Electrical Energy & Charge Calculator (kWh, J, C)

Understanding the electrical energy and charge calculator — concept and principle

Electrical energy is what you pay for on an electricity bill and what a battery stores. This calculator converts an appliance's power and running time into energy in kilowatt-hours (kWh) and megajoules (MJ), and, given the supply voltage, works out the current drawn and the total charge moved in amp-hours. Use it to estimate running costs, compare appliances or size a battery or generator.

Each hour adds another 1.5 kWh block: 3 kWh in two hours, equal to 10.8 MJ.

How it is calculated

Energy: E = P × t. In kWh: P (W) × t (h) ÷ 1000.
Energy in MJ: P × t × 3600 ÷ 10⁶, since one hour is 3600 seconds and 1 J = 1 W·s.
Current: I = P ÷ V.
Charge: Q = I × t, shown in amp-hours (A·h); 1 Ah = 3600 coulombs.

The principle

Power is the rate of energy transfer, so energy is power multiplied by time. The watt-hour is a practical unit: 1 kWh = 1000 W for 3600 s = 3.6 million joules exactly. Voltage is energy per unit charge (1 V = 1 J/C), so dividing power by voltage gives the rate at which charge flows, which is current; multiplying current by time gives charge. The same relations hold for DC circuits and for AC circuits with purely resistive loads, such as kettles and heaters, where power factor is 1.

Worked example

Inputs: P = 1500 W, t = 2 h, V = 230 V.
Energy = 1500 × 2 ÷ 1000 = 3 kWh.
In joules: 1500 × 2 × 3600 = 10,800,000 J = 10.8 MJ.
Current = 1500 ÷ 230 = 6.52173913 A.
Charge = 6.5217 × 2 = 13.04347826 Ah.

Each point is the calculator re-run with only “Power (W)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Power (W)	Energy (kWh)
375	0.75
750	1.5
1,125	2.25
1,500	3
1,875	3.75
2,250	4.5
3,000	6

How to read the result

Multiply kWh by your tariff to get cost: at a price of p per kWh, this example costs 3 × p. The current shows whether a circuit or extension lead can carry the load; compare it with the rating of the breaker, fuse or cable. Typical values: a phone charger uses tens of watts while charging and a kettle 2–3 kW.

Common mistakes

Confusing kW (a rate) with kWh (an amount).
Using an appliance's maximum rated power when it actually cycles on and off, as fridges and air conditioners do.
Applying I = P ÷ V to motors or electronics with a power factor below 1; the real current is P ÷ (V × PF).

Keep in mind: The current result assumes a power factor of 1 and constant power. Electrical installation sizing must follow local wiring regulations and be checked by a qualified electrician.

Frequently asked questions

How many joules are in 1 kWh?

Exactly 3.6 × 10⁶ J, or 3.6 MJ.

Does the Ah figure mean the same thing on AC?

On AC the current reverses, so it is the RMS current multiplied by time, not a net transfer of charge. It is meaningful mainly for DC and battery systems.

Why does the same appliance draw more current at 120 V than at 230 V?

For the same power, current is inversely proportional to voltage: 1500 W draws about 12.5 A at 120 V but 6.5 A at 230 V.

Sources: BIPM, The International System of Units (SI Brochure), 9th ed., 2019 · NIST Special Publication 811, Guide for the Use of the International System of Units

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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