What is the solar panel output? Volts, Amperes, Watts and KW-Hour made easy

The solar electric panel converts Sun radiation to electrical power. The solar panel output is electrical power. The energy input to the solar electric panel is sun light radiation. Sunlight is available only during day time, reaching it peak value at noon time.

Shading and partial shading, clouds, dust accumulation over the panels, non-uniformity of the panels in the array can reduce the actual output. My own system built from the highest quality components, that was calculated to deliver more than 10 kW at peak hours, in actuality delivers a bit more than 8 kW on a bright day.

Electrical Terms related to Solar Panel Output

How to read the spec of the solar electric panel?

I copied the spec of a commercially available solar photovoltaic panel that will serve me as an example on how to read solar panel electrical specs

Maximum Power: 130 Watts; this is the power in Watts the panel will provide under ”standard sun conditions” into ”the best load conditions”; the underlined terms are explained in the next paragraph

Tolerance: +10%/-5%

Maximum Power Voltage: 17.6 volts; The voltage when ”the best load conditions” is connected to the panel output terminals

Maximum Power Current: 7.39 amps; The current delivered by the panel to ”the best load”

Open Circuit Voltage: 21.9 Voc; The voltage that appears between the two panel output terminal (the plus terminal and the minus terminal) when no load is connected (electrical engineers call it Open Circuit, because no current flows unless the circuit is closed)

Short-circuit Current: 8.02 amps; The current that will be measured if the plus terminal is shorted (connected by a wire) to the minus terminal

Explanation to the underlined terms

”Standard sun conditions”: Standard Test Condition STC is under irradiation (solar radiation density) of 1000Watt/square meter (93 Watts/square feet) at ambient temperature of 25 degrees C (77 degrees Fahrenheit). For all practical purposes, it enables you to compare one product to another.

”The best load conditions”: The power delivered to the load is the multiplication of the Voltage on the load multiplied by the current the load draws. The two extremes are: Open Circuit (maximum voltage, zero current, zero power delivered) and Short Circuit (maximum current, zero voltage, zero power delivered to the load). Once the load (e.g. a resistor, DC to AC inverter, a deep cycle rechargeable battery, a DC motor) is connected, it takes its (bigger than zero) current and the voltage drops to less than the Open Circuit Voltage. One particular load (for measurement purposes the test laboratory uses a variable resistor) the panel deliver the “Maximum Power Voltage” and the “Maximum Power Current” resulting the Best Load Conditions Delivered Power (“Maximum Power”). Trying to increase the current above the “Maximum Power Current” will result in lowering the voltage in a way that the delivered power (voltage multiplied by current) will be less than the “Maximum Power”. Trying to reduce the current bellow the “Maximum Power Current” will result in increasing the voltage in a way that the delivered power (voltage multiplied by current) will be less than the “Maximum Power”. Bottom line – the “Maximum Power” is the highest power the panel can deliver

A note about MPPT DC to AC solar inverter

An MPPT DC to AC solar inverter is a smart solar inverter that is designed to maximize the performance of the residential solar power system. In real life, many panels (a typical 3 kW solar residential system will have 24 panels each rated to 130 Watt) are served by one solar inverter. Because the panels involved are not identical (there is a spread in the work lines of the panels) the inverter find an average working point, that point is a compromise.

You might get less than the Expected Solar Panel Output

The following factors might reduce the Maximum Power (and hence the solar panel output):

Tolerance: +10%/-5%; this reflect the variation you might find over a high volume production quantities between individual panels. You might end up with a panel delivering 5% less or if you are lucky with up to 10% more. For many panels working together, the MPPT algorithm might cause the output to be governed by the weaker panels.

Ambient temperature will reduce the power by about 0.5% per degree C for crystalline silicon cells and 0.25% per degree C for amorphous silicon cells.

Aging will reduce the power production capability The typical warranty given by panel manufacturers is for a period of 25 - 30 years, wherein the output shall not fall below 85% of the rated capacity (this equates to roughly 0.5% drop per year)

The optimum installation is when the panel is orthogonal to the sun direction. For residential application the panel is installed on the south side of the roof, therefore a slight degradation from the optimum solar panel output can be expected.

Power (Watts) versus Energy (KW-Hour)

Watts is a measure of power. Another measure of power in other units is the Horse Power (HP). Horse Power is equivalent to 745 watts and to 33,000 ft-lbs per minute. Anyhow, power is energy divided by time. Electrical Energy is measured in KW-Hour; the energy (produced or consumed) delivered by a 1 KW (1000 Watts) power source during 1 hour is 1 KW-Hour. The utility charges its customer based on the consumption as measured in KW-Hour. The panel in the example, if operated under standard conditions 24 hours would deliver 130 Watts multiplied by 24 hours or 3.12 KW-Hours. However, there are only 6 hours of useful day light during 24 hours and taking into account the non optimal installation, it is agreed that only 20% of the 3.12 KW-Hour (0.624 KW-Hour) is produced in 24 hours. During a year it amounts to 0.624 multiplied by 365 days = 228 KW-Hour

Installation can affect the solar panel output

Shading and Partial Shading

Trees and buildings that cast shades on the solar panels, has an adverse effect on the solar panels output because they block the sun radiation impingement on the panels. Partial shading on some of the panels might cause an adverse effect on the MPPT inverter and cause the algorithm "to get confused". Bottom line: avoid shading on the solar panels.

Temperature effects

Most of the impinged radiation is converted to heat, only a minor part (10% to 20%) is converted to electrical power. The generated heat heats up the panels and cause a reduction in the solar electric generated power. Allow air to flow around the panels, do not pack them close to each other.

Efficiency

The efficiency is the proportion between the produced electrical power to the irradiated power. The panel in our example is 1.42 m times 0.65 m = 0.92 square meter. Under standard conditions this oanel gets 0.92 multiplied by 1000 Watts/square meter = 920 Watts of sun radiation. The bottom line efficiency is therefore 130/920=0.141=14.1%.

$ per Watt

What really counts when you buy a panel is how much you pay per one Watt production capability. (The price of the panel divided by the solar panel output production capability in Watts) Present time cost for cheap solar panels range between 1$/Watt for to 3$/watt. It depends on the manufacturer and the quantity of panels you buy