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How to Size your Solar Pv System

For a solar PV system to work properly, the size of the panels and the battery must be matched with the energy needs of the appliances. Because panels and batteries are expensive, people often try to save money by installing too few panels or too small a battery. This is very poor practice and does not really save money, because a system that is too small for the appliances do not work well and the battery will have to be replaced, often at a high cost. Sizing PV systems for homes is not difficult if you know what appliances will be used and how long they will operate each day. Because all the power must come from the solar panels, it is most important that they are large enough to provide the energy needed even on cloudy days.

See what we wrote about Battery Sizing.

The most common reason for the failure of a PV system is that the panels are too small. Designs are usually based on new components used under ideal conditions. As all parts of a PV system degrade over time, the system becomes less efficient and the panels must supply more energy as the system ages. Because panels that are too small do not charge the battery enough each day, battery life will be shorter than in a system with enough panel capacity. Trying to save money by using panels that are too small results in spending much more on battery replacements over the life of the system. It is usually cheaper to add extra PV panels because battery life is increased and few battery replacements will be needed.

Source: Semantic Scholar


The energy used by appliances is measured in watt-hours and the energy produced by the panels is also measured in watt-hours. Watt-hours of energy are like litres of motor fuel. When 5 litres of fuel are needed to go from one place to another, if only 4 litres of fuel are provided the motor will stop before the trip is completed. In a PV system, if an appliance needs 100 watt-hours a day to work properly and if the solar panels only produce 80 watt-hours the appliance will stop working early in the day.

  • Example 1

If a 10 W light is turned on 2 hours a day and a 120 W fan is operated 3 hours a day, how much energy is used by each appliance in one day? What is the total number of watt-hours used by both appliances in one day?

(Light) 10 W × 2 hours = 20 Wh/day

(Fan) 120 × 3 = 360 Wh/day

(Total usage) 360 + 20 = 380 Wh/day

Calculating appliance watt-hours used each day

To calculate the number of watt-hours needed each day from the panels, first calculate the number of watt-hours needed each day by the appliances. Then increase the result to cover the watt-hours lost in the wiring and battery before the energy reaches the appliances.To calculate appliance watt-hours, multiply the number of watts needed to operate each appliance by the number of hours each appliance is used per day Always calculate the energy used on a ‘per day’ basis because the solar panels provide energy on a daily cycle. Sometimes an appliance is used more on some days than on others. So far, we have assumed that there is only one appliance. Usually, there are several. You have to find the total energy needed by all the appliances each day. So first find the watt-hours needed per day by each appliance, then add them all together.

Calculating total watt-hours needed from the panels each day

When you know the daily energy in watt-hours needed by all the appliances, the total watt-hours that the panels must provide each day will be that number of watt-hours plus the watt-hours lost in the wires, battery and controller. A reasonable estimate is that for every 100 Wh used by the appliances, the panels have to produce 130 Wh. Therefore to find the total watt-hours that the panels must provide each day, multiply the total watt-hours used by the appliances by 1.3.

  • Example 2

A house has three lights. One is 20 W and is used 3 hours a day. The second is 10 W and is used 4 hours a day. The third is 2 W and is used 9 hours a day. What is the total watt-hours used by the three lights in a day?

20 W × 3 hours = 60 Wh/day

plus: 10 W × 4 hours = 40 Wh/day

plus: 2 W × 9 hours = 18 Wh/day

The total energy used is: 60 + 40 + 18 = 118 Wh/day.

  • Example 3

The appliances in a house require 100 Wh per day. How many watt-hours per day must the panels produce?

100 Wh × 1.3 = 130 Wh/day of which 30 Wh will be lost in the system and 100 Wh used by the appliances.

Finding the number of panels needed

  • If two panels are joined together, twice as many watt-hours will be produced. Three panels will produce three times the watt-hours, and so on. The watt-hours produced are the same whether the panels are connected in series or in parallel. To find the total peak-watt rating for the PV panels needed to operate the appliances, find the number of watt-hours that the panels must provide and divide by the Panel Generation
  • Factor. For a tropical coastal climate the factor is typically 3.43.
  • To find the peak-watt capacity that will be needed in a system follow these steps:
  • Step 1. Calculate the watt-hours per day for each appliance used.
  • Step 2. Add the watt-hours needed for each of the appliances to find the total watt-hours per day needed by the appliances.
  • Step 3. Multiply the total appliance watt-hours per day by 1.3 to find the total watt-hours per day that the panels must provide.
  • Step 4. Divide the total watt-hours per day by the Panel Generation Factor for your climate
  • (3.43 is typical).
  • Step 5. Divide the total peak-watt capacity by the peak watts of the panels available to you.

This will give you the exact number of panels needed. Usually the result will not be a whole number, but of course you cannot install only part of a panel. You must increase any fractional part of the result to the next whole number to find the number of panels.

  • Example 4

A house has the following appliances in use:

One 18 W light used 4 hours per day

One 60 W fan used 2 hours per day

One 75 W refrigerator that runs 12 hours per day

The system will be powered by 110 Wp panels. How many panels will be needed if the climate is typical tropical coastal (Class 2)?

(Step 1)

  • Light: 18 × 4 = 72 Wh/day
  • Fan: 60 × 2 = 120 Wh/day
  • Refrigerator: 75 × 12 = 900 Wh/day
  • (Step 2)
  • Total appliance use = 72 + 120 + 900 = 1,092 Wh/day
  • (Step 3)
  • Panel energy needed = 1.3 × 1,092 = 1,419.6 Wh/day
  • (Step 4)
  • Wp of panel capacity needed = 1,419.6 ÷ 3.43 = 413.9 Wp
  • (Step 5)
  • Number of panels needed = 413.9 ÷ 110  = 3.76 panels.
  • Actual requirement = 4 panels


The amount of energy from the panels will be greater than our estimate if the climate is sunnier than a tropical coastal climate. Also, the energy from the panels will be less than our estimate if the climate is cloudier than a tropical coastal climate. Some climates are seasonal, with many more cloudy days in one season than in another. The size of the system has to fit the cloudiest season if it is to give service all year round.

To make a reasonable estimate of the panel output for different climates, you can use the following guidelines:

  • Climate Class 1: Sunnier than the tropical coastal climate with many days of clear skies and few cloudy periods longer than four days. A desert location may be in this class. For this climate,use a Panel Generation Factor of 3.86.
  • Climate Class 2: A tropical coastal climate with most days partly cloudy. Fully cloudy periods are usually no more than five days long. For this climate, use a Panel Generation Factor of 3.43
  • Climate Class 3: Cloudy periods of five to seven days occur regularly but are typically followed by three or more clear days. For this climate use a Panel Generation Factor of 3.0.
  • Climate Class 4: Cloudy periods of ten or more days occur regularly and fully clear days are unusual. For this climate use a Panel Generation Factor of 2.57. Remember that you must use the cloudiest season for this calculation even though part of the year the climate may be very clear and sunny. If the system is not sized to allow for the cloudy season, then it will not work properly at that time of year

See what we wrote about The Major Types of Solar Panel

Do get in contact with us for detailed load analysis and quotation.

1 comment

  1. […] buying a battery, the voltage and the ampere-hour rating must be known. For a solar PV system in a home the voltage will usually be either 12 V or 24 V. The size in ampere-hours will depend on […]

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