Calculation Method

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How to Calculate Solar Power

You can do easiest Solar Power/Energy Calculation with simple Method. The purpose of the information on this page to provide a basic understanding of the major components in a basic solar power system, draw with diagrams and to help you identify and select the correct size components for your system.

There are two Methods of Solar Power Calculation

First Method of solar Power Calculations

Following show important components in draw-diagram

A Solar Panel convert Sunlight into Electricity power without any Fuel, Gas and Global Warming. This DC electricity is fed to the battery via a solar Charger Controller which ensures the battery is charged properly and not damaged.DC Battery provide 240 Volts power through panel & inverter rather regulator control over charging.

Detailed Component Description

Solar Panels

Solar panels are classified according to their rated power output in Watts. This rating is the amount of power the solar panel would be expected to produce in 1 peak sun hour. Different geographical locations receive different quantities of average peak sun hours per day.

Solar panels can be wired in series or in parallel to increase voltage or current respectively. The rated terminal voltage of a 12 Volt solar panel is usually around 17.0 Volts, but through the use of a regulator, this voltage is reduced to around 13 to 15 Volts as required for battery charging.

Solar panel output is affected by the cell operating temperature. Panels are rated at a nominal temperature of 25 degrees Celcius. The output of a typical solar panel can be expected to vary by 2.5% for every 5 degrees variation in temperature. As the temperature increases, the output decreases. With this in mind, it is worth noting that, if the panels are very cool due to cloud cover, and the sun bursts through the cloud, it is possible to exceed the rated output of the panel. Keep this in mind when sizing your solar regulator.

Solar Regulators

The purpose of solar regulators, or charge controllers as they are also called, is to regulate the current from the solar panels to prevent the batteries from overcharging. Overcharging causes gassing and loss of electrolyte resulting in damage to the batteries.

A solar regulator is used to sense when the batteries are fully charged and to stop, or decrease, the amount of current flowing to the battery.

Most solar regulators also include a Low Voltage Disconnect feature, which will switch off the supply to the load if the battery voltage falls below the cut-off voltage. This prevents the battery from permanent damage and reduced life expectancy.

A solar regulator also prevents the battery from back feeding into the solar panel at night and, hence, flattening the battery.

Solar regulators are rated by the amount of current they can receive from the solar panels.

See section below for information on correctly sizing a solar regulator.

Inverters

An inverter is a device which converts the DC power in a battery to 240V AC electricity. Inverters come in two basic output designs, pure sine wave and modified sine wave (square wave).

Most AC devices will work fine on the modified sinewave inverter, but there are some exceptions. Devices such as laser printers can be damaged when run on modified sinewave power. Motors and power supplies usually run warmer and less efficiently, and some things, like fans, amplifiers, and cheap fluorescent lights, give off an audible buzz on modified sinewave power. However, modified sinewave inverters make the conversion from DC to AC very efficiently, and they are relatively inexpensive.

Pure sine wave inverters provide AC power that is virtually identical to, and often cleaner than, power from the grid.

Inverters are generally rated by the amount of AC power they can supply continuously. Manufacturers generally also provide 5 second and 1/2 hour surge figures. The surge figures give an idea of how much power can be supplied by the inverter for 5 seconds and 1/2 an hour before the inverter's overload protection trips and cuts the power.

Deep Cycle Solar Batteries

Deep cycle batteries that are used in solar power systems are designed to be discharged over a long period of time (e.g. 100 hours) and recharged hundreds or thousands of times, unlike conventional car batteries which are designed to provide a large amount of current for a short amount of time.

To ensure long battery life, deep cycle batteries should not be discharged beyond 70% of their capacity. i.e 30 % capacity remaining. Discharging beyond this level will significantly reduce the life of the batteries.

Deep cycle batteries are rated in Ampere Hours (Ah). This rating also includes a discharge rate, usually at 20 or 100 hours. This rating specifies the amount of current in Amps that the battery can supply over the specified number of hours.

As an example, a battery rated at 120Ah at the 100 hour rate can supply a total of 120A over a period of 100 hours. This would equate to 1.2A per hour. Due to internal heating at higher discharge rates, the same battery could supply 110Ah at the 20 hour rate, or 5.5A per hour for 20 hours. In practice, this battery could run a 60W 12VDC TV for over 20 hours before being completely drained.

There are many factors that can affect the performance and life of a battery bank. It is highly recommended that you speak with an experienced solar power system installer or solar battery provider prior to making any significant battery purchase.

Solar Regulator Sizing Information

A solar regulator must be able to handle the maximum current that can be produced by the solar panels.

Reflected sunlight and specific temperature conditions can increase the output current of a solar panel by as much as 25% above it's rated output current. The solar regulator must be sized to handle the increased current.

Solar regulators often short the solar panel input when regulating. This does not damage the solar panel, but it does mean that the solar regulator must be sized to handle 125% of the solar panel's rated short circuit current.

Example: A BP Solar 80W solar panel has a rated output current of 4.55 Amps and a rated short circuit current of 4.8 Amps.

Minimum solar regulator size for a single BP Solar 80W panel would be:  4.8 Amps x 1.25 = 6 Amps.

It is recommended that the regulator selected is even slightly larger than this figure to ensure that it is not constantly operating at 100% of its rating, particularly in regions with higher ambient temperatures.

Sample Sizing Calculation

In order for you to size the system correctly, you need to note the power rating of each appliance that will be drawing power from the system.

For this example, we will calculate the power requirements for a campervan with:

• 2 x 15W 12VDC Fluorescent Lights
• 1 x 60W 12VDC Water Pump
• 1 x 48W 12VDC Fridge
• 1 x 50W 240VAC TV
• 1 x 600W 240VAC Microwave

(Note that a 600W microwave will consume approximately 900W of power)

1. Calculate Loads

Calculate total DC and AC loads:

DC Loads

·  Lighting - 2 x 15W DC Lights - each used 2 hours per day = 60Wh/day

·  Pump - 1 x 60W DC Pump - used 1/4 hour per day = 15Wh/day

·  Fridge - 1 x 48W Fridge - runs 8 hours per day = 384Wh/day

Total for DC Loads = 459Wh/day

AC Loads

·  Television - 1 x 50W - used 2 hours per day = 100Wh/day

·  Microwave - 1 x 900W - used 15 min per day = 225Wh/day

Total for AC Loads = 325Wh/day

Allowing for inverter efficiency of 85% = 382Wh per day (ie. 325 / 0.85)

Total for AC and DC Loads = 841Wh per day

2. Calculate Required Solar Input

In Central to Northern NSW expect a usable average of around 5 peak sun hours per day.

Required solar panel input = (841Wh / 5h) * 1.4 = 235W

Note: The 1.4 used in this formula is a factor we have found that can be used to simplify the calculations for basic systems.

To ensure that adequate power is produced in the winter months, use a figure of around 4.0 to 4.5 peak sun hours per day instead of 5.

3. Select Solar Panels

Select solar panels to provide a minimum of 235W. Always best to go bigger if possible:

·  2 x 123W solar panels chosen which, when connected in parallel, will provide 246W or 14.32 Amps.

4. Select Solar Regulators

The rated short circuit current of the 123W solar panels is 8.1 Amps each, giving a total of 16.2 Amps.

Select a solar regulator that is more than capable of handling the total short circuit current: 16.2 x 1.25 = 20.25 Amps

·  Steca 30Amp regulator chosen.

Note that, as described in the notes above, you must allow 25% extra capacity in the regulator rating as solar panels can exceed their rated output in particular cool sunny conditions. A 30A regulator will allow for an additional panel in the future.

5. Select Inverter

Select an inverter that is more than capable of supplying the maximum anticipated combined AC load required. In this example, maximum load would occur if the microwave and TV were running at the same time. Load in this case would be 900W + 50W = 950W.

Note that this calculation assumes that the inverter selected has a suitable surge rating to cope with the start-up surges of the microwave or other loads. A 1000W inverter would appear to be suitable, but a 1200W - 1500W inverter would be recommended.

·  1200Watt pure sine wave inverter chosen.

Note: A pure sinewave inverter is the preferred choice, but if the budget is tight, a modified sine wave unit could be used.

6. Select Battery

Select a battery, or a matched combination of batteries, that is capable of supplying the total power usage without being discharged more than 70%.

In most cases it is recommended that the batteries are sized such that they have around 3 to 4 days back-up capacity. This allows for days with low sunlight and reduces the daily depth of discharge resulting in longer battery life.

With 3 days storage capacity, the battery sizing would be as follows:

·  Ah Required = (841Wh * 3 / 12V) / 0.7 * 1.1 = 330Ah.

Note: The 1.1 is used in this formula as batteries are generally only about 90% efficient.

The appliance ratings used in the above examples may not be accurate. They have been used for example purposes only. Check the ratings on your appliances before performing any calculations.

Second Method of Solar power system components

Let’s start with a brief revision of the major components found in a basic solar power system. This should help you to understand then correctly identify and select the correct size components for your solar power system.

The following diagram shows the major components in a typical basic solar power system.

A basic solar powered system:

The Solar Panels converts sunlight into DC power or electricity to charge the Batteries This DC electricity/(charge) is controlled via a Solar Regulator/Charger Controller which ensures the battery is charged properly and not damaged and that power is not lost/(discharged). DC appliances can then be powered directly from the battery, but AC appliances need a Power Inverter to convert the DC electricity into 220 Volt AC power.

Description of individual solar power system components

Solar Panels

Solar panels are classified according to their rated power output in Watts. This rating is the amount of power the solar panel would be expected to produce at STC (standard testing conditions) of sunlight intensity 1000w/metre at 25 degrees centigrade

Different geographical locations receive different quantities of average peak sun hours per day.

As an example, in areas of the Highveld in South Africa, the annual average is around 5.6 sun hours per day. This means that an 80W solar panel based on the average figure of 5.6 sun hours per day, would produce a yearly average of around 450W.H per day.

Solar panels can be wired in series or in parallel to increase voltage or current respectively. The rated terminal voltage of a solar panel is usually between 17-22 volts, but through the use of a regulator, this voltage is reduced to around 13 or 14 volts as required for safe battery charging.

Solar panel output is affected by the cell operating temperature. Panels are rated at a nominal temperature of 25 degrees Celcius. The output of a solar panel can be expected to vary by 0.25% for every 5 degrees variation in temperature. As the temperature increases, the output decreases.

Solar Regulators

The purpose of solar regulators, or charge controllers as they are also called, is to regulate the current from the solar panels to prevent the batteries from overcharging. Overcharging causes gassing and loss of electrolyte resulting in damage to the batteries.

A solar regulator is used to sense when the batteries are fully charged and to stop, or decrease, the amount of current flowing to the battery.

Most solar regulators also include a Low Voltage Disconnect feature, which will switch off the supply to the load if the battery voltage falls below the cut-off voltage. This prevents the battery from permanent damage and reduced life expectancy.

A solar regulator also prevents the battery from backfeeding or discharging into the solar panel at night and, hence, flattening the battery.

Solar regulators are rated by the amount of current they are able to receive from the solar panel or panels.

See section below for information on correctly sizing a solar regulator.

Power inverters

The power inverter is the main component of any independent power system which requires AC power. The power inverter will convert the DC power stored in the batteries and into Ac power to run conventional appliances.

DC to AC power inverters

Just over a decade or so ago, DC AC power inverters were so inefficient and unreliable, many people restricted themselves to 12V lights and appliances.

If you have recently tried to shop around for 12V DC appliances, you will see that there is a very limited selection available.

Today, the efficiency and reliability of the latest DC AC power Inverters, are a far cry from the inverters that were available 15 to 20 years ago.

There are three waveforms produced by modern solid state power inverters. The simplest, a square wave power inverter, used to be all that was available. Today, these are very rare, as many appliances will not operate on a square wave.

True Sine wave inverters provide AC power that is virtually identical to, and often cleaner than, power from the grid.

Power inverters are generally rated by the amount of AC power they can supply continuously. Manufacturers generally also provide 5 second and ½ hour surge figures. The surge figures give an idea of how much power can be supplied by the inverter for 5 seconds and ½ an hour before the inverter’s overload protection trips and cuts the power.

For more info on solar inverters go check out our power inverter website - 

Solar Batteries

Deep cycle batteries are usually used in solar power systems and are designed to be discharged over a long period of time (e.g. 100 hours) and recharged hundreds or thousands of times, unlike conventional car batteries which are designed to provide a large amount of current for a short amount of time.

To maximize battery life, deep cycle batteries should not be discharged beyond 50% of their capacity. i.e. 50 % capacity remaining. Discharging beyond this level will significantly reduce the life of the batteries.

Deep cycle batteries are rated in Ampere Hours (Ah). This rating also includes a discharge rate, usually at 20 hours. This rating specifies the amount of current in Amps that the battery can supply over the specified number of hours.

As an example, a battery rated at 120A.H at the 100 hour rate can supply a total of 120A.H over a period of 100 hours. This would equate to 1.2A per hour for 100 hours. Due to internal heating at higher discharge rates, the same battery could supply 110Ah at the 20 hour rate, or 5.5A per hour for 20 hours. In practice, this battery could run a 60W 12VDC TV for over 20 hours before being completely drained.

There are many factors that can affect the performance and life of a battery or bank of batteries. It is highly recommended that you speak with an experienced solar power system installer or solar battery provider prior to making any significant battery purchase.

Choosing the right size solar regulator

A solar regulator must be able to handle the maximum current that can be produced by the solar panels. Reflected sunlight and specific temperature conditions can increase the output current of a solar panel by as much as 25% above it’s rated output current. The solar regulator must be sized to handle the increased current.

Example: An 80W 12V solar panel has a rated output current of 4.55 Amps and a rated short circuit current of 4.8 Amps.

Minimum solar regulator size for a single 80W solar panel should be:
4.8 Amps x 1.20 = 5.76 Amps.

It is recommended that the regulator selected is even slightly larger than this figure to ensure that it is not constantly operating at 100% of its rating, particularly in regions with higher ambient temperatures.

A good rule of thumb is a margin of between 20 and 30%.

Sizing your solar power system

In order for you to size your solar system correctly, you need to note the power rating of each appliance that will be drawing power from the system.

Let us take some common household appliances like lighting, a TV, and a fridge to see how one calculates the correct size solar system:

10 x 12W globes = 120W operating 10 hours per day. = 1200W.H

2 x TV x 250W = 500W operating 6hrs per day            = 3000W.H

Fridge x 250W = 250W operating 24/7                         = 6000W/day

Power inverter sizing

Appliance total power draw = 120W(for the 10 lights) + 500W(for the 2 TV’s) + 250W(for the fridge) = a total power draw of 870W.

To provide a small buffer or margin your minimum size inverter choice should be around 1000W.

A modified sine wave inverter with a 1500W continuous power rating will therefore be your obvious choice in this specific solar system design.

Determining the size and number of solar panels

Here we take the total power usage daily = 1200W.H + 3000W.H + 6000W.H

This = a total of 10200W.H

Divide the total daily power requirement by the number of charge hours for that geographic region eg. 10200/5.5Hrs = 1854.54W

Add 20% for inefficiencies = 2225.45 W

This total power value determines the size and number of panels eg. 2225.45/75W panels = 30 x 75W panels.

If you fancied say 125W panels , then 2225.45/125W = 18 panels.

How many batteries?

Well the 75W panels produce 4.4Amps, thus 30 x 4.4 A = 132A x 5.5 Hrs = 726Ah

105Ah batteries, should be discharged to no more than 50%, thus we divide total amps by 105A x 50% = 50A.H

726/50A = 14.5 x 105Ah batteries.

For ease of possible 24V or 48V configuration, this would mean 16 batteries.

What size regulator do we need ?

Let’s say we had 20A regulators at our disposal.

One 75W panel produces around 4.4Amps.

3 x 4.4A = 13.2A

So 30 solar panels would need 30/3 = 10 x 20A solar regulators.

Complete the solar power system

Well we have the following:

• 30 x 75W solar panels
• 10 x 20A solar regulators
• 16 x 105A.H deep cycle batteries
• 1 x 1000W modified sine wave power inverter

Thus we have calculated the solar power system components that you would need in order to cater for your current power usage

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