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Number of solar panels required under the conditions given:

2029.30 watts/102 watts  = 19.89 panels, round it to 20 panels

Required Solar Panels Plus Other Components, Battery and Charge Controller


<< Continue from previous page



Battery and Charge Controller


Power Ratings Required for the Battery Bank

It was said before that the nominal maximum power at any given time required by the house will be 6500 watts, which is the  sum of all devices turned on at the same time, this is the worst case scenario for power demand, it was also said that a safety margin must be used, here a 30% safety margin was selected, or 6500 x 1.3 = 8450 watts, in other words the calculated nominal power consumption for the house was 6500 watts, but we want to add an extra 30% margin.

Applying the same power safety margin the minimum power that the battery or battery bank has to deliver in any given time to the DC to AC converter is 8450/0.90 X 1.3 = 10985 watts, and must handle a current level of  457.70 amps without damage, (Which is calculated from 10985 watts divided by 24 volts = 457.70 amps).


Please do not confuse the instantaneous power which is simple the voltage multiplied by the current, with the concept of  Watt/hour  which is the instantaneous power consumed in one hour.



A 100 watts light bulb consumes an instant power of 100 watts

A 100 watts light bulb  left turned on for one hour consumes a 100 Watt hour

The same bulb left turned on for five hours will consume 500 Watt hour, but if a 500 watts light bulb is left on for one hour will consume the same power in one hour or 500 Watt hour, but the instant power and current rating in each one is totally different, in the first case assuming 120 V applied voltage the instantaneous  current will be  100 watts/120 volts= 0.833 amps, in the second case will be 500 watts/ 120 volts = 4.16 amps


Calculating the Battery Storage Capacity

From the previous page “summary of power requirements” the total watts/hour required after adding all appliances and equipments was 13150 watts/hour, in other words, this is the total wattage to be used in 24 hours, therefore the average watts/hour is 13150/24 = 547.91 watts/hour. Because increase in power storage means also increase in cost, it will be assumed that no additional storage will be required at this time, always storage capacity can be installed later as required at additional cost. To properly calculate the battery power capacity is necessary to know the battery efficiency, for this example it will be assumed to be 90%, then the total power in Watt/hour that the battery needs to receive from the charge controller to compensate for the battery 90% efficiency is:


13150/0.90 = 14611 watts/hours total

or 14611/24 = 698.79 watts/hour for 24 hours


Because the battery nominal voltage is 24 volts, the amps capacity required will be:


P = V x I


Then the amps/hour storage capacity for the battery at the specified voltage is:


I = P/V = 14611/24 = 608.8 amps/hour


The average hourly current consumption in 24 hours will be 608.8/24 = 25.36 amps per hour.



Charge Controller Requirements

The charge controller is the interface between the solar panels and the battery or batteries bank, the main purposes of this equipment is to provide a regulated and stable voltage and current source to the battery. Solar panels power output is affected by the sunlight variation during the day, when the sunlight is insufficient the charge controller avoids power from the battery to feedback the solar panels specially at night, further more the charge controller prevents the battery from overcharging or deep discharging, both are undesirable conditions that can severely damage the batteries or short their expected life.  


As any other equipment, charge controller operates at lower than 100% efficiency, for this case 90% efficiency will be assumed, in an actual project the efficiency must be taken from the manufactures specification sheet, but a 90% is a realistic one. Because not all power from the solar panels is delivered to batteries duet to this 90% efficiency, then the power loss from the converter has to be taken in account to calculate how much power need to received from the panels.


The batteries require  14611 watts/hour to supply the DC to AC converter to provide an average of 608.79 watts/hour during 24 hours or in one complete day.


The input power in Watt/hour that the charge controller must receive from the solar panels is:


Power = 14611/0.90 = 16234.44 watts/hour


Important Note:

This power must be supplied by the solar panels during the effective sunlight hours, per say eight hours, it is important to know that solar panels wont generate power at maximum efficiency during all day long, because in the morning and some hours afternoon the sunlight is weaker. The effective sunlight timing changes during the season, in summer the days are longer and normally the sunlight is stronger as compared to winter, where the days are shorter and the sunlight is weaker, the best thing to do is to consult your local sunlight index to know what is the sunlight radiation level during the day  and by season, for the lack of the exact sunlight radiation index for your area a good overall approximation is to use a 85 percent solar panels power output for eight hours per day as a yearly average for an effective sunlight radiation, if you live way at north or at south this figure wont be very helpful perhaps north of Canada or at the end of South America’s Tierra del Fuego or the north and south poles.  


The minimum power that the charge controller has to handle is:


16234.44 watts/hour divided by 8 hours

= 16234.44/8 = 2029.30 watts/hour for 8 hours


Amps/hour will be:


P=V x I


Because battery voltage is 24 volts




I = P/V


I = 2029.30 watts /24 volts


I =84.97 amps/hour


The charge controller must be capable of supply the calculated amp/hour.



Calculating the Required Solar Panels


Electrical Panel Specifications


Solar panel nominal power in watts at 100% sun light and rated current of 3.64 amps

= 120 watts,  Manufacturer data  


Solar panel open circuit voltage = 33 volts,  Manufacturer data  


Solar panel nominal voltage (volts) at 100% sun light and at rated current 3.64 amps

=  29 volts,  Manufacturer data  


Power output in accordance with local sunlight conditions:

Average solar panel power for a  8 hours sun light day 85.00% or change this percentage in accordance with your location

Nominal solar panels output at max sunlight = 120 watts nominal  

Effective  average  watts per panel = 102 watts = 120 x 0.85  (85% for eight hours sunlight)

Effective  average  current per panel = 3.09 amps

Volts = 29 volts  at the specified current load












Continue tutorial on batteries >> GO

Always select a charge controller with a minimum of plus 30% margin over the calculated power value