W, kW, Wh, kWh and Ah: solar energy units explained

September 29, 2026 · 5 min read · Danilo Nascimento

Tell power from energy, understand each unit and learn the calculations that link panels, inverter and battery, with everyday examples.

Power and energy are not the same thing

The biggest source of confusion in solar energy is mixing up power and energy. An analogy helps: power is the flow of the tap, how much comes out right now; energy is the volume of water in the bucket, how much has come out over time.

Power

How much, at this instant

Units: W and kW
A 10 W lamp, a 5 kW inverter
Like the flow of the tap

Energy

How much, over time

Units: Wh and kWh
A 10 W lamp on for 5 h = 50 Wh
It is what the electricity bill charges for
Like the volume in the bucket

The units, one by one

W and kW

power

The watt measures an appliance’s power. 1 kW = 1,000 W. An LED lamp uses about 10 W; an electric oven, 2,000 W (2 kW).

Wp and kWp

peak power

The “p” stands for peak: the panel’s power under test conditions. One 550 Wp panel; ten panels add up to 5.5 kWp.

Wh and kWh

energy

The watt-hour measures energy: the appliance’s power times the time it stays on. 1 kWh = 1,000 Wh. It is the unit on the electricity bill.

Ah

battery capacity

The ampere-hour measures the electric charge a battery holds. On its own it does not give the energy: you have to multiply by the voltage.

V and A

voltage and current

The volt is the electrical “pressure” and the ampere is the current. Multiplied, they give the DC power: W = V × A (in AC, the power factor also enters the calculation).

The calculations that tie it all together

Three calculations settle almost every doubt. The first links voltage, current and power:

48 Vvoltage×10 Acurrent=480 Wpower

The second turns power into energy by multiplying by time:

1,500 Wpower×3 htime on=4.5 kWhenergy used

The third gives the energy stored in a battery (divide by 1,000 to go from Wh to kWh):

48 Vvoltage×100 Ahcapacity=4.8 kWhenergy stored

Convert and compare

Choose the power and time of a load and the system’s battery. The calculator shows the energy used, the energy stored and how long the battery would power the load, without losses.

Everyday examples

Energy of a few uses (in kWh)

10 W LED lamp for 5 h0.05 kWh

Refrigerator, in one day≈ 1.2 kWh

550 Wp panel, in 5 peak sun hours (80%)≈ 2.2 kWh

48 V, 100 Ah battery bank4.8 kWh

Note that a 550 Wp panel does not “generate 550 W all day”: it delivers close to 550 W only in its best conditions (the nameplate value is a reference, not a ceiling) and, on a day with 5 peak sun hours and 80% efficiency, about 2.2 kWh.

Common mix-ups

Do

Write kW for power and kWh for energy, always
Multiply Ah by the voltage to get the battery’s energy
Remember that a system’s kWp is the sum of its panels, not what it generates
Check the unit before comparing two numbers

Avoid

Say the panel “generates 550 kWh”: 550 is power, in watts
Compare batteries by Ah alone, without looking at the voltage
Mix up an appliance’s power with the consumption on the bill
Add W and Wh in the same calculation

Draw it in Editor Solar

This is Editor Solar itself, already loaded with the diagram from this article. Move the equipment, edit labels, connect new parts and export to PNG, SVG or PDF. It is free and runs right in your browser.

A 48 V, 100 Ah bank supplies a 1,500 W load: the units show how much energy there is and how long it lasts.

Try it: Replace the load with one of your own appliances, write its power in watts and redo the two calculations: energy used per hour and the bank’s ideal runtime.

Ready-made templates: see all diagram templates

Open this diagram in the full editor ↗

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