Power and energy
The unit of electrical power is watts (W). The power unit watt is equal to the product of voltage (volt) and current (ampere).
Power (W) = voltage (V) x current (A) P=VxI
Kilowatt (kW) is a unit of high power, such as 1000 W = 1kW.
Electric energy is defined as the working capacity or power over a period of time.
Electric energy (Wh) = power (W) x time (h) E=P×T
A large amount of electrical energy or electrical work is expressed as kWh, which is simply the power multiplied by the time of use. The unit of solar radiation data is usually kWh/m2, which is the amount of solar energy received in 1h on the surface of 1m2 under the full sun (solar radiation 1000W).
The difference between the concepts of power and electrical energy is important because energy utilization is the
The basic conditions for the output of the energy system.
For example:
The 60W lighting lamp keeps lighting for 12h, the bulb will consume 720Wh or 0.720kWh:
60W x12h = 720Wh

Peak solar hours (PSH): The amount of daily radiation is usually called daily PSH (or full sunshine hours). The PSH of a certain day is the number of hours worked at the rated power (kW/m2), which is equivalent to the total energy of the day. The term peak sunshine hours may also be used.
Radiation: The total amount of solar radiation energy received per unit area in a given time (daily, monthly or yearly).
Sunshine: Another term for radiation. The amount of sun received on a surface over a period of time. Dayang peak hours (kWh/m2/day) is a measure of daily sunshine.
Illuminance: The solar radiation on a surface at any time, in W/m2.
For example:
If the received sunlight is 2h with an irradiance of 100W/m2, 1.5h with an irradiance of 600w/m2, and 1h with an irradiance of 20W/m2, the total amount of radiation received on this day is 3.1 PSH ：100W/m2×2h +600W/m2×1.5h + 200W/m2 ×1h = 3100W/m2/day
3100W/m2/day÷ 1000W/m2=3.1 PSH The solar peak hours are very useful in the calculation of system output. 1PSH means to irradiate 1h at 1kW/m2. Since the sun does not shine evenly throughout the day, the solar peak hours are usually less than the number of hours in a day

Direct radiation: Sun radiation transmitted directly to the surface of the earth.
Scattered radiation: Solar radiation that is first scattered and absorbed by clouds and gases in the atmosphere, and then emitted. Scattered radiation is not as strong as direct radiation.
Atmospheric quality: The distance that radiation travels through the atmosphere to a point on a surface. For a location, the air quality changes throughout the day.

Sun angle
Sun altitude angle: The angle between the sun and the horizon. The altitude angle is generally 0°-90°.
Azimuth of the sun: The angle between the position of the sun on the compass and the direction of true north. As the sun crosses the sky from east to west during a day, the azimuth of the sun also changes. Generally, the azimuth angle is clockwise from 0° (true north) to 359°.

Magnetic declination
When installing an optical system, it is important to consider the field declination. The magnetic declination is the difference between the true north direction (the direction of the north pole) and the magnetic north direction (the direction indicated by the compass). The photovoltaic system should face true north or true south, so the magnetic declination angle of the site should be considered.
In New Orleans, the magnetic angle is close to 0%, so the compass indicates true north. However, Seattle’s magnetic declination is about 17° east, so when the compass points 17° east, it actually faces true north, or the direction of true north is 17° west. The photovoltaic system installed in Seattle should face 197° east (or 163° west) so that it can face true south.