Solar Power and method to calculate panels

This text is written for common people without usage technical terminology and in very simplified manner. For those who is very technical, you could skip o the bottom of this page, straight to Solar Power Calculator.

Very often common people face the same set of questions when they wish to use solar energy.

Q. How big the solar panel shall be?

Answer: Starting point is to calculate power consumption of your house or devise.

Normally Power P=V x I , where V is voltage and I is current. So Volts multiply by Amps will give you Watts. Example 220V household multiply by 1A give you 220 Watt of Power Usage.

Another example for battery usage. We have an LED lamp, which is 12V nominal and 0.5A. This yield 6Watt of Power, consumed by this lamp.

If this is all what needs to be powered by the Solar Panel, then next step to calculate total power consumed by this device: P xH, i.e.

Or simply Power calculated earlier multiply by total time of usage will give you Watt Hours. Total time of usage determine by period of day light and most likely it will be calculated during one day. For example: light is used during dark time and only to eliminate porch during night. In this case, during winter Sun rise is around 8Am and Sun set around 5pm. Thus we need 15hrs of 6 Watt so in total 90 Watt hours

If we have more loads or usages, we will have to calculate Watt hours for each of them and after that sum them together. Let say as an extra, we would like to read for 2 hours a good book but light is very small: only 3 watts. Thus it is 3W x 2W= 6 Wh and plus the light from porch another 90Wh: total 99Wh.

Luckily we have very trivial loads like LED lights. But things gets very complex when we deal with water pumps, compressors or refrigerators. They do not work constantly and you might have to ether consult with manufacturer or simply measure their power consumption per hour in an average usage pattern.

Let's say we got estimate for daily power usage in Watt hours.

Next step is to estimate Power Usage but with consideration of all inefficiencies of our system. This is usually connected to efficiency of our charger controller, losses in charging/discharging batteries and also charge controller does consumes power even if nothing is connected or turned on on the output (no load currents)

Finally we know the overall Power which we would like to extract from sun.

Then we need to size our battery to that: Let's say we come up with 100Wh (watt hours). In this case the battery can be sized also in watt hours or simply V x Capacity (Amp Hours). In most of cases it is 12V lead acid battery , which is around 12.4V and it will have very important characteristic Capacity in Ah. Multiply them and match to calculated load in Watt hours. Please remember that battery shall be slightly bigger capacity. So if we have 100Wh load, I would recommend 150Wh battery. I will omit the reasons for that to avoid lengthy reading material.

Then we ready to choose the solar panel. It shall be no less then total power needed to run our all loads during complete solar charging cycle.

Normally solar charging cycle can be measured during one day: For example in the winter we have 9hrs of light (see the above example of sun rise and sun set). But normally even during those 9hrs we have very hazy light, which gives less then 50% of normal energy.

So thus we need to match needed total power usage to TOTAL power generation during day (24hrs) period. In the above example we need 100Wh total daily usage produce by solar panel within 9hr at poor light conditions (30...50% light). So we need (100/9)x2 = about 22W solar panel.

Here comes the part where you need to research what is real solar power generated by the panel. Most of manufacturers present their readings from the best conditions. You need to be realistic and count on the worst.

Below we present very simplified calculator for known loads and with multiple simplifications and assumptions. So please treat it as very rough calculator, which needs to be proven by practical testing.

Calculator of Solar panel size for different loads Solar time, (hr)
Load Power (W)
Load use (hr)
Visibility (%)

  Panel size 0: Watt
  Panel area 0: m2
  Panel area 0: cm2
  Panel area 0: ft2

Panel area is very crude estimate based on common silicone panel with 15% efficiency and if sun is always perpendicular to panel.

 Solar time= how many hours per day we have light. It is vary from geographic location and by season. Please see here for explanations.

Load Power= How big is your load in Watts. For example we need to power a water pump with rating 400Watt

Load use=how long is your load turned on during an average day. For example your water pump is about 20 times turned on and each time it is about 15min. Therefore it is 300min total or 5 hours.

Visibility (%)=Number in percentage to represent your typical weather condition. Note: normally winter and summer vary so much from each other that we propose to split characterization of Solar panel usage by two simplified yearly period- Fall(Autumn)+Winter AND Spring+Summer. Haze factor here is in %. Example- typical winter day without clear sun, but without heavy snow is 50%, Summer day with bright sun is 100%, Heavy rain or snow with overcast is 15%. It is hard to predict this number from day to day, but if you don't want to be bothered by thinking, please put 50 in Visibility (%).

Please note: ALL calculations done with assumption your solar panel is ALWAYS perpendicular to sun light. For more information about angular dependency solar energy and how to calculate solar panel angles please look here.
If you buy smaller then calculated panel, it will most likely won't have enough energy to power your device. If you buy larger then calculated value, then you might have more problems where to put it or overspend your budget. But as of recommendation, we think it is always the best to buy larger panel if you could find a spot to put it without having shade.

Luminatek Inc, Inc. All rights reserved.

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