Sizing a Solar Photovolatic (PV) Array and System

Determine the size of solar panels, solar regulator, inverter or battery
to meet your off-grid system power requirements

Solar Panels - Background information

Solar panels are classified by 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. Note: geographical location will affect peak sun hours and this has to be borne in mind when making your calculations.

Solar panels can be wired in series or in parallel to increase voltage or current respectively. We have covered these connections in our product catalogue.

Solar panels are rated according to Peak Power [W], the Voltage at Pmax [V], the Current at Pmax [A] etc. All solar panels have a rated terminal voltage of about 17 volts.

The output of a solar panel is affected by the cell operating temperature. Panels are nominally rated at 25 degrees Celsius and can vary by up to 2.5% for every 5 degrees increase in temperature. Thus, as temperature rises, output from the solar panel decreases.

Regulating Solar Panels

A solar regulator, or charge controller, is used to regulate the flow of current [A] from the solar panel to the battery.  Batteries must never be overcharged or they are damaged.  When a battery has been fully charged the solar regulator will stop the flow of electricity to the battery. Solar regulators can be purchased with all sorts of features and functions an essential one being a Low Voltage Disconnect. 

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

Inverters

Storage batteries use and store DC - Direct Current and have a low voltage output usually in the range of 12 - 24 volts. Virtually all modern appliances operate on AC - Alternating Current and work on 240 volts. An inverter is a device that takes the power from your DC battery source and through special technology boosts it to household AC electricity giving you the power to run appliances such as televisions, lights, computers, and power tools wherever you may be. Simply, an inverter increases your 12/24/48-volt battery power to 110/240 AC power.

Read more here

Solar Batteries

Solar panels should be used in conjunction with Deep cycle batteries. These batteries are designed to be charged and discharged over a long period of time. They are not the same as car batteries which provide a large amount of current for a short period of time.

To ensure long battery life, deep cycle batteries should not be discharged beyond 50% of their capacity. Discharging beyond this level significantly reduces the life expectancy of the battery.

Deep cycle batteries are rated in Ampere Hours [Ah]. This rating specifies the amount of current in Amps that the battery can supply over a period specified in hours.

Read about battery sizing here

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.

When rating or sizing a solar panel it must not be forgotten that the output of a solar is affected by temperature. Thus, temperature can increase the output of a solar panel by up to 25% above it's nominal rated current. For this reason solar regulators must be sized accordingly to be able to handle the increased short circuit current. Typically regulators are rated 125% higher to enable the short circuit current to be handled correctly.

For example, a Kyocera 85W solar panel has a rated output of 5.02 Amps and a short circuit current of 5.34 Amps. The minimum size solar regulator for this panel would be 6.67 Amps or 5.34 Amps x 1.25 = 6.67 Amps.

Solar Array Sizing

To quickly determine the size of your solar array use the following worksheet. or you can scroll down to read more about the steps to take sizing your solar array, controller, inverter and battery sizes - if required.

NOTE:Always work in two's. That is 2X 12v MODULES will be required for a 24V sistem etc.

Step 1. Determine the average yearly solar isolation for your LOCATION.

Step 2. Determine the daily total loads in Watt Hours (Wh).

Step 3. Divide step 2 by step 1. This is the number of watts we need to generate per each sun hour.

Step 4. Select your solar panel. Power = V x I or (volts x amps)

Step 5. Divide step 3 by step 4. The result is the number of modules required for your system.

System Sizing - Steps to Success

Before sizing a system, we need to know the power rating in WATTS of each and every appliance that will be used. The easiest way to do this is to walk around your property make a note of the power rating of each appliance. Add up the wattage of all of the appliances you want to run off your renewable energy system.

Example: A barn that is not connected to the electricity grid requires low voltage DC lighting and 240V mains voltage to power some small electric appliances such as TV, Laptop and some other small appliances.

Q) How many solar panels do I need and what size should I buy?

The first step is to work out your load requirements. In our example there are two types of load - DC appliances and AC Appliances.

6 x 11W 12v DC Lights - used for 4 hours per day

1 x 150W 240V AC Television. Example is based on Philips 32PF7321 32" Widescreen LCD TV - used for 6 hours per day

Our calculation begins as follows.

1. Calculate both the DC and AC Loads

Determine the DC Load

- Lighting - 6 x 11W DC Lights - used for 4 hrs per day = 264Wh per day

Total DC Load in Watt Hours = 264Wh per day

Determine the AC Load

- Television - 1 x 80W - used 6 hrs per day = 480Wh per day

Total AC Load = 480 Wh per day

Total Load = DC Load + AC Load =744Wh per day.

There will be energy losses to account for so add 20% to the load as this will account for the losses and emergency power use outside of the specified times.

Total Load + 20% Energy Losses = 892.8Wh per day.

2. Sizing the Solar Panel

Due to UK weather conditions and for this example, we shall use 1.5hrs of Peak Sunshine. Please note: you do need to know the weather conditions for your area as this will affect the size of the panel or array.

Required solar panel input = (892.8 Wh / 1.5h) = 595.2W. You will need solar panels that will generate 595.2 watts per hour.

3. Selecting the Solar Panel and Regulator

Select the solar panels to provide a minimum of 595.2 or 600W. Always round to the nearest 10.

Any combination of solar panels can be used to provide the required 600W

3 x 200W solar panels. Each solar panel will provide an output of 200W [Pmax] at 7.61 Amps [Current at Pmax]

Depending upon the required final configuration the solar panels may be connected in series or parallel.

As the output of solar panels vary with temperature we do need to know what the rated short circuit current of the chosen panels.

Rated Short Circuit Current of solar panel

3 x 200W solar panels. Each solar panel has a rated short circuit current of 8.21 Amps Sizing the Solar Regulator

The purpose of the solar regulator or charge controller is to regulate the current from Solar panels to prevent batteries from overcharging. A solar regulator senses when the batteries are fully charged and stops the current flowing to the battery and also prevents the battery from feeding back into the solar panel at night when it is dark.

Most solar regulators include a Low Voltage Disconnect feature, that senses the battery voltage and if the battery voltage drops below a a pre-determined level (cut-off voltage) the solar regulator will switch off the supply. Solar regulators are rated by the amount of current they can receive form the solar panels.

NOTE: the solar regulator should be capable of handling the total short circuit current of a solar panel.

From the example shown above we have 2 x 200W and 1 x 130 W solar panels and the solar regulator must be able to handle the increased rating of the short circuit current. Thus, the short circuit current = 2 x 8.21 + 1 x 8.02 [A] x 1.25 [increased rating] = 30.55 Amps

NOTE: Always increase the solar regulator and add an additional 25% capacity to allow for growth and the fact that the solar panels may exceed their rated output.

Solar Regulator = must be able to handle 30.55 Amps.

NOTE: always allow for future growth so size the regulator accordingly.

4. Sizing the inverter

Inverters should always be chosen that would be more than capable of supplying the maximum anticipated AC load. This is always taken to be the combined maximum load for all AC appliances running at the same time. Always allow for loads that have a surge rating, such as motors and fluorescent lights etc. It is advisable to use pure sine wave inverters where possible.

In our example of 150W inverter would be suitable but a 300W rated inverter allows for further growth.

Generating AC from a DC supply source requires an inverter.

Converting DC to AC results in a loss of efficiency for the inverter and energy losses are assumed to be 20%.

We therefore achieve an inverter efficiency of 80%. Thus 892.8Wh / 0.80 equates to 1,071.36Wh per day.

5. Sizing the Battery

Most batteries will last longer if they are shallow cycled - discharged only by about 20% of their capacity. A conservative design will save the deep cycling for occasional usage. This implies that the battery bank should be about five times the daily load. It also means that the system will be able to provide power continuously for five days without any sun or recharging.

The simplest way to determine the total battery amp hours required is to determine the total watt-hours required by all loads and then divide by the DC system voltage. This will result in the amount of amp hours needed to operate all loads for a given period.

The battery size is determined by the DAILY WATT-HOUR requirements and the desired number of DAYS of storage capacity required AND the assumption that the battery will never be discharged less than 20% - (80% remains of its capacity).

The Average Daily Load  = 892.8Wh per day. Add 20% for system losses and safety. Thus load = 1071Wh per day.

If no inverter is used then Average Daily Load (1071Wh)  / System Voltage (12V) = Average Amp-hours per day (89.2Ah per day)

Now take the Average Ah per day (89.2Ah) x Days of Battery Storage (5 days) / Battery Discharge Limit (0.8) / Battery Ah Capacity of your choice = Number of batteries in parallel connection.

DC System voltage / battery voltage = Number of batteries in series.

Total Number of batteries required = batteries in series + batteries in parallel

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NOTE: This solar panel sizing example is provided for information and guidance only. Please seek expert advice before purchasing any items based on the above example.