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How to build a small home photovoltaic system with less than 100 dollars

Is it possible to  “make  a small photovoltaic installation  at home” for  less than 100 euros  ? Here is an interesting home energy experiment that can be done with a mini investment, specifically:  98 euros  , to buy all the necessary components.

Obviously, the plant is a small plant  independent  and can not connect to the mains. Despite this, thanks to the use of a 7Ah battery, it can directly power some small appliances or, with the addition of some devices, it can support the home electrical system, reducing consumption on the bill.

In this article we will explain step by step how to create a small low-cost domestic photovoltaic installation by   purchasing and assembling all the necessary materials independently.

It sounds difficult, but it is not that complicated. The only precaution is that, speaking of electrical current, due precautions must be taken. Also because the mechanism is scalable and, with proper precautions, it is possible to easily achieve a higher power and intensity of output energy. So be careful.

Here is the general scheme for assembling the mini home system. In our experiment, we will use a small monocrystalline photovoltaic panel with only  20 watts  of power: a common module measuring 47 × 34 cm and 2.5 cm thick and about 2 kg in weight.

charger regulator


This image illustrates the general installation diagram of our low cost solar project   . As you can see: it’s nothing particularly complicated, you just need the correct components and the correct assembly steps.

Let’s look at the components one by one.

Photovoltaic panel

solar panels

Obviously the function of the solar panel is to receive radiation in an optimal way to transform solar energy into electrical current. The monocrystalline silicon panel will be facing the sun (south with a 30-35 degree tilt) and away from passing shadows.

In our little experiment, the photovoltaic panel is made of  monocrystalline silicon  , the most efficient in the smallest space, if placed with the right inclination and orientation. The panel is rated at  20 watts  , is 47 × 34 cm in size, and will produce direct current at  12 volts  and  20 watts as  long as it is exposed to direct sunlight.

How to tilt the photovoltaic panel optimally

The advantage of using such a small module is that it can be easily placed in the best place and position.
The optimal orientation is, of course, always  south  .
The incline, on the other hand, changes according to the latitude.

A brief note to identify the optimal inclination   of the photovoltaic panels: at each latitude the best inclination changes. The higher  you “go up”  on the globe, the more vertical the panels need to be to make the best use of sunlight. The further you go down towards the equator, on the other hand, the more  “horizontal”  the panels will have to optimize radiation.

In Italy, the optimal tilt is 30-35 degrees, but, as in this case we can  “play a lot”  in the position of the panel, this is the formula to identify the best (optimal) tilt at any latitude:

Optimal incline  = 3.7 + (0.69 x  latitude  )

Optimal incline in  Rome  = 3.7 + (0.69 x 41.89) = 32.6
Optimal incline in  Milan  = 3.7 + (0.69 x 45.27) = 35
Optimal incline in  Agrigento  = 3.7 + (0.69 x 37.32) = 29.45

If you want to recover a little more energy in winter at the expense of part of the summer production, you can slightly increase the inclination of the panels or, depending on the season, slightly change the inclination of the panel: in winter increase it slightly, in summer decrease it.

Charge controller

This little device is the “beating heart” or better: the  “brain”  of our little installation. In fact, the charge controller decides, through a  microcontroller  ,  if  and  when to  recharge the battery, when to discharge it and when to  “divert”  the energy produced to allow direct power (in direct current) of the connected electrical appliances. .

The device also has the fundamental function of regulating the battery charge: it will prevent excessive discharge of the accumulator or its overcharging, and it will prevent the battery current from flowing to the solar panel at night.

Charge controller

                                                                                                        Charge controller for photovoltaic panels with batteries

The charge controller will provide the following information using bright LEDs:

  • Level of battery charge
  • “Battery charging”
  • availability of energy for consumption

If the indicator LED is on it means that we can consume, if it is off it means that there is no current from the panel or the battery.

To avoid damage from  overload  or  overvoltage, make  sure the regulator supports input voltage and current compatible with the connected PV panel. For example, for our little 12 volt 20 watt panel, the regulator will have to support a 12 volt voltage input and at least 20-25 watts of power.

The charge controller has 6 connection ”  slots”  for cables:

  • the first  and  second  are the input / output of the photovoltaic panel
  • third  and  fourth  I entered / left the drums
  • fifth  and  sixth  are input / output of the mini-inverter
    or directly of devices that operate in direct current


The  “charge”  regulator is also a “discharge” regulator. 

The charge controller is also a “discharge controller”. Batteries, in fact, have a specific “depth of discharge” indicated by the manufacturers. Each battery, in fact, to be recharged several times over time, must never be completely depleted, but must always retain a minimum amount of charge.

Lead acid batteries typically have a  “depth of discharge” of  around  50 percent  . This means that if at nominal level they   have a capacity of 3 kWh, in practice they have a useful capacity of around 1.5 kWh.

When the maximum “discharge” threshold is reached, the regulator  automatically ”  disconnects ” each supply to preserve the battery and its ability to recharge.

Inverter (if necessary)

The investor  is not  always necessary, whenever possible it is better to prevent devices operating in DC “suffer” double conversion between  current  continuous  -alterna-continuous  . The inverter is the device that is responsible for raising the output voltage, to 12 Volts, to bring it to 220-230 Volts like the one we use every day in common household systems: it converts  direct current  , panel output and batteries, in  alternating current  , compatible with common mains current.

The inverter used in our little experiment is a  mini-inverter  that converts the 12 Volts produced in direct current, and present in the battery, into  220 Volts  in alternating current with which most household appliances work.

In this inexpensive experiment   we use a small portable inverter  for a car or caravan  . To be clear: it is a small  “plug”  like the one that connects to the cigarette lighter of the machine (12 Volts) and connected to a device that contains the common 220V plug.

The mini-inverter has very small dimensions, is equipped with a small cooling fan (which, beware, consumes energy) and is generally very easy to connect through small cables. Beyond certain powers, the fan is necessary to avoid overheating and permanent damage.

This inverter allows you to have a real 220V portable plug, connecting it to a common 12V battery and a few amps. Usually inside the package there is a cable to connect the inverter to the car battery and, more frequently, a cable to connect it, alternatively, to the car cigarette lighter. An excellent product for our use at an  economical price  : if there is only the cigarette lighter socket, you will need to cut the socket and connect the cables directly to the battery or charge controller in the two   “outlet” slots used to connect the ‘Username.

200 watt inverter that converts the 12 volt direct current present in the battery into 220 volt alternating current that can be used by ordinary electrical users using a transformer (this model is suitable for the Italian plug).

In this experiment, the inverter used has  200 watts  of power and is perhaps slightly larger compared to the maximum power of the photovoltaic panel of 20 watts. In any case, this calibration is completely irrelevant, what matters is that the “input” voltage must be  12 Volts  (not 24). With proper precautions you can increase the power of the panel, but it is good to always stay below 100 watts / peak. Similarly, as the power of the photovoltaic panel increases, the power of the charge controller must also be increased.

This type of inverter creates a modified sine wave, so it is not very suitable for electronic devices with direct power (such as TV), but for lighting, chargers and the like.

This type of inverter is also equipped with two USB sockets, useful for charging, for example, smartphones, ipads, tablets, directly taking advantage of the direct  current  provided by the battery or the charge controller.

Attention, however, that this inverter consumes current even when it is in stand-by, and the consumption is anything but insignificant: in just over a day the entire battery can be consumed if it remains on without producing power.

That is why we have put a  switch  in our diagram: to avoid consumption when the system is not working.


Batteries: How many solar panels do i need to charge a 100 ah battery?

For this experiment, we considered using a new lead-acid battery, but when you change your car or motorcycle battery, you can try reusing it for this experiment. When a battery can no longer start a car engine, it does not mean that it is completely unusable: the engine requires a high enough “peak” power when starting that, when the battery is partially discharged, it cannot achieve. This does not mean that the battery has completely stopped working.

A car battery is at least 40-50 Ah. In our case, 6 times less “capacity” will suffice.

In this case, the cost of the battery will not exceed 20-30 euros: a 12 Volt battery with a “capacity” of 7 Ah will suffice   .

12 Volt 7 Ah Lead Acid Battery

We have already seen previously how the depth of discharge  of the battery must be preserved  to ensure the longest possible operating period and a greater number of recharge cycles. The charge / discharge regulator plays a fundamental role in this.

Therefore, the actual storage capacity, expressed in watt hours, depends on and is proportional to the depth of discharge indicated on the nameplate data. Thus, for example, a battery with a “nominal” capacity of 4 Watt hours and a depth of discharge of 50 percent will have an effective storage capacity of approximately 2 Watt hours.

In principle, to calculate the nominal capacity of a battery, the Volts must be multiplied by the Ah indicated on the label. Thus, for example, our battery and our small domestic photovoltaic system will have a nominal storage capacity of about 84 watt hours of which, actually usable, about 42 watt hours.


Cables and fuses for safety

In this it is better not to skimp: your security, the entire system and the devices connected to it are at stake. In fact, the circuit operates at 12 Volts and the current is much higher than if it were operating at 220 Volts. For safety, it is better to use a  cable with a section of at least 2.5 mm  . Also, even if the charge controller has protections, it is better to insert two fuses: one between the photovoltaic panel and the charge controller, and the other between the controller and the inverter.


What am I doing with this inexpensive 20 watt mini-system?

With this mini photovoltaic system I have a storage capacity of about 42 Watt hours. That’s not much, but it can power, for example, a 10-watt lamp for about 4 hours.

As for recharging, in winter we can fully recharge the battery in one day (with a 20-watt panel). In summer, the same recharging is done in half the time. Obviously the times are cut in half by increasing the power of the photovoltaic panel.

Obviously, the entire system is completely scalable  : you can increase the storage capacity by connecting other batteries in parallel or even use a higher power photovoltaic panel. In this case, care must be taken to correctly size all other components, the charge controller first.

Components and cost of the small domestic photovoltaic system:


    Cost: about  30 dollars on Amazon 
  • CHARGE REGULATOR  for Photovoltaic 12/24 Volt 15A.
    Cost: about  12 dollars on Amazon
  • 200 WATTS INVERTER  to convert 12 volt direct current to 220 volt alternating current.
    Cost: about  27 dollars on Amazon
  • LEAD BATTERY  12 Volts and 7.2 Ah.
    Cost: about  10 dollars on Amazon
  • Cables, fuses + other expenses  .
    Price: around  20 dollars