A few simple steps for a brief, quantitative assessment of how well a solar array performs, incorporating:
1) The physics and mathematics of PV output--an abstracted PV output model (Sandia National Lab.) using actual measured solar irradiance and cell(panel) temperature estimated from measured temperature of the bottom side the panel); and 2) Low cost tools for making measurements on the system.
Bag of tools: clock-wise Compass with scales, HodgePodge from
school supplies & Auto-Dash-top add-on sensor, RadioShack Digital
Compass, Fluke Current Probe, Angle Finder, Electronic Range Sensor,
VOM with AC/DC Current Probe, GPS reciever IR Temperature Sensor,
DayStar Pyranometer.
Physical inspection:
- Dust, leaves, or bird dropping on PV panels.
- Shadows from trees or adjacent building.
- Loose or corroded wiring connections.
- Defects, damage, or even wiring errors on panel(s).
DC parameters of PV Panel/Array (for example)
All data-sheets contain these parameters.
Maximum power output: 210 Watts (your number)
Max. power voltage: 40.0 Volts
Max. power current: 5.25 Amps (your number)
Open Circuit Voltage: 47.3 Volts
Short Circuit Current: 5.65 Amps
AC parameters of PV system output --Typical specification an Inverter
Max Power Point(MPP) voltage range 150 - 400 V
Max. input voltage (at 1000 W/M2;-10C) 500 V
PV array output Wp: 1800-2700
Nominal output 1800W (your system number)
Max. power output: 2000
Max. efficiency: 94.3% (Your number)
Theoretical Available Output - based on current condition and set of common spec sheet parameters Mathematical Model for Actual Performance: (very abbreviated version of the Sandia Model)
Power output = S*Cos(Φ)*No-of-panels*Watts*(1-KT)*(over-all system efficiency)
Here S is the measured solar irridiance in Watts/meter squared. Incident Angle Φ is the current angular quantity the Sun ray makes relative to the vertical (normal) line from the PV pane1 plane. A rough estimate can be obtained with tools described below. For the serious minded, the National Oceanic & Atmospheric Administration provides a calculator for any location. K is the de-rating temperature coefficient, approximately 0.005. T is measured cell temperature increment over 25 degree Celcius. Over-all System-efficiency is nominally 80%Actual Output: as reported by the inverter. or can be measured with Digital Volt Meter (DVM). optionally a non-intrusive clamp-on current measurement device like the Fluke i410 Current Clamp, may be used to measure the DC current on the bundle. or individual DC wire between the array and the inverter.
Some tools for measuring a live system (Most cost a few dollars to less than $150)
Compass. Protractor. Bubble Level --are used to determine the attitude of the solar panels, and indirectly the incident angle of the solar ray to the PV panel.
Digital solar meter by DayStar. Inc. Digital Volt/Amp meter, with AC/DC current clamp, infra-red temperature sensor/probe--the Fluke 80PK-IR. or similar, may he used to determine the cell temperature by aiming it on the top or bottom surface of the PV panel. A few degrees of differential maybe added to the reading when applying it in the formula. Path Finder. GPS receiver(map servers). rubber gloves. etc. (Lethal voltage is present, DO NOT attempt if not familiar with equipment and its proper use!!)
What accounts for the difference? A 20% to 40% degradation in performance is typical. Contact your solar contractor if you observe a consistent and large degradation in performance.