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Window mounted solar charger. Components include brackets and glass mount adhesive

Window mounted solar charger. Components include brackets and glass mount adhesive

    Do Solar Panels and Chargers Work Indoors?

    Solar panels are an almost free way to generate electricity, but in some cases, you don’t want to keep your panels outdoors. This could obviously lead to efficiency issues, which is what I want to investigate and test out in what follows.

    The question I’m trying to find an answer to today is: Do solar panels and chargers work indoors?

    Yes, they can work indoors, although not as efficiently as outdoors. Solar panels are made for outdoor use, but they can work if set up near a window. They can also work under indoor lights, but that’s not efficient at all – or useful.

    However, some sources of indoor lighting have a similar spectrum to that of the sun, making it possible to power solar panels inside. Exposed to this indoor lighting, solar panels, and solar chargers can produce electricity.

    Electricity is created by photovoltaic cells that are exposed to light. The light does not necessarily need to be direct sunlight.

    It is possible to use solar panels and chargers indoors in two different ways. They can be used by placing them in the light that is entering through the Windows. They can also work by exposing them to the light from certain types of light bulbs.

    To understand this effect, let’s first look at how they work behind the glass. We can then see how they function when exposed to the light from light bulbs.

    Power From Interior Light Sources

    During the nighttime, we light our homes with light bulbs. Is this ambient lighting a suitable source to create electricity in a solar device? Yes – however, several factors affect their efficiency when used in this way.

    Light Intensity

    Our eyes tell us that the level of light indoors is like the levels outside. In reality, there is a huge difference between the light levels.

    Lux levels are a method of quantifying the levels of light that enter the eye. Using Lux levels, it is possible to measure the difference. We would expect exterior light to measure approximately 100,000. 1,000 could be seen as the order of the level of light that would be found indoors.

    Remember, it is the light levels that create electricity. Therefore, such a large difference in the light levels will translate into a large difference in electricity production.

    Color of the Emitted Light

    The sun emits light that covers the whole spectrum from blue to red, and into the infrared range. Interior lighting has a restricted range of the spectrum.

    Solar panels are sensitive to the light spectrum and produce different levels of electricity from different colors of light.

    The Size of the Panel or Device

    Solar panels are constructed by combining a series of photovoltaic cells that each produce approximately 0.5 Volts.

    Some panels for outside use will have 60 cells to create a usable voltage. This size is impractical for use indoors. It is simply too large.


    Defects introduced to the panel as part of the construction process will reduce its efficiency when exposed to low-intensity light indoors.

    Types of Bulb with the Most Power for Indoors Solar Devices

    So, what light bulbs can produce the best wavelength and intensity of light that can be used to power a solar panel? Here are a few options for you to consider.

    Incandescent Bulbs

    An incandescent light bulb produces light from an electric current that passes through a fine wire. This is in a sealed glass chamber holding certain, specific types of glasses.

    The light produced by these bulbs is close to the ultraviolet light from the sun. Provided that the light is bright enough, the lamp will produce electricity in a solar cell.

    The bulb, or in some cases, several bulbs, will need to be supported and suspended approximately 20 inches from the panel. They must be 100w or more for increased efficiency.

    LED Light Bulbs

    Light-emitting diodes within the LED bulb produce light when connected to an electricity supply. The more diodes within the light the higher the intensity of light is produced. This type of bulb is more energy efficient.

    The light produced by these bulbs is in the long infrared and ultraviolet range. This wavelength is like that produced by the sun and will produce electricity in a solar cell.

    An LED bulb or several bulbs should also be suspended approximately 20 inches above the cell.

    Which Bulb is Best to Use?

    LED bulbs will generate a wider spectrum of light and use less energy to achieve the same intensity, making this a slightly better choice for the vast majority of people.

    The smallest bulb size needed to generate power efficiently would be:

    Both bulbs will produce the same 1600 Lumens of light – however, the LED will consume less power.

    Does This Really Work?

    There are several smaller devices whose power requirements allow them to function from the power produced by an indoor solar panel:

    • Charging cell phones
    • Charging portable radios
    • Security cameras
    • Wi-Fi
    • Solar lights
    • Tablet
    • E-reader
    • Battery bank

    How do solar panels work?

    Solar panels are made up of lots of ‘photovoltaic cells’ which are housed between a material which is semi-conductive. The semi-conductive material is usually silicon (the material common inside all your electronics) or in older models could even be glass. An electrical field needs to be established in these photovoltaic cells. To do this the silicon is altered by adding some other elements, phosphorus to one end to give a negative charge and boron to the other to give a positive charge. This creates an electrical equivalent to a magnet with one end being positive and one being negative and this is an electrical field. So you now have electrons flowing around within these photovoltaic cells. So how does this generate power? Well, light is made up of millions of tiny particles known as photons. These photons from sunlight hit the photovoltaic cells and this knocks one of the electrons flowing around free. These electrons are then collected by metal conductive plates and can then be utilized to create electricity.

    • With more and more portable options now available such as small battery packs for charging phones or laptops, you may want to leave these charging in your car or caravan whilst driving.
    • If you live in an area where they could get stolen. Solar panels aren’t cheap and if you just want to use a small panel but don’t want to fix it securely to your roof, you may be worried about leaving it outside in the garden where it could get stolen.
    • One of the most common places people use solar panels behind glass is in a caravan or motorhome. Many people simply don’t have the room to install panels, or don’t fancy drilling a hole in the roof to fix them. However, when parked up the large Windows can make a great place for a panel.
    window, mounted, solar, charger

    How do glass Windows affect this process?

    You might think that being behind glass might actually be better for solar panels. We’ve all been in a greenhouse or building with lots of Windows on a sunny day and the temperature can get much much higher than outside. This is mainly due to a lack of airflow inside making it feel much hotter than if you were standing outside with a breeze.

    But regardless of this fact, to get solar panels to work we aren’t interested in temperature and the amount they heat up, we are interested in the number of photon particles which reach the photovoltaic cells (as explained above).

    Reflection/ Refraction

    So now you know solar panels are reliant on these photons reaching the photovoltaic cells to work it will be no surprise to learn that the more photons that reach the panels, the more effective they will be.

    Putting something such as a sheet of glass in the way will affect this. Even though glass lets some photons through it also reflects some as well. Those photons that do pass through the glass slow down and therefore change direction (are refracted).

    This reflection and refraction is shown simply in the image below, the incident ray is the light coming from the sun and then once that hits the glass (the light blue section) some light is reflected back at an angle and some that passes through is refracted and changes direction slightly. :

    This will always occur no matter how clean or transparent the glass but by different amounts. So for tinted glass more reflection will occur.

    Those photons that are reflected away never reach the solar panel and therefore this decreases the number of photons and thus the likelihood of an electron being knocked out of the electrical field in the photovoltaic cells.


    If your Windows are in a house or another fixed building the orientation will cause further issues.

    Outdoor solar panels are usually fixed to a south facing surface in the northern hemisphere. This is so they are exposed to sunlight for the most amount of time. Studies showed this actually makes quite a big difference to efficiency, with east and west facing panels averaging around 20% and north facing around 40% less efficient than south facing panels. In the southern hemisphere the opposite is true and panels should face north to get the most sunlight and thus generate the most energy.

    In a house you are unlikely to have Windows that face directly south and even they are unlikely to be in the roof (or facing directly up) and so will only receive sunlight through them either earlier in the day or later in the day when the sun is not at its strongest. The sun is at its strongest (highest irradiance) when at its highest point, i.e directly overhead.

    A car or caravan will give you slightly more flexibility as you can turn it to face the correct direction, even moving it to maximise the amount of sunlight entering throughout the day. But if you are driving then this is going to keep changing and Windows tend to be small in size not letting much light through.


    Being inside means you are more likely to encounter shading for many reasons. This might be a window frame or a branch of a tree outside the window. Every time your solar panel ends up in the shade this is time when it will not be able to generate electricity.

    Outside, a south facing panel will still receive some sunlight even when the sun is shining to the east or west. But inside, unless you are in a greenhouse or room with Windows all around you are likely to find the panel is in the shade for a large proportion of the day.

    Higher temperatures can make the panels less effective

    You may or may not have heard that solar panels actually work better in lower temperatures. This is partly true and very high temperatures can actually cause solar panels to work less effectively. This is because the silicon in the photovoltaic cells is an excellent conductor of heat. Studies have shown that efficiency decreases by 0.5% per degree increase over 25 o C.

    So as you can imagine inside a car or house (without the air-conditioning) it can get extremely hot due to the insulation of the external materials. It is not unusual for temperatures inside a parked car to get very high indeed and therefore this will cause even more inefficiencies on top of all the other factors already mentioned in this article.

    How to optimize panels for use behind glass?

    If you still insist on trying to use your solar panel behind glass then there are a few things you should do to optimize your chances of getting useful amounts of electricity:

    • Use a south facing window if you have the option to get as much direct sunlight onto the panel as possible.
    • If the panel is a small portable one that can be easily moved around then you can move it to different Windows depending on where the sun is at that time of day. This will give you the maximum amount of sun throughout the day.
    • Place the panels as close to the glass as possible to minimise loss from refraction and also to prevent shading from window frames or other objects in the room.
    • If you can open a window and allow the sunlight through then that is, of course, the best option but you are unlikely to be asking the question if that was an option.
    window, mounted, solar, charger


    So there you have it, solar panels will work when placed behind glass but don’t expect any great results.

    That said it really depends on what you need. Obviously, if you are attempting to power your entire house, then putting panels behind glass isn’t really feasible. However, if you just want to charge a mobile phone or some small appliances in a caravan then you should be able to do that.

    Rob is the head writer at Innovate Eco sharing knowledge and passion cultivated over 10 years working in the Environmental Sector. He is on a mission to build a community of people that are passionate about solving environmental problems.

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    Step 2: Circuit Diagram

    The solar charger circuit board comes with a USB port, DC jack for the solar panel, and two JST ports already attached to the board. The battery comes with a JST plug and will attach to the JST port labeled BATT. The solar charger comes with a JST pigtail cable which will connect to the LOAD port and be soldered directly to the PowerBoost input terminals.

    The power switch (at the top of the diagram above) should be attached to the PowerBoost pins labeled EN and GND. Flipping it will turn on and off the PowerBoost. This switch does not have to carry the circuit’s current load, so choose almost any on/off switch you like. I chose an illuminated on/off pushbutton, which also needs to be connected to the PowerBoost’s 5V and GND pins, with a 220ohm resistor in series. The illuminated portion of the switch is optional, but it is a nice indicator that the device is ready to charge your USB devices.

    Step 3: Test Fit Components

    You’ll want to pick an enclosure that fits all your components snugly, without too much squishing. I had an extra Moo business card box that fits the length of the battery and the height of the solar charger perfectly, and even has a little extra space left over for business cards still.

    It closes with magnets embedded in the layers of cardboard and paper. If you can’t find a stiff paper/cardboard box, you can choose an enclosure made from wood, plastic, or metal, however these harder materials will require different tools for creating port openings, such as a drill with a step bit.

    In addition to physically fitting inside, you must also plan out where to create the openings so that your device is useable. I chose to put the illuminated power button next to the USB port, since the light indicates it’s ready to charge. This area of the box is recessed, making the button less likely to get accidentally triggered while the device is in my bag. Opposite the button and USB A port (PowerBoost) are the solar panel DC port and USB mini B port (solar charger).

    Step 4: Solder Capacitor to Solar Charger

    The solar charger board comes with most of the components soldered to the board already, with the exception of the large filtering capacitor. Look for the large circle on the circuit board, with holes matching the capacitor’s lead spacing.

    The capacitor’s polarity is important! The negative side of the capacitor is labeled with a white stripe and minus symbol, and the negative lead is typically shorter. The positive side of the capacitor is not labeled, and the leg is typically longer.

    Line up the positive lead to the hole marked. and the negative lead to the hole marked

    If your enclosure doesn’t have enough space to fit the height of this large capacitor, you may bend it over slightly before soldering, or use wires (and heat shrink tubing) to move it to another part of your enclosure. According to the official assembly instructions, you should be careful to avoid contact with the hot chip in the center of the board.

    To learn how to solder, check out the soldering lesson in Randy’s free Instructables Electronics Class.

    Details and specifications

    Introducing the Somfy Li-ion Solar Panel, the perfect addition to your Somfy motorized window treatment system! Harness the power of the sun to keep your window treatments fully charged and ready to go. With this innovative panel, you’ll never have to worry about your window treatments running out of power again. Say goodbye to pesky cords and power outlets, and hello to a sleek and sustainable solution.

    Upgrade your home with a Somfy Solar Panel

    With the power of the sun, your blinds will be powered year-round, with zero maintenance, making your shades and blinds more efficient.

    Wire-Free Efficiency

    Eliminate the need for electrical wiring and increase energy efficiency by using the Li-ion Solar Panel. Perfect for hard-to-reach areas and allows you to feel good about your environmentally-friendly choice.

    Sleek Design

    Experience the convenience of a solar panel with a low-profile design that preserves your window‘s look while still taking advantage of the benefits of solar energy.

    Reliable Power

    Reliable performance day and night with safe, long-life solar powered lithium-ion battery technology. Enjoy worry-free operation and peace of mind knowing your batteries will automatically recharge.

    Unbox Solar Panel Kit

    Open the package and take out the Somfy Solar Panel, mounting kit, and glass mounting adhesives. Check that all the components are included and in good condition. Visualize the setup before you begin mounting the panel.

    Explore Mount Options

    Determine where to mount your Somfy Solar Panel for optimal sun exposure. Options include brackets (2 or 4) for interior window frame, or mounted directly to a window covering headrail where applicable. There’s also an option the mount the solar panel directly to the window glass.

    Test Placement Mount

    Check the placement of the Somfy Solar Panel. Ensure that it will not make contact with the window coverings when opening or closing. If using the glass adhesive, please allow 24 hours for the adhesive to fully bond. Tuck away any excess wire to maintain a neat and tidy installation.

    Connect Panel to Battery

    Simply connect the Somfy Solar Panel to a Li-ion battery pack or Motor with built-in Li-ion battery using the cables and connectors provided. Now you can enjoy uninterrupted solar power energy for your Somfy blinds!

    Compatible with Somfy WireFree Motors Li-ion Batteries:

    WireFree Li-ion motors (with embedded Li-ion battery)

    Sonesse 40 WireFree RTS Ref. 1240485, Sonesse ULTRA 30 WireFree RTS Ref. 1003310, Sonesse 28 WireFree RTS Ref. 1003297

    Roll Up 28 WireFreeTM RTS V2 Ref. 1241150, Roll Up 24 WireFreeTM RTS Ref. 1240278

    External Battery motors (using Li-ion Battery Pack 9021217)

    Sonesse 28 WireFree RTS Ref. 1241230

    Roll Up 28 WireFreeTM RTS V1 Ref. 1240292, Roll Up 28 WireFreeTM RTS V2 Ref. 1241162

    Cord Lift WireFreeTM TL25 RTS Ref. 1003294, Cord Lift WireFreeTM CL32 RTS Ref. 1002422

    Tilt 50 WireFreeTM Through-Shaft RTS Ref. 1240276

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