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Readers have told me they like to build small-scale photovoltaic installations like those that power Low-tech Magazine''s website and office. However, they don''t know where to start and what components to buy. This guide brings all the information together: what you need, how to wire everything, what your design choices are, where to put solar panels, how to fix them in place (or not), how to split power and install measuring instruments. It deals with solar energy systems that charge batteries and simpler configurations that provide direct solar power.
Conventional solar PV installations are installed on a rooftop or in a field. They convert the low voltage direct current (DC) power produced by solar panels into high voltage alternate (AC) power for use by main appliances and rely on the power grid during the night and in bad weather. None of this holds for the small-scale systems we build in this manual. They are completely independent of the power grid, run entirely on low voltage power, and are not powering a whole household or city but rather a room, a collection of devices, or a specific device. Small-scale solar is decentralized power production taken to its extremes.
Most of the work in building a small-scale solar system is deciding the size of the components and the building of the supporting structure for the solar panel. Wiring is pretty straightforward unless you want a sophisticated control panel. You only need a limited set of tools: a wire stripper, some screwdrivers (including small ones), and a wood saw are the only essentials. A soldering iron, pliers, and a multimeter are handy, but you can do without them.
Low-voltage DC power does not carry a risk of electrocution (a fatal electric shock). That is especially so for 12V systems. Depending on the electric conductivity of your body (and other factors), you could go up to 20-50V before an electric shock may kill you. 1
Nevertheless, solar power systems have their risks. The main dangers are electric shock (non-fatal but painful), fire, battery explosion, and component damage. However, if you stick to some simple rules, you will be OK. During seven years of solar power experiments, I have never caused a fire or received an electric shock, although I have fried some components.
Electric power (expressed in watts) equals current (in ampère) multiplied by voltage (expressed in volts). Consequently, electric power (W) can refer to a low voltage (V) with a high current (A) or a high voltage with a low current. Conventional solar installations for households always use an inverter, which converts the low-voltage DC power from a solar panel into the high-voltage AC power used by main appliances. You can do the same for a small-scale solar installation, but it''s better to omit the inverter and build a low-voltage DC solar system. 23 That is the type of electrical installation in cars, trucks, sailboats, caravans, and motorhomes.
Power (watts) = V (volt) x A (ampère)
Directly coupling a low-voltage DC device to the low-voltage DC power produced by a solar panel avoids these energy losses and results in a more energy-efficient system. Practically, you can power the same device with a smaller solar panel. However, this implies that you use low-voltage appliances. Of course, you could plug in an inverter occasionally to power a mains appliance if there is no alternative. Make sure to buy one that is not too powerful, because it has to be operated on high capacity to be efficient. I have not found inverters with less than 150 watts of power capacity.
The sun doesn''t always shine. That is particularly so at night. You can add a battery and charge controller to your solar PV installation, and then you can use solar power when the sun is not shining. However, batteries are expensive, energy-intensive, and have short lifetimes. 4 Across the entire life span, batteries account for 80-90% of total costs and energy invested in an off-grid solar system. 5 They also introduce charge and discharge losses, which have to be compensated for by larger solar panels. For lead-acid batteries, the most cost-efficient option, these losses can be as high as 20-30%.
Batteries account for 80-90% of total costs and energy invested in an off-grid solar system.
This guide does not argue against battery storage, which is handy for some applications. However, you can often build a photovoltaic solar installation without battery storage. Such "direct" or "direct-drive" solar systems are cheaper, quicker, and easier to make. A direct solar power system allows you to use a wide variety of appliances during the day, even powerful ones. Examples are power and workshop tools, sound systems, and ventilating fans. Other devices, such as refrigerators, cooking stoves, and heating systems, can use direct solar energy in combination with heat or cold storage as a cheap and sustainable alternative to batteries. 6
Part of the money saved on batteries can be spent on larger solar panels, increasing the power supply in less optimal weather. A direct solar system can thus perfectly work in cloudy weather, even if it does not work between sunset and sunrise. It also works particularly well for powering devices with batteries, such as smartphones, tablets, laptops, bicycle lights, portable power tools, and power banks. You can only charge these devices during the day. However, you can use them after sunset.
The distinction between solar panels with or without energy storage isn''t always obvious. For example, you can connect a solar panel to a USB power bank (with a DC-DC converter in between). The system then becomes a battery storage system based on lithium-ion, taking advantage of the power management already available in the power bank. If you charge portable LED lights with batteries, a solar panel can even keep the lights on at night – a modern approach to the torch.
Solar panels are the main component of all systems we build here. Solar panels come in different voltages, usually 12V or 24V, sometimes 36V, 48V, or higher for grid-tied systems. For small-scale systems, 12V or 24V is what you want, especially to start with. You can also find small solar panels with voltages below 12V.
People often ask which solar panels to buy, but there is little advice to give. You have a choice between mono- and polycrystalline solar panels. The first are more powerful and expensive, but there is little difference. Almost all solar panels are made in China, no matter where you buy them. 7 A good advice is to compare prices and buy one that is not unusually cheap or expensive.
The other components depend on the type of installation you want to build. A solar installation with battery storage also needs a charge controller and a battery. A batteryless, direct solar system only requires a DC-DC converter. Both systems also need electric cables, fuses, and connectors. Optional components are on/off buttons and measuring devices.
Solar panels can be used individually or connected in parallel or in series. When you wire solar panels in parallel, the voltage output remains the same, but the current output doubles. That is the most common set-up. If you need (for example) 50W of 12V solar power, you can buy one 50W solar panel or several smaller panels (2x25W or 5x10W) and wire them together in parallel.
Using several smaller panels rather than one large panel is not the cheapest option because smaller solar panels cost more per watt of peak power. However, it can be the only way to fit panels where you want them. For example, my window sill is too narrow for a 60W solar panel, but I can install three 20W solar panels mounted next to each other. It would be cheaper and easier to have one 60W solar panel with a size that corresponds to the window sill, but that format is not available.
When you wire solar panels in parallel, the voltage output remains the same, but the current output doubles.
You can also wire solar panels in series. The voltage output doubles, but the current output remains the same. Wiring solar panels in series allows you to power 24V devices with 12V solar panels. Of course, you may just as well get a 24V solar panel. It''s best to connect solar panels of the same type, whether you wire them in series or parallel. Different types of solar panels may have different current outputs, which decreases efficiency.
You can also wire solar panels in series and parallel. For example, you can connect two groups of three 12V panels in parallel and then wire the two groups together in series. The result is a 24V system with the combined current output of three solar panels. You can also wire batteries in series or parallel to the same effect.
When building a solar power system with battery storage, you need a solar charge controller and a battery. Most off-grid solar installations run on lead-acid batteries. For portable solar systems with batteries, lithium-ion is the most practical option. Otherwise, lead-acid batteries are still the safest and most affordable option. They require less complex battery management controls than lithium-ion batteries. There are many other less common battery types, I''m not going to elaborate on those here.
Never connect a solar panel directly to a battery. If you want to store solar power for later use, install a solar charge controller in between. A solar charge controller regulates the voltage output of the solar panel in the function of the voltage that the battery needs during its different charging phases. It also provides a stable 12V output from the battery and shuts down the system if the voltage falls below a determined level. Most solar charge controllers offer a menu to adjust these values. Some come with a more elaborate second screen.
There are hundreds of different types of solar charge controllers available. For small-scale solar scale systems, my experience is that anything goes. The cheapest solar charge controllers work fine, but they should work on the correct voltage and have sufficient capacity (see how to size a solar system). More expensive solar charge controllers (like MPPT) are not worth it for small-scale systems. If your system works with lithium-ion batteries, you need a different solar charge controller, which is more expensive. If you are handy with electronics, you can build your solar charge controller. 8
The type of lead-acid battery you need for a small-scale solar system is a sealed lead-acid battery. If you use a 12V solar panel, you need a 12V battery. If you use a 24V solar panel, you need a 24V battery. Handle lead-acid batteries well because not doing so can ruin them quickly. Most importantly, their voltage should not drop too much, and you should fully recharge them regularly. Never leave a lead-acid battery without charging for a prolonged time. Keep it connected to a solar panel, including when you are away from home.
Handle lead-acid batteries well because not doing so can ruin them quickly.
When coupled to a solar panel and battery, the charge controller will disconnect the battery when the voltage drops below a specified level, usually 12V. You can adjust this value in the menu. You can go as low as 11V, at the expense of a shorter battery lifetime. If you want a longer battery lifetime, you can set the value to 12.2 or 12.5V, for example. The price you pay is a lower energy storage capacity.
Solar charge controllers connect all other components: the battery, the solar panel, and the electric load (the devices you will power). A solar charge controller should have six wires sticking out: two to the battery, two to the solar panel, and two to the electric load. You should always join the components in the order described below.
In a direct solar power system, there is no need for a battery or a charge controller. The solar panel is either directly connected to the powered device or has a DC-DC converter in between. Some DC devices can work on fluctuating voltages, for example, fans, pumps, and other devices with a DC motor. The motor will run faster or slower depending on the voltage. Heating elements can also work at different voltages. However, other appliances – such as all electronics – need a precise and steady input voltage. A DC-DC (buck or boost) converter is essential to provide that stable voltage input.
A DC-DC converter is an electronic module that converts the input voltage from a solar panel (or other power source) into a steady output voltage for a device, for example, 5V for USB gadgets and 12 to 20V for power tools. "Step down" or "buck" converters lower the output voltage compared to the input voltage. Boost converters step up the voltage in a similar fashion. A DC-DC converter introduces energy losses, but they are smaller than the losses of batteries, inverters and AC/DC adapters.
For solar systems without battery storage, you should know that 12V solar panels produce more than 12V. In full sun, the voltage output will be closer to 20V. The same goes for 24V solar panels, which will have a voltage output of around 32V. The 12V or 24V indication only refers to the type of battery system you are supposed to use it for. Consequently, if you want to run 12V devices directly on a solar panel, you need a DC-DC module that converts the 20V input into a steady 12V output (unless the appliance is happy to work on different voltages). If you want to run 5V devices, you need a module that has a steady 5V output.
Take care to get the correct electronic module. The most versatile DC-DC converter accepts a wide range of input voltages and converts these into any output voltage you want. This type of DC-DC converter can be directly connected to a solar panel and power all devices, no matter the voltage they run on. Such modules allow you to adjust the output voltage by turning a tiny screw or pressing a button. Some buck and boost converters have a small digital screen that shows the output voltage. Otherwise, use a multimeter to adjust the voltage output.
Wiring a direct solar system without battery storage is straightforward. If there is no DC-DC converter, screw the + and the - of the solar panel to the + and the - of the appliance. Put a fuse in between. Optionally, add an on/off button. Make sure the device you power can take the voltage that the solar panel supplies to it.
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