Should You Shovel the Snow Off of Your Rooftop Solar Panels?

When a solar installer designs a photovoltaic installation, they take into account a number of factors to ensure an accurate prediction of the system's energy production. Weather, of course, is one of the main influences on power output of a solar installation. A cloudy day lowers the output of solar panels considerably. An even more drastic effect can be seen if the panels become buried in snow.

But exactly how much of the potential solar energy is lost due to snowfall? To find out, a range of factors can be considered. The configuration of the solar array, including tilt angle and distance from surrounding building geometry is important and can affect melting and snow shedding. Weather factors include the quantity of snow as well as its moisture content; the recurrence pattern of storms; the amount of solar irradiation, the speed and direction of wind; the relative humidity; and especially important: the temperature pattern after a snowfall. If the temperature persistently lingers below freezing, the panels are likely to remain covered.

Tim Townsend and Loren Powers of DNV KEMA Energy & Sustainability formulated a generalized monthly snow loss model based on some of the factors above as well as experimental data to estimate the loss of power generation due to snow-covered panels. The formula is as follows:

Snow loss, % = C₁*Se'*cos2(T)*GIT*RH/T_AIR2/POA^0.67

Where:

C₁ is a fitted coefficient, 5.7x10⁴
Se' is the 6-week rolling average effective snowfall in inches, with
Se = S (monthly snow, inches)*0.5*[1+1/N], where N is the number of snow events per month
GIT = ground interference term, defined in detail below
RH = average monthly relative humidity, %
T_AIR = average monthly air temperature, C
POA = monthly plane of array insolation, kWh/m2

GIT = 1-C₂/exp(ϒ); C₂ is fitted from data as 0.5; ϒ is the dimensionless ratio of snow received divided by snow dissipated, such that whenever the amount of snow received exceeds the ability of the array geometry to deposit it on the ground, shadow-like interference will quickly reduce array output by a factor of 2 to 1. BEW defines ϒ as:

R*cos(T)*Se'*2*tan(P)/(H²-Se'²)

Where:

R is the row plane of array dimension, inches
T is the tilt angle, degrees
P is the stabilized snow pile angle, nominally assumed to be 40 degrees
H is the drop height, inches

In coming up with their model, Townsend and Powers used actual snow loss figures for a solar installation site in Truckee, California near Lake Tahoe. The site is one of the U.S.'s snowiest urban areas, receiving an annual average snowfall of 200 inches (5 meters). For panel tilt angles ranging from 39° to 0° (flat), energy losses during winter were 40% - 60%, with annual losses of 12% - 18%.

The highest solar irradiance occurs during the summer, which helps alleviate some of the losses incurred by snow in the winter. So even if the panels are missing more than half of the potential solar irradiance during winter, annual losses are limited to less than a 20% shortfall after taking into account the summer bounty.

So should you attempt to shovel the snow off of your rooftop solar panels after every bout of winter weather?

Safety experts (and perhaps common sense) would recommend not clambering up on your roof to shovel snow off solar panels. The obvious argument is that the slippery winter conditions could easily lead to injury or worse, not to mention potential damage to the panels. And losses in production due to weather, including snowfall, should have been accounted for at the design stage of the solar installation, so you need not worry that the system's output will fall short of expectations. Of course, extremes are possible and you could have a particularly snowy winter and cloudy summer. If you are truly fanatical about eking out every last bit of efficiency from your solar installation, removing snow from the panels is the only way to get the energy flowing.