## The units related to the solar panel

Just as we buy food in euros per kilogram [€/kg], solar panels are bought in euros per Watt peak [€/Wp]. Manufacturers must therefore measure the peak power of their solar panels according to an international measurement standard: the STC (standard testing conditions). These conditions are set at 1000W/m², at a temperature of 25°C, which corresponds to optimal conditions.

Watt peak [Wp]: The maximum electrical power that a solar panel can achieve, under laboratory conditions. This is the value that is presented when selling a solar system or product.

Watt [W]: The actual electrical power output from your solar panel. Indeed, between lab measurements and what you’ll get in real life, the difference can be significant.

In fact, a solar panel is sensitive to the heat and to the light intensity to which it is subjected. A solar panel with a stated peak power of 100Wp could very well provide a power of 30W or less, if even the smallest cloud wanders overhead, if the solar panel is not properly tilted, if it is very hot etc.

The power of a solar panel is not the same as the power of the sun. The actual power of your solar panel is then quite different to the created power that you paid for.

## Units related to the battery

A battery will store an electrical charge via a reversible chemical process. By injecting energy into the battery, it will recharge, more or less quickly depending on the power (energy flow) injected.

Watt [W]: Measures the electrical power flowing into or out of the battery – directly related to its charging and discharging rate.  For example, will output up to 18W when charging a smartphone.

Watt hours [Wh]:A measure of the total capacity of the battery. By multiplying a flow rate and a duration, you get a capacity. So this measurement indicates how many hours the battery is going to be able to provide a certain power output. For example, a  20 external battery has a capacity of 74Wh, so it will be able to charge a device for 4.11 hours with 18W of power, or for 7.4 hours with 10W of output power.

Milli-Ampere Hour [mAh]: Another measure of battery capacity, often used for smaller capacities such as an external battery – powerbank. It can also be converted to Wh.

## How long does it take to charge my portable solar battery?

Now that you have mastered the concepts and units, you can calculate how long it takes to charge your portable solar battery:

time needed to charge [h] = battery capacity [Wh]
solar panel power [W]

However, you will most likely have available to you the peak power measurement of the solar panel, not the measurement of the actual power it produces under the conditions to which it is exposed. This calculation will therefore give you the minimum charging time, possible only if it is exposed under optimal conditions.