Checklist for Choosing Solar Panels

The following are the key parameters you should evaluate while selecting a solar module:

  • Rated Power at STC (Standard Test Conditions)
  • Rated Power Tolerance (%)
  • Type of Cell
  • Module Efficiency (%)
  • Normal Operating Cell Temperature (NOCT)
  • Maximum Power Temperature Coefficient (% per °C)

Key Parameters to Consider While Selecting Solar Modules

Specification What to look for Notes Location where it is specified
Rated Power (Pmax) Should satisfy the power requirement you are looking for. This is noted on the modules, usually in Watts (W) On the module
Rated Power Tolerance A positive tolerance is good, negative tolerance is not. Look for a high positive number and a low negative number. Look for tolerance in the range +5%/-3% On the module
Type of Cell Multi Crystalline– less expensive, less output for same area
Monocrystalline – more expensive, higher output per area
Thin film – low efficiency panels, usually not preferred for rooftop solar.
Monocrystalline is more expensive than Multi Crystalline, so please consider the cost benefits before deciding. Datasheet
Module Efficiency (%) Efficiency should be as high as possible for your budget. Check for module efficiency, not just the cell efficiency. A panel whose performance is good has efficiency in the range as mentioned below:

Monocrystalline panel efficiency range: 18-22%

Polycrystalline panel efficiency range: 15-18%

On the module
NOCT As low as possible. NOCT should ideally be less than 50°C The best module operated at a NOCT of 33°C, the worst at 58°C and the typical module at 48°C respectively Datasheet
Maximum Power Temperature Coefficient As close to zero as possible. Look for a value between -0.5%/°C and zero. Note that this is usually given as a negative %, eg., -0.465%/°C Datasheet

Details of Parameters
Rated Power at STC (Standard Test Conditions)

It is the power output of a panel when it is receiving 1000 watts per square meter of solar irradiance at 25°C.This value will be equal to the “advertised” power of the panel. In other words, a 180 watt solar panel will have its rated power at STC as 180 watts.Higher the module wattage, higher the electricity generated by it. So, depending on the number of units of electricity (N) you need, choose your wattage (W) accordingly. A rough thumb rule to estimate the number of units of electricity per day generated is: N = W*4. That is, if your module has a rated power of 300 Wp, it can approximately generate 300*4 or 1200 Wh (1.2 kWh) per day. Note however that “4” is just an indicative multiplier; it could vary from 3 to 5, depending on the amount of sunlight in your region.

Rated Power Tolerance (%)

It is the range within which a panel will over perform or underperform. In other words, if a panel rated 180 watts has a rated power tolerance of +/-5%, the power obtained would be 189 watts to 171 watts in ideal conditions.For maximum power output, panels with a small negative number or only a positive number must be selected.

Type of Cell
There are two types of solar cell
Thin film solar cells – made from amorphous silicon, Cadmium Telluride or Cadmium Indium Gallium Selenide (CIGS)
Crystalline solar cells – these are usually made from silicon. Crystalline silicon cells can either be monocrystalline cells or multi crystalline cells. Monocrystalline cells have higher efficiency than multi crystalline cells but are also more expensive.

While evaluating cells, use this simple thumb rule. Unless you have serious space constraints and would like to generate as much as possible from your rooftop or farm within a restricted area, go for multi crystalline solar cells. While their output could be about 25% lower for the same area, they cost almost 40% less than many monocrystalline cells.

Module Efficiency (%)

It is the efficiency with which the solar panel generates electricity with the sunlight it receives. If a panel generates 200 watts of electricity when hit with 1000 watts of sunlight, the module efficiency is 20%.

For two different panels with same rated power, the panel with higher module efficiency will generate more electricity for the same area.

Crystalline modules have efficiencies in the range 16%-22%. Thin film solar modules have efficiencies in the range 10%-15%. If you are space constrained, look for modules with high efficiency. However, as can be expected, high efficiency modules can cost much higher than modules with lower efficiencies.

Normal Operating Cell Temperature (NOCT)

NOCT (Nominal Operating Cell Temperature) indicates the temperature developed in each panel in high radiation fields. NOCT can give an idea of the performance of the solar panel at high temperatures, i.e., higher than 25 degrees Celsius. This represents a more real world scenario than STC. The testing conditions are

  • Irradiance of 0.8 kW/m2
  • 20°C ambient temperature
  • Average wind speed of 1 m/s

The lower the NOCT, the more efficient the panel is in high temperatures over time.
Maximum Power Temperature Coefficient (% per °C)

The solar panel temperature coefficient gives information about the variation of panel’s power with respect to temperature. Output will decrease as the temperature rises (Solar panels create electricity from the light of the sun. Heat reduces their power output).If the maximum power temperature coefficient of a panel is -0.5% it means 0.5% of output power will be lost for every degree the temperature rises above 25°C.The Maximum power temperature coefficient should be as low as possible since the output should not be affected due to temperature variations.

Other Parameters Specified for the Module
These parameters need not be evaluated as critically as key parameters but check the datasheet for the following parameters also.
Open Circuit Voltage (Voc)

The open-circuit voltage, VOC, is the maximum voltage available from a solar cell, and this occurs at zero current.Any equipment connected to the panel (meters, charge controllers, etc.) should be capable of handling the full VOC of the string of solar panels. Otherwise, equipment damage may occur when the panels are irradiated with sun rays and they’re not loaded.

Short Circuit Current (Isc)

The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited).Short-circuit current rating is useful for determining the sizing of wires, fuses and controller.

Maximum power voltage(VMP)

The voltage at which maximum power is produced by a solar panel. This value is usually around 80% of the Open Circuit voltage.

  • When different panels are wired in parallel, the VMP of the modules must be matched so that they work well together.
  • The panel output voltage under load must be as close to VMP as possible.

Maximum power current(IMP)

It is the current at which maximum power is produced by a solar panel.When different panels are wired in series, the IMP of the modules matched for optimal performance.

Series Fuse Rating

The current reading at which the fuse will open and cause the circuit to break to protect the solar panel from damaging.