Understanding panel efficiency, getting the most for what you pay for

By Ben Delman on April 6, 2018

Not all solar panels are the same. They can vary by size, color, cost, and efficiency. Efficiency refers to how much electricity the panel creates from the sunlight it captures. The electrical capacity of solar panels is measured in watts (W). The typical solar panel is rated at 250-300 W. The higher the rating, the more electricity a panel produces.

What makes a panel more efficient?

There are several factors that make a panel more or less efficient. These include the type of material that is used in the panel as well as the quality of the panel’s construction. Monocrystalline silicon for example, is typically more efficient per square foot than polycrystalline silicon. Monocrystalline cells use a single, purer form of silicon in their cells whereas Polycrystalline cells are composed of several forms of silicon. Both types are commonly available.

External environmental factors also impact a panel’s efficiency. Heat and humidity can reduce a panel’s electric output and so can the amount of shade that falls on a panel over the course of a year. How susceptible a panel is to local conditions like shading and weather-related factors depends upon the materials used to make the panel, the inverter equipment selected, and other design considerations. Your installer will try to design for the maximum efficiency possible given the local conditions, the space available and your budget.

Why not always use the most efficient panels?

Having the most efficient panel may not be your best option. You may find it makes more sense to install more panels that are lower efficiency, if you have plenty of space for a large system. High-efficiency panels come at a higher cost and are designed for systems where space is limited. Your installer will estimate how many panels can fit on your roof or in yard given the space available and how shady it is over the course of a year to determine the ideal size of your system.

To estimate how much solar you can install on your roof (and how much electricity it will produce each year), we recommend using the PV Watts tool (see tutorial below). To get a sense of how much your solar will offset your electricity needs, divide the annual kWh production estimate from the tool by your annual kWh consumption of utility electricity (the sum of 12 monthly bills). Many utility bills will list the kWh consumption for your property over the last 12 months.

PV Watts tutorial