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Solar panels work wherever there is sunshine, most of us know that, but what determines how much electricity they produce and how efficiently do they work in winter? Does the temperature determine a solar panel’s efficiency or the amount of direct sunlight? Let’s dive in.
As a general rule solar panels do work in the Winter but not with the same efficiency. In the continental USA an array of solar panels facing due South generates approximately 53% less electricity in Winter than in Summer.
This value is derived from actual irradiation data based on 4 States in the geographic extremes on the U.S. Probably one of the most surprising things is that the difference isn’t more, which hints that there is more to consider than just temperature or sunshine.
Seasonal solar panel performance
While direct sunshine has a strong impact on output power, length of daylight hours, depth of cloud cover and precipitation affect solar panel efficiency. Solar databases use historical data for finding irradiation levels in various geographic locations and this is used for solar panel system design and sizing.
- Irradiation – how much of the sun’s power in W/m2 is falling onto the earth. Basically, it is a measure of the sun’s strength in any location.
- Insolation – how much irradiance over time, normally in the solar industry kWh/m2, sometimes known as a ‘sun-hour’.
One of the most important values for any solar system is how much energy it will produce over time and not instantaneously. Such values are affected by irradiance, atmospheric conditions, cloud cover, humidity, rain, snow and temperature.
Solar panel output winter vs summer
The chart below displays 4 U.S. cities from all 4 points of the compass and the amount of energy they receive between Winter and Summer.
Difference - what % is winter energy
Winter is 72% less
Winter is 40% less
Winter is 56% less
Winter is 45% less
Paradoxically, solar panels are more efficient in the cold. It isn’t the sun’s heat that generates electricity, but energy conversion inside the silicon crystal structure of the solar cells. In fact, high temperature is the enemy of photovoltaic action.
Solar panel efficiency vs temperature
In general the efficiency of a solar cell goes down as the temperature goes up – it will lose 0.5% of it’s output power for every degree above it’s rated temperature of 25 degrees C.
Solar panels are commonly rates using the STC standard, which states the power output at an irradiance of 1000W/m2 at 25 degrees.
In actual fact, in most seasons solar panel surfaces get much hotter than this, 65 degrees not being uncommon in summertime. For a 100 watt panel, the output would look like this:
- 100 watt – ((65-25) x 0.5%) = 100 – 20% = 80 watts
For the average American domestic solar installation of 5kW that amounts to 1kW of power, so not negligible. Strangely enough, a system could easily produce more power on a bright sunny winter’s day in North Dakota than on a hot summer’s day in Texas. It all depends on temperature, irradiance and local conditions.
Manufacturers test their solar panels down to -5 degrees Centigrade so there is little danger of hitting any lower efficiency limit. Peak or optimum output falls off after the temperature passes 25 degrees C, so anything lower than that is good. Not only does snow keep solar panels cool, but it reflects more light so more energy is produced.
Do solar panels work when covered with snow?
Solar panels don’t work when covered in snow because sunlight will be blocked from the surface of the solar crystals. However, the panels surfaces tend to absorb heat and as they are mostly mounted at an angle, snow readily slides off. This also has the effect of cleaning the surfaces, so it’s a good thing.
Do solar panels work in the dark or on overcast days?
As long as there is some sunlight coming from somewhere, solar panels do generate some electricity. It has been said that even moonlight generates a tiny fraction of electricity but I wouldn’t rely on that!
The light level is very important. On a dark and cloudy day electricity generation may be 10% of full rating, while on a bright but cloudy day, 30 to 40% may be achievable. This is due to the light diffusion, or photons bouncing around between water molecules. As light photons become more diffused they lose a little more of their energy.
Solar panel temperature coefficient
In general solar panels work better at lower temperatures. The very best conditions are cooler, with a wind for keeping panels cool and bright so there’s plenty of sunshine energy convert. It’s often forgotten that it isn’t heat that gets converted to electricity by the sun’s rays. It just so happens that bright sunshine often goes hand in hand with heat.
There is a limit to everything and even solar panels start to stop converting so well when the temperature moves very low.
Solar work very well in the colder regions of the U.S.
Statistics show that brighter, Northern states of America produce very good quantities of PV energy and are among some of the leading installers of solar panel arrays.
The difference between geographical areas is not so great as you might think – in some southern states the extra sunshine in the summer actually causes production to go down due to the previously discussed temperature coefficient effect.
In many states the most productive time of year for solar panels is the spring, when there is abundant sunshine and temperatures are lower.
For example, in Glastonbury, CT the irradiance for May is 5.23 kWh/m2/day while in August in high summer it is 5 kWh/m2/day.
In recent times bad winter weather has made people seriously consider alternative power generation techniques such as solar PV. They have a lot going for them. There are no moving parts, they are robust, withstand all kinds of weather and have a long life.
Most solar panels have a 25 to 30 year warranty and will continue to produce power in reducing amounts for up to 50 years! (80% guaranteed after 25 to 30 years.)
A medium sized solar system coupled up to a battery bank can give complete peace of mind if ever there’s an outage. If the system feeds into to the utility grid then it can pay for itself over 7 or 8 years, automatically disconnecting and powering the home through batteries until the mains supply returns.
Afternote: During my research I found that many ‘official’ articles such as the one below and also commercial solar panel info sheets often state that solar panels will produce electricity with a light covering of snow, as light diffuses through it.
I have to say that out of all the accounts of winter production I read from actual users, no one reported significant production through even a light dusting of snow. However, the irradiance level wasn’t given so overall it’s difficult to quantify.
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