400 watt kettle draws about 10 amps on a 240 volt supply and about 20 amps on 120 volts. Motors draw substantially more than this briefly at startup, which is why circuits serving them are sized above the running current.
- Can I do my own electrical work in Australia?
- No, for fixed wiring. Installing, altering or repairing wiring, switchboards, outlets or hardwired appliances requires a licensed electrician and compliance with AS/NZS 3000. Unlicensed work is illegal, commonly voids home insurance, and its failure modes, electrocution and fire, appear after the fact rather than during installation.
- Why does the grid use such high voltage?
- Because losses in a conductor rise with the square of current. Delivering the same power at a higher voltage means proportionally less current, so far less energy is wasted as heat over long distances. Transformers then step the voltage down near the point of use.
- How do I work out what an appliance costs to run?
- Take the power in watts, divide by 1,000 to get kilowatts, multiply by hours of use to get kilowatt-hours, then multiply by your tariff. A 2,000 watt heater for six hours at 30 cents per kilowatt-hour is $3.60 a day. Use the rate from a recent bill rather than a headline figure, and remember many plans have separate peak and off-peak rates.
Sources
- Energy Safe Victoria: electrical safety and licensing. Australian guidance on licensed electrical work