I have been asked several times what is the energy produced by a 1 kW panel in a day or how many units of energy are produced over 24 hours. Let me first state that the product of power and time gives you the energy produced. So for a 24-hour period, we have 24 kWhr of energy produced, if the solar panel was producing peak power throughout the day. But we know that that is not the case. So for the time component of the above equation, we can use 5 hours, also called peak sun hours. This gives a total of 5 kWhr (1kW x 5 hours) of energy produced over a 24-hour period i.e. 5 units of energy are produced. In a month you can possibly produce 150 units. If the per unit rate is Rs.10, you can produce Rs.1500 worth of electricity with a 1 kW panel in a month.

Now let us take a deep dive and see how peak solar hours are calculated.

Solar radiation falling on a panel continuously changes throughout the day with a peak of around 1000 Watts per meter squared around noon-time. This is shown in the above figure as well. But the peak radiation is of no use, we want to know the average radiation, that we can use to calculate the units produced. Another way is to calculate the effective duration of the radiation if it remained at its peak throughout the emission cycle. This can be done by first finding the total energy, which is the area under the curve above, and then dividing it by peak power which is 1000 Watts (both the quantities are assumed to be given per meter squared). The calculation is shown below.

Total Energy = Power x Time = (1000×1)+(800×2)+(600×2)+(400×2)+(200×2) = 5000 Watts Hours

Peak Sun Hours = Total Energy/Peak Power = 5000 Watt Hours/1000 Watts = 5 hours

But peak sun hours change with geographical location, season, and weather ranging from about 3 to 6 hours. The average peak sun hours map for the USA is shown below.

Note:

1. Note the above figure of solar radiation as a function of time is a simplification to an actual curve that looks like an inverted bell. The exact calculation of the area under the curve is a bit more complicated than what is shown above.

2. Solar panels are generally only about 20% efficient i.e. they convert 20% of the solar energy to electrical energy. So in the above example, the solar panel size would be about 5 meters squared to produce 1000 Watts (or 1 kilowatt) of peak power.