Watts vs Volts vs Amps vs Wh Explained
Volts are a measure of electrical pressure, amps are a measure of current flow, watts are a measure of instantaneous power (volts × amps), and watt-hours (Wh) are a measure of stored or used energy (watts × hours). The key to proper battery, power station, inverter, or solar system selection is understanding the difference between volts, amps, watts, and watt-hours.
The water pipe analogy
The easiest way to understand electricity is to compare it to water flowing through a pipe:
- Volts (V) – water pressure. The higher the voltage, the "harder" the electricity is pushed through the wire, like water pressure in a garden hose.
- Amps (A) – quantity of electricity ("water") flowing through the circuit (garden hose) at any given moment.
- Watts (W) – electricity (water) multiplied by pressure (flow). Watts represent the amount of work per second – similar to water flowing from the hose – the higher the pressure and flow, the more work is performed.
- Watt-hours (Wh) – quantity of electricity ("water") collected in a circuit over time ("in the hose"). Not a rate, but a quantity. Similar to how much water was in the hose at a certain moment.
The core formulas
Three simple equations connect all four units:
Watts = Volts × Amps
Amps = Watts ÷ Volts
Watt-hours = Watts × Hours
What each unit actually measures
Volts (V) — electrical pressure
Voltage is a push force that drives electricity through the circuit. A typical AA battery has 1.5V, 12V in your Car battery, 120V in a US wall outlet, and most home power station units use 12V, 24V, or 48V battery packs to provide power.
Amps (A) — rate of current flow
Amperage is the measure of electricity flow at any given moment. A cell phone charger could use 1-2A while a hair dryer might use 10A or more on a 120V circuit.
Watts (W) — rate of power
Watts tell you what the device needs to currently run, or how much electricity it produces at this exact moment. This is usually the number written on your appliances, like a microwave being 1000W and a lightbulb being 9W. This means how many Watts it can output right now.
Watt-hours (Wh) — total energy over time
This is where many are making a mistake. Not kW is the amount of energy, which is equal to the number of watts multiplied by the number of hours: 1 kWh equals 1000 W per hour. The battery's capacity in the power station is expressed in Wh, because this value fully reflects how much energy can be stored in the power station, regardless of how quickly this energy is released.
Worked examples
Example 1 — Finding watts: A power station outputs 12V and delivers 5A to a device. Watts = 12V × 5A = 60W
Example 2 — Finding amps: A 100W solar panel is charging a 12V battery. Amps = 100W ÷ 12V = 8.3A
Example 3 — Finding watt-hours: A 50W fan runs for 6 hours. Watt-hours = 50W × 6h = 300Wh of energy used
Example 4 — Runtime from a power station: A 1000Wh power station runs a 50W fan. Runtime = 1000Wh ÷ 50W = 20 hours (before accounting for inverter losses)
Quick reference table
| Unit | Measures | Analogy | Typical use |
|---|---|---|---|
| Volt (V) | Electrical pressure | Water pressure | Battery/circuit voltage (12V, 24V, 120V) |
| Amp (A) | Rate of current flow | Water flow rate | Charging current, appliance draw |
| Watt (W) | Rate of power (right now) | Power of the water jet | Appliance rating, inverter output |
| Watt-hour (Wh) | |||
| Total energy over time | Total water collected | Battery/power station capacity |
Why this matters when buying a power station or battery
- Watts (W) indicate the power capacity of the station, meaning that if the required wattage exceeds the rated wattage of the power station, it will not be sufficient.
- Watt-hours (Wh) indicate the amount of energy the power station can provide, which means that when comparing two power stations, it is this parameter that is decisive in determining the power capacity.
- Volts (V) indicate the voltage of the photovoltaic panels and the battery, so that the panels and the battery are selected according to their rated voltage.
- Amps (A) indicate the current, so that the appropriate cable cross-section and fuse rating can be selected. The higher the amperage, the faster the battery can be charged, but a larger cross-section of the cable is required.
Frequently asked questions
What is the difference between watts and watt-hours?
Watts are the unit of measure for electricity used at a specific point in time, whereas watt-hours are the electricity used or stored over a period of time. If you have a 100W device running for two hours, it will consume 200Wh of energy.
How do I convert watts to amps?
Divide watts by volts to get amps. For example, a 1200W appliance on a 120V circuit draws 10 amps (1200 ÷ 120 = 10).
How many watt-hours are in a kWh?
One kilowatt-hour (kWh) equals 1,000 watt-hours (Wh). This is the unit used on home electricity bills.
Does higher voltage mean more power?
Not by itself. The amount of power, or watts is also dependant upon volts and amps. Volts times amps equals watts. So high voltage with low amperage can equal low voltage with high amperage.
How do I know what size power station I need?
Calculate the watt-hours of your devices (device wattage x hours of use) and get a power station with a Wh capacity larger than that number by a decent margin to account for losses.