A kilowatt (kW) measures power: how fast electricity is being used or produced at a given moment. A kilowatt-hour (kWh) measures energy: the total amount used or produced over a period of time. Think of a tap and a bucket. The kilowatt figure is how far the tap is open; the kilowatt-hours are how much water ends up in the bucket. Electricity bills charge mostly for kilowatt-hours, while solar systems, inverters and battery power ratings are described in kilowatts. Mixing the two up is the most common reason spec sheets and quotes seem confusing.

Power and energy: the core relationship

The link between the two units is time. Energy equals power multiplied by the number of hours it runs, and the same formula can be rearranged to answer different questions:

• kWh = kW x hours. A 2 kW heater running for 3 hours uses 6 kWh.
• Hours = kWh / kW. A 6 kWh store of energy can supply a 2 kW load for about 3 hours.
• kW = kWh / hours. Using 12 kWh evenly across 24 hours works out to an average draw of 0.5 kW.

One kilowatt is 1,000 watts, so smaller devices are usually labelled in watts (W) and their energy use in watt-hours (Wh). Larger quantities step up the same way: 1,000 kWh is one megawatt-hour (MWh), the unit used in wholesale electricity markets and for renewable energy certificates.

Everyday examples

The table below uses typical appliance ratings. Real products vary, so check the rating label on your own equipment.

The kettle is a useful lesson. It has one of the highest power ratings in the house, yet uses little energy because it runs for minutes. The pool pump draws less than half the power but, running for hours, uses more than thirty times the energy. High power matters when sizing cables, inverters and batteries; high energy use is what drives the bill.

Reading an electricity bill

A typical residential bill in NSW mixes several units, and each is charged differently:

• Usage charges (c/kWh): the energy imported from the grid, sometimes split into peak, shoulder and off-peak periods.
• Supply charge (c/day): a fixed daily cost for being connected, regardless of how much energy is used.
• Feed-in credit (c/kWh): the energy a solar system exported, credited at a rate that is now commonly a few cents per kWh.
• Demand charge (c/kW or $/kW): on some tariffs, a charge based on the highest average power draw over a short window, usually half an hour. This one is measured in kW, not kWh.

Most bills also show average daily use in kWh, which makes seasons easy to compare. A household averaging 16 kWh a day in autumn and 24 kWh a day in winter has a heating load worth investigating. On time-based pricing, when those kilowatt-hours are used matters as much as how many, as explained in understanding time-of-use tariffs.

Solar systems: rated in kW, judged in kWh

A "6.6 kW solar system" describes the combined nameplate power of the panels under standard laboratory test conditions. It does not mean the system produces 6.6 kWh every hour. Output rises from zero at dawn to a midday peak and falls again, and the peak itself is usually below nameplate because of heat, dust, wiring losses and the angle of the sun.

What matters for savings is the energy total. As a typical range, a 6.6 kW system in Sydney produces roughly 24-27 kWh a day averaged across the year, more in summer and less in winter. A quote may also show two different kW figures: the panel array (DC) and the inverter (AC). A common pairing is 6.6 kW of panels on a 5 kW inverter, because panels rarely deliver their full rating at the same moment.

Monitoring apps usually show both units side by side: a live figure in kW for what the system is producing right now, and a running daily total in kWh.

Battery specifications: two numbers, two questions

Battery spec sheets are where the kW and kWh distinction matters most, because each figure answers a different question:

A 10 kWh battery with a 5 kW continuous rating could, in theory, supply 5 kW for two hours or 1 kW for ten hours. It could not run an oven, kettle and air-conditioner drawing 8 kW together, however full it is. In practice, the inverter connected to the battery often sets the real power limit, and conversion losses mean slightly less energy reaches the appliances than the capacity figure suggests. How battery storage technology works explains those losses in more detail.

When comparing batteries, compare usable capacity with usable capacity. The federal home battery discount is also calculated per kWh of usable capacity, so it is the number that matters most on a quote.

Quick conversions worth remembering

1. 1 kW = 1,000 W, and 1 kWh = 1,000 Wh.
2. 1 MWh = 1,000 kWh.
3. Running cost = kW x hours x price per kWh. At an illustrative 35 c/kWh, a 2 kW heater running for 3 hours costs about $2.10.
4. kVA (kilovolt-amps) appears on some inverters, generators and business bills. It measures "apparent power" and is close to kW for most household loads, but can be noticeably higher for motors and some commercial equipment.

The solar technology glossary covers related terms such as depth of discharge and round-trip efficiency.

Next steps

Seeing a household's real kW and kWh patterns is more useful than estimating them. Two services on the energy market are a practical start: the Electricity Bill Audit ($99 for up to four bills) and an Energy Monitoring Dashboard (from $590 per site, installed and set up). Prices are indicative and confirmed after a site assessment. To have a solar or battery system sized from your actual usage, request a free assessment from Blue Energy Solar.

Frequently asked questions

What does kWp mean on a solar quote?

kWp stands for kilowatt-peak, the rated DC output of the panels under standard test conditions: sunlight of 1,000 watts per square metre with the cells at 25°C. A 6.6 kWp array simply has 6.6 kW of panel nameplate capacity. It lets system sizes be compared on equal terms, but it is not a prediction of output, because real rooftop conditions are usually hotter and less intense than the test.

Why is an EV charger rated in kW but a car battery in kWh?

The charger's kW rating is the rate at which it can deliver energy, while the car's kWh figure is how much energy its battery holds. Dividing one by the other gives a rough charging time: adding 21 kWh through a 7 kW charger takes about three hours, plus a little extra for charging losses. The car's own onboard charger can also cap the rate below the wall charger's rating.

How do I read a watt-hour rating on a portable power station?

Divide watt-hours by 1,000 to convert to kWh, so a 2,000 Wh unit stores about 2 kWh. Then divide by the power of what you want to run. A steady 100 W load could run for roughly 17 to 18 hours once inverter losses are allowed for. Also check the continuous output rating in watts, because it limits how much can be powered at the same time.