How to calculate solar panel size and output

Solar power is measured in kilowatts per hour or kWh. To find out how much appliances use per day, simply multiply their power rating in watts by the amount of time they are used for. For example, a 1000 W electric radiator turned on for an hour will consume 1000W (or 1 kWh); to use the radiator for 6 hours a day, you will use 6kWh daily. Another place you'll find kWh used is on your energy bill as utility companies charge for electrical energy by the kilowatt-hour (kWh), which is equal to 1000 watt-hours.

To estimate the energy needs of the devices or appliances you want to power, simply multiply the power rating of each by the duration of use in hours and add up all the totals. This will provide you with the minimum kWh of energy required to power your appliances. It's important to note that the power produced by a solar panel will be higher at midday than in the morning or evening. As a result, you will probably have to invest in a battery to store electricity as well as an inverter to use the stored power for your appliances.

The energy production of a solar panel depends on the quality of the equipment, the environment surrounding the panel and the circuit it is used to power.

Solar panel power output

Solar panel power is measured in peak watts (Wp) or peak kilowatts (kWp) which describes the power output of a panel provided in ideal sunlight and temperature conditions. Depending on the technology used, one square metre of solar panel can produce 60 to 150 Wp. You can then use this value to calculate the total area of solar panels required.

Sun exposure

It goes without saying that a solar panel in the south of the country will be more efficient than one in the north. Sun exposure is a useful factor as it allows you to evaluate energy production on an annual basis taking into account the location of the panel using peak power.

Efficiency and installation location

If your panels are not south-facing with an ideal tilt angle of 30-35° from the horizontal, they will produce less electricity than they would in optimal conditions since the absorbing surface of the panel is less exposed to sunlight. You will therefore need more efficient solar panels in sub-optimal conditions.

The total amount of energy produced (E) represents the number of kilowatts per hour (kWh) produced by a panel over the course of a year. It can be calculated using the following formula:

• E = A x r x H x PR

Whereby:

• E = Energy (kWh)

• A = Total solar panel Area (m2)

• r = solar panel yield or efficiency (%) 

• H = annual average solar radiation on tilted panels

• PR = Performance ratio, coefficient for losses (default value = 0.75)

Photovoltaic cells

The efficiency of a solar panel also depends on the materials used to make it. For example:

• amorphous silicon has an efficiency rating of around 6%;

• polycrystalline silicon has an efficiency rating of around 15%;

• monocrystalline silicon has an efficiency rating of around 16 to 24%.

Performance ratio estimates the total output of a PV installation that is produced as AC current. A number of factors are used to calculate the potential losses as energy undergoes its transformation from DC to AC power. Below are some examples of the types of losses that can occur to provide the PR value:

• Inverter losses

• Temperature losses

• DC cables losses

• AC cables losses

• Shading losses

• Losses at weak radiation

• Losses due to dust, snow, etc.

• Other losses

To simplify things, PR is provided as a default value of 75% to compensate for any number of potential losses.

The easy formula to calculate solar output is as follows:

• Average hours of sunlight × solar panel watts x 75% = daily watt-hours

Let's take an example using this formula.

The 75% in the formula refers to the PR value described above. Let's say your solar panel receives 6 hours a day of sunlight on average and you have opted for a solar panel with a 300W rating. The formula will therefore look like this:

• 6 hours x 300 W x 0.75 = 1350 daily watt hours.

You can convert daily watt hours into kilowatt hours by dividing the total by 1000. This means that you can therefore expect to get 1.35 kW per solar panel.

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