first time here and still learning.

I have just ordered a 100W solar panel with these specs:

  • V:21.61V
  • I:5.73A

I am aware that to calculate the power output i can multiply V x I.

However the sun is not always shining and the panel is not mounted on a sun tracking system, thus sometimes the solar panel will output more than other times. The questions is: is there a way (like a digital device) to read how much energy (or power) has been produced by the panel at the end of the day?

I plan to connect the panel to a 12V car battery and then keep an LED light turned on for about 14-16 hours per day.

  • 1
    \$\begingroup\$ Note that the values you've given are for open circuit voltage and short circuit current. You will never get them together. Additionally there are many off the shelf systems that will meter your panel output. Are you looking to design and build one or buy a ready made one? \$\endgroup\$
    – Samuel
    Nov 17, 2015 at 17:54
  • \$\begingroup\$ Thanks Samuel, I wasn't sure which nominal value to share. Here it is the whole label from the back of the solar panel: Product Specification = Peak Power (Pmax):100W; Peak Circuit Voltage(Voc):21.61V; Max Power Voltage(Vmp):17.43V; Short Circuit Current(Isc):6.13A; Max Power Current(Imp):5.73A; I am planning to build one. Can't afford to buy a whole system nor to have it installed, so I am kind of experimenting with just one panel. \$\endgroup\$ Nov 17, 2015 at 18:01
  • \$\begingroup\$ Have you build electronic circuits before? Also what kind of LED do you want to use (nominal voltage, power rating, socket/package) ? \$\endgroup\$
    – Grebu
    Nov 17, 2015 at 19:11

2 Answers 2


Product Specification =
Peak Power (Pmax):100W;
Peak Circuit Voltage(Voc):21.61V;
Max Power Voltage(Vmp):17.43V;
Short Circuit Current(Isc):6.13A;
Max Power Current(Imp):5.73A;

Most useful values are Vmp, Imp, Wmp and Isc.
Wmp = wattage out in full sun at optimum load.
Vmp = loaded voltage in full sun at optimum load.
Imp = loaded current in full sun at optimum load.
Isc = current into short circuit in full sun at optimum load.

Isc is max current you will ever get.
In a simple system Vload is < Vmp or << Vmp and Iout approximates Isc x Zsun
Where Zsun is fraction of max solar output.

To properly determine energy delivered across a day you need to calculate Vload x Iload at regular intervals and sum these to get daily result. This is "not too hard" to do with eg an Arduino or any other microcontroller with 2 x ADC inputs.
Vload can be measured with a simple resistive divider.
Iload can be measrured by plaving a resistor in series with the panel feed and measuring the voltage drop across it. (There are a number of other ways but that is easiest. If you use a say 10 bit ADC with 1024 step resolution and max input of 5V then you get ~= 200 readings per Volt input. If you allow I_series_R to reach 0.5V max at Imax then you get about 1% steps. Current measurement accuracy will probably be in the +/0- 2.5% to 5% range (using a 1% series resistor).

You can allow higher voltage drop for more accuracy or use an opamp or instrumentation amplifier to increase the voltage into the ADC. A lower ADC Vref will allow greater accuracy and resolution

An "18V" panel such as you have is often used to charge a 12V lead acid battery. If a linear charging circuit is used the excess vopltage above battery voltage is wasted. In such a case, adding a 0.5V or even 1V drop in a sense resistor does not affect available charging energy.
If eg a buck converter is used to reduce panel voltage to battery voltage themn any extra drop represents a reduction in available energy.

Any of several simpler methods may be good enough:

You can measure and sum only Iload at regular intervals. This can be done with discrete ICs if desired. If charging a battery Vload may be approximately constant (depending on battery chemistry) and Iload will then approximate charge rate. In many cases the total mAh of charge is of more interest than the charge energy.

A VCO (Voltage Controlled Oscillator) can be used to produce a frequency output proportional to load current and this can be summed by an IC counter or by a microcontroller.

Iload can be used to charge a capacitor to a preset voltage at which point the capacitor is reset to zero and a count of the rests indicates mAh of charge. This is essentially a very simple Vco. If eg a comparator is used the rest threshold voltage can be low and available voltage is not significantly reduced.

Panel mean output if optimally loaded will be
Energy ~= (average equivalent sunshine hours per day) x (Panel max power out)
= Wmp x SSH ("sunshine hours")
Solar insolation is expressed in kWh/m^2 with "1 sun" = 1 kWh/m^2.
It happens that the typical midday solar output is approximately 1 sun.
This varies with site and may be higher or lower and depends on geographical factors, time of year, weather conditions and air purity.

An excellent site which gives mean monthly SSH plus many other related parameters is GAISMA.COM. Here is the page for Kabul in Afghanistan. A good way to access pages is to web search for gaisma city_name. This is usually faster than using the sites own search system.

This chart from the above page for Kabul hives "sunshine hours" or kWh/m^2/day for Kabul.
eg in June (month VI) there are an average of 7.4 equivalent full sun hours per day. This is an exceptionally high figure not achieved at most locations. In December (month XII) there are 2.17 SSH/day on average.

These SSH figures may be multiplied by panel Wmp rating to get the average maxi,mum output that can be expected. This needs to be reduced to allow for non-steering - both per day and across a year.

The sunpath chart on the same page shows sun angle across a day and across a year and sunrise and sunset times and is useful for predicting likely performance.

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I plan to connect the panel to a 12V car battery and then keep an LED light turned on for about 14-16 hours per day.

Due to factors such as

  • the voltage mismatch when charging and

  • battery charge/discharge inefficiencies and

  • panel pointing losses,

a rule of thumb is that you can expect the battery to output ABOUT one third to one half of the max possible panel energy per day.

  • Available energy ~~~= SSH x Wmp / 3

eg if you have a 100 W panel and 2.7 SSH per day then you van notionally get 100 W x 2.7h = 270 Wh/day from the panel but after charging the battery and then using the battery at night the likely energy is 270/3 to 270/2 = 90 to 135 Wh/day.

  • Load Watts ~~~= SSH x Wmp / 3 / Operating_hours

If you wish to operate a load for 16 hours then the available Wattage is ~~= 90/16 to 135/16 = 5.6 to 8.4 Watts of load.
If operating LEDs strips or similar that accept 12V directly then a 5 to 8 Watt load can be supported with 2.7 SSH of insolation. If the LEDS require constant current at about 3 to 4V then use of a buck converter or equivalent is required if efficient use of the battery energy is required.


Note: solar panel also generate the power by reflecting of sun light by other objects like sky,home, buildings, moon light,any light source, etc.

1. As your solar panel have 21.61V it will give around 12v(might be 10V) when the sun light is not directly contacting with the solar cell.(if it's giving more than 12v great don't worry about 2,3)

2. As your car battery is 12V it won't charge unless voltage of solar cell is more than 12V. sad luck :(

3. Possible time frame for charging car battery will be around 11AM to 3PM.(around 4 hours)

4. As solar panel has I=5.73A car battery will charge around 30% to 40 % in 4 hours.

5. You can turn on 4 LED(3V each) in series for 15 hours easily.

I have solar panel with 12v 300mA:

I get around 6V and i'm using voltage regulator(L7805) to charge my old cell phone battery(750mAh). i can glow 3V LED till 8 hours easily. (might be more never checked)

  • \$\begingroup\$ So would it be better if I used a different battery instead of a car battery? \$\endgroup\$ Nov 17, 2015 at 18:37
  • \$\begingroup\$ I think car battery will be better as it will store more charges. You need to buy another solar cell around 12v so it can give 6v. total volt =10+6 . Now you can charge your 12v battery easily. also you can charge your battery around 9pm to 5pm (11 hours) Full charged :) Note: use voltage regulator L7812 to charge your car battery. \$\endgroup\$ Nov 17, 2015 at 18:56
  • 2
    \$\begingroup\$ @simplemind Some of editinit's advice is incorrect. Your panel will make well over 12V for insolation levels above about 10% of full sunshine level. At levels of light below that the amount of energy available is minimal. See the gaisma site I mentioned for average equivalent full sunhours in your area. \$\endgroup\$
    – Russell McMahon
    Nov 17, 2015 at 19:15
  • \$\begingroup\$ @RussellMcMahon answer updated. actually my solar cell was cheap it might not giving me the right voltage :( \$\endgroup\$ Nov 17, 2015 at 19:42

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