solar-panels-windmills

It's a recurring theme on SolarReviews that people ask questions about solar hoping for a simple answer but the correct answer is usually…….it depends.

Sadly, for those looking for a quick read the question “how much electricity does a solar panel produce” is another such question.

However, it is less complex than some other solar questions and we have some great supporting articles on SolarReviews that means we can explain it quite simply.

What factors affect the amount of electricity produced by a single solar panel?

There are several factors that affect the amount of power that will be produced by a solar panel but it is useful for analysis to group them into two categories being:

  • Characteristics of the solar panel itself; and 
  • The amount of access to solar irradiation the solar panel has where it is installed.

What features of a solar panel that affect its production of electricity?

The most important feature of a solar panel that affects the amount of power it produces is the wattage of the solar panel.

The peak wattage of a solar panel refers to the amount of DC power it produces at standard test conditions.

Standard test conditions are when 1000 Megajoules of solar radiation per square metre is falling on the panel and the cell temperature of the solar panel is 25 degrees celsius.

These exact condition don’t occur that often in the real world but they give us a way to measure and compare solar panels power output.

Solar panels are made by joining a number of solar cells together in series. Generally speaking residential solar panels available in 2018 will either fall in the range of 260-300 watts and will contain 60 solar cells joined together, or they will fall into the range of 360-400 watts and comprise 72 cells joined together.

Obviously, the higher that wattage of the solar panel the more electricity it will produce given a fixed amount of access to solar irradiation. See these related articles www.solarreviews.com/solar-panels/efficiency-of-solar-panels/ 

Measuring the annual power output from a full array of solar panels in different climatic conditions?

Here is a map showing the average daily electricity production from 1kW of peak DC solar panels installed in each state of the United States.

american states, average daily production

 

If we take California as an example the map shows that on average 1 kW of DC peak capacity of solar panels produces 4.5 kWh per day or 1642 kWh per year.

From the example of a standard residential polycrystalline solar panel we used above we know that if one 275 watt solar panel is 18.85 square feet then 1 kW of panels will require 3.63 of these panels and so will take up 68.42 square feet.

"Given 1kW of panels produces 1642 kWh per year in CA and 1kW of panels takes up 68.42 square feet solar panels installed in California on average produce 23.99 kWh (kilowatt hours) per square foot per year."

You can use the table of solar power production per kW for each state above to do the same math for your state.

solar efficiency 1953 to 2015

Source: Solar Power Rocks

How big are the most commonly used residential solar panels?

Most solar panels used in home solar systems in 2018 are either made up of monocrystalline or polycrystalline solar cells. Whilst monocrystalline solar panels are slightly more efficient than polycrystalline panels the difference in size is usually not more than 5%.

The standard residential solar panel is about 39 inches wide and about 65 inches high.

Comparing Canadian Solar and Sunpower panels

 Solar panel #1Solar panel #2
Brand Canadian Solar Sunpower
Model CS6K-275P SPR-X21-345
Rating 4.82 based on 104 reviews 4.80 based on 396 reviews
Wattage (W) 275 345
Type Poly-crystalline Monocrystalline cells
Module Efficiency (%) 16.80 21.50
Product Warranty (Years) 10 25
Power Output Warranty (Years) 25 25
Power Output Warranty Type Unavailable Unavailable
Power Output Warranty Derating Unavailable Unavailable
Weight (lb) 40.10 41.00
Height (inch) 65.00 61.30
Weight (lb) 39.10 41.20

 

Man standing next to solar panel

 

You can notice on each panel the smaller squares within the larger panel. Each of these is a solar cell and they are joined in series such that together they output the electrical characteristics of the panel.

How much does the efficiency of the solar cells inside the panel affect the power production of the panel?

The meaning and importance of solar panel efficiency is often misunderstood by consumers evaluating solar and it is a misunderstanding that is sometimes exploited by makers of premium panels.

The take home point to remember is that solar panel efficiency is taken into account when rating the wattage and output of a panel and so theoretically, if all other solar panel characteristics are the same, a 280 watt panel with a less efficient cell will produce the same amount of power in the same conditions as another 280 watt panel with more efficient panels. Some manufacturers of high efficiency panels make outlandish claims about their panels producing far more power than standard panels of the same wattage but such claims are usually exaggerated in an attempt to justify very high module prices. Some manufacturers that claim very high efficiencies have clauses in their warranties that say that their panels can be 3% less efficient than what they claim to be before a warrantable event occurs.

How much energy can solar panels produce per square foot?

The reverse of this question is obviously how many square feet of roof space do I need to install enough solar panels to produce enough energy to zero out my electricity bill.

This is really the question where solar panel efficiency becomes most relevant. As I have already said above efficiency is already considered before a solar panel is rated at a specific number of watts so if you buy (or are faced with the choice of buying) two different solar systems each of 6kW then if all other things are equal they will produce the same amount of power each year. However, the more efficient solar panels could be smaller and produce more electricity per square foot.

There are two ways to work out solar power production per square foot. The first is to look at the peak possible instantaneous solar power production per square foot for a particular solar panel. Make this a snippet “So for a 275 watt solar panel with dimensions 65 inches x 39 inches (1.752 metres squared or 18.85 square feet) then the peak instantaneous electricity production per square foot at STC is 14.58 watts per square foot. This is typical for residential solar panels being sold in the US in 2018.

What is the annual energy produced by a solar panel per square foot?

When working out the annual electricity produced by solar panels per foot we go away from looking at just the characteristics of the panel and need to look at the climatic conditions in which the panel is being installed. For now we will assume the installations are on a perfect south facing roof with an optimal tilt angle given the latitude at which the panel is being installed. However, if you want to look at the effect on power production of installing solar panels at a sub-optimal azimuth or tilt angle then this article will help you. However, even eliminating the variance associated with different roofs to answer this question we still need to consider the climatic conditions of the location in which the solar panel is to be installed because this will affect how much sun the panel gets.

What’s the power rating for an average solar panel?

The reason that residential solar panels are generally sold in the range of 260-300 watts is because of the simply practicality that they need to be lifted onto roofs and handled by installers and this is the largest practical size in which this handling can be done safely. These are panels containing 60 cells. There are also less commonly used solar panels that contain 72 cells and are sold in the wattage range of 340-400 watts but these panels become large and difficult for installers to handle.

How to calculate solar panel Solar panel output for your home (kWh)

There are three ways to calculate the electricity production you are likely to get at your home from solar panels, how many you need and how many square feet of roof space they will take up. There is the lazy way, the very lazy way and the science nerd way.

The science nerd way is to look up the meteorological data for your location, measure the direction and tilt of your roof and examine your power usage patterns and net metering scheme available from your utility to work out how much solar you need.

The easy way to to use one of the free online solar panel calculators that already have all of this information programmed into them so that when you enter your zip code, power bill and utility provider they automatically work out all these things for you as well as estimate the cost of solar based on solar company pricing in your area. The downside of this is that you have to enter your details and they will try and offer you quotes for solar but let’s face it, with net metering and a 30% solar tax credit now available it is probably a good thing to get proper quotes for solar anyway and online estimates are not as accurate as a quote where a solar company has viewed your roof online because the direction, tilt of your roof and any shading affecting your roof will affect the energy production you will get from solar panels and this is the only real way to know you have accurate information on which to make a decision about solar.

How many solar panels are needed to power an average house?

Here is a link to an article that looks at the average number of solar panels required to power the average home in each state. The answer to this question is more complex than you might first think because not only do solar panels produce different amount of energy in each climatic location but the use of energy by the average house if different in each location.

How much power do my specific devices use on average per year?

 

How much energy is used chart

*2009 is the most recent year for which data are available.

Source: US Energy Information Administration, Residential Energy Consumption Survey (RECS) 2009.

 

The above pie chart from the Energy Information Administration shows the basic breakdown of electrical usage by US homes.

Since the time this chart was compiled the percentage spent on lighting has fallen because we now have most new lighting being either CFL or LED lights.

This has taken the usage from the average light from around 60 watts to around 11-15 watts.