Do solar panels need direct sunlight to work?

Updated

Trees cast shade on rooftop solar panels
Many homeowners have roofs that are partially or fully shaded. Will solar panels still work on them?

There are many situations in which solar panels may not get direct sunlight. They may be covered by shade from surrounding buildings or trees, are turned away from the sun, or simply affected by weather conditions like clouds, rain, or snow.

What happens when solar panels - also known as photovoltaic (PV) modules - don’t receive direct sunlight? And how much importance should you place on shade and weather conditions when trying to decide if you should get solar panels?

We answer both these questions in this blog. 

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    Do solar panels need direct sunlight?

    Solar panels work best in direct sunlight, but can also work without it. 

    Solar panels produce electricity using a combination of direct and indirect sunlight as inputs. Both forms of sunlight carry photons, which is what the solar panels convert into electric current.

    If there is no direct sunlight available, solar panels will produce electricity using indirect sunlight alone. 

    There will, however, be a drop in performance in the absence of direct sunlight. That’s because solar panels need 1000 W/m2 of sunlight to reach their peak output; that much sunlight can only be achieved when there is direct sunlight shining.

    You can learn more about direct sunlight (technically, ‘direct beam solar radiation’) and indirect sunlight (or ‘diffuse solar radiation’) in this Department of Energy resource.

    See your potential solar output based on historical solar data for your location

    Do solar panels work in the shade?

    Yes, solar panels can work in the shade, but they will generate less electric current than they would under optimum conditions. 

    The exact impact of shading on your solar power system depends on these factors:

    • Duration of the shading: The longer your solar panels are under shade, the bigger the drop in electricity production. Bear in mind that sunlight and shade levels will vary day to day due to the constantly changing path of the sun’s rays.
    • Technology used in the solar panels: Newer solar panels that use half-cut photovoltaic cell technology are designed to reduce the impact of partial shade. However, this is not the case with standard solar panels (i.e. those that don’t use half-cut PV cells).
    • Inverter setup: If a central string inverter is used, shade on a single panel will reduce power output for the entire solar array. By contrast, with systems using MLPEs - microinverters or DC power optimizers - shade on a single panel has no impact on other panels and causes only a small reduction in the amount of energy produced. (The difference between string inverters and MLPE is explained here). 

    If you plan to install solar panels, you should first determine how much shade there will be on your roof. If you expect limited or partial shade, minimize your output losses by installing solar panels that use half-cut solar cells and that are connected to MLPEs. And if there’s heavy shade on your roof, you may want to evaluate if it’s a good idea to install solar panels over there at all. 

    A professional solar installer can calculate how much shade a particular roof section will receive over the year, as well as help you calculate solar panel output and your solar payback period based on that. 

    Weather conditions can also impact sunlight availability

    Weather conditions can have a big impact on solar panel production. Clouds, rain, and snow can reduce both direct and indirect sunlight, hampering solar power production. 

    Do solar panels work on cloudy days? 

    Yes, solar panels do work on cloudy days — but not as effectively as they would on a sunny day. That’s because clouds block some of the sun’s energy from reaching the ground (or in this case, your roof). 

    Under overcast conditions, your solar panels will produce anywhere between 10% and 60% of their regular power output, depending on how thick the cloud cover is.

    Do solar panels work in the rain? 

    Rain itself doesn’t affect solar panel output, but the heavy cloud cover that accompanies rain does. When rain clouds are blocking sunlight and darkening the sky, system output will be 40–90% lower. 

    Despite that, though precipitation can cause short-term output losses, rainfall actually has a positive side effect: it helps to clean the solar panels. If you live in a dusty climate, the solar output boost from a good downpour can be significant

    Do solar panels work when it snows? 

    Yes, solar panels do produce power in snowy conditions - as long as the snow isn’t too heavy. 

    Sunlight can pass through a light dusting of snow, so your solar panel system will generate solar electricity during light snowfall. And cold weather is actually good for solar panels, as it prevents them from getting too hot and losing efficiency

    A heavy accumulation of snow, on the other hand, would block sunlight and greatly reduce energy production. The good news is that this situation rarely occurs: solar panels are great at shedding snow. This is because they are slick, warm up as they absorb heat, and are usually installed at an angle. 

    The Department of Energy also points out that, similar to rain, snow is effective in cleaning panels, as it melts away. 

    How much sunlight do solar panels require to be economic?

    The general rule of thumb is that an average of four peak sun hours per day is enough sunlight to make a solar renewable energy system worthwhile. 

    Four peak hours is equal to 4000 watt-hours of cumulative solar radiation over a day. We explain peak sun hours, and the numbers for each state, in this blog.

    However, the amount of sunlight is not the only factor homeowners should consider when evaluating the cost-effectiveness of solar. You’ll also want to take local utility rates and state incentives into account when calculating your payback period.

    Find out what solar tax credits, rebates, and incentives are available to you here.

    The higher your local utility rates and the better the local incentives, the higher the potential savings from solar. In fact, tese factors make Massachusetts - which receives an average of just three peak sun hours a day - one of the best places in the nation to go solar, with payback periods as low as 3.2 years.  

    Calculate your solar payback period based on the amount of sunlight hitting your roof

    Key takeaways

    • While solar panels perform best under direct sunlight, they can still produce solar energy in the shade, during cloudy weather, in the rain, and while it snows. Performance will be affected, but not by much.
    • The impact of shade can be mitigated by using half-cell solar panels and MLPE (microinverters and power optimizers).
    • The average amount of sunlight you receive is an important factor in determining whether or not solar panels are worth it for you.
    • The SolarReviews calculator can estimate solar output on your roof based on your roof direction and sunlight at your location.
     - Author of Solar Reviews

    Zeeshan Hyder

    SolarReviews Blog Author

    Zeeshan is passionate about promoting renewable energy and tackling climate change. He developed these interests while studying at beautiful Middlebury College, Vermont, which has a strong focus on sustainability. He has previously worked in the humanitarian sector — for Doctors Without Borders — and in communications and journalism.

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