First Point: "Full Score" in the Laboratory VS "Performance" in the Real World
You see "1 kilowatt" (1 kW) on the board label, which has a scientific name: "nominal power".
How was this number obtained? It was measured under extremely ideal laboratory conditions, officially known as "Standard Test Conditions" (STC).
Perfect light intensity: 1,000 watts of sunlight per square meter-about as strong as the sun directly overhead at noon on the equator in summer.
Exactly the right temperature: The temperature of the panel must be kept steadily at 25°C (warm to the touch).
Ideal spectrum: The spectral composition of the sunlight must also match a specific standard.
Think about it-in real life, how often do we have such a "perfect score" environment?
At sunrise, and sunset, the angle of the sun is low and the intensity is low. In cloudy, overcast or the day is hazy -- the amount of sunlight is, for the most part blocked. Even on sunny days the angle of the sun continuously changes, so, while it is likely some days the panel maybe not perpendicular, it is however very difficult to keep the panel perfectly perpendicular, even on the best of days.
This is similar to an athlete performing on an ideal windless track, doing their best performance, but then has to compete with the wind, rain, bad surface and therefore their performance changes.
As a result, the actual output power of the panel is rarely 1 kilowatt.
Second Point: Peak Sunshine Hours
This is the key concept for understanding solar power generation.
Regardless of whether conditions are good, the sun does not shine at full intensity for twenty-four hours a day, we get intermittent intensity of sunlight hours each day from sunrise to sunset.
This is why we use the concept called "Peak Sunshine Hours".
In basic terms, it converts the total solar energy received during the day into a solar energy equivalent of a number of hours of full sunlight (i.e., 1000 watts/m² as per STC).
For example, on a given day at a given location, sunlight energy is equivalent to 4 hours of full sun. That is 4 peak sunshine hours.
So, the theoretical power generation for your 1 kW-labeled panel on that day is:
1 kW × 4 hours = 4 kilowatt-hours (4 kWh)
This does not mean the panel only operates for 4 hours, it means the variation in available sunlight is equivalent to a full powered 4 hour operation.
Third Point: System Losses Exist
The 4 kWh we calculated above is theoretical. By the time it reaches your home meter, the electricity has passed through a few more "checkpoints," each one involving some loss:
Line Loss: The electricity has moved from panel, through wire to inverter, and finally to the meter. Because wires have resistance they will lose some energy to heat. The longer, thinner, and lower quality the wire, the more loss there will be.
Inverter Loss: The panel produces DC electricity, but our homes use AC. This conversion is done by an inverter. Even the best inverters aren't 100% efficient-most are around 97%-98%, meaning 2%-3% of the energy is lost in conversion.
Temperature Losses: The panels will get hot under direct sunlight. Panel temperatures will reach 60 - 70degrees Celsius under sunlight, at high temperatures; the efficiency of the semiconductor materials that comprise the panels decreases with increased temperature-output is lower with increased temperature-this is a fundamental law of physics that cannot be disputed-therefore, the panel's rated output (NOMINAL POWER) should be considered only at an ambient temperature of 25 degrees Celsius. On very hot days (e.g., hot summer days) outputs of panels may be as low as 80 -90 percent of their rated output.
Dust and Shadowing: The output of a complete string of solar panels and/or inverter can be significantly impacted by dust and shadow covering objects like roofs, trees, and other structures. To ensure maximum output, solar panels must be cleaned and maintained on a regular basis.
Other Minor Losses: These include poor connector contacts, inverter standby power consumption, etc.
The combined factors will decrease the amount of actual power produced by between 5% and 15% or more. As a result, the 4.0 kWh that you theoretically produce may only be reflected on your home meter as 3.6–3.8 kWh, depending on the design and installation of the system. If a system is not designed well or installed improperly, greater losses than listed above will occur!
Solar panels can lose efficiency when exposed to extremely warm temperatures, but typically, the additional exposure to radiant energy, or sunlight during the summer months, mitigates much of this loss associated with heat. Therefore, even though colder temperatures in the fall/winter periods generally do produce better efficiencies from solar panels, they produce much less energy due to lower daily solar radiation levels (especially, due to shorter days and lower sun angles).
