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. 

Fig 1: The map shows the total average solar radiation falling on one square metre surface inclined at 30 degrees to the horizontal, measured in kilowatt hours. 

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

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

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 o 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 flourescent 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% of its capacity.
 
 
The load is 892.8 watt hours per day.  Note: losses are taken in account.

Hence daily load in Amp Hours is Load (Watt hours) / System Voltage (Volts) or 892.8Wh / 12volts  = 74.40 Amp Hours

74.4 Amp Hours x 5 Days Backup (no sun) = 372.0 amp/hours for 5 days without sun.

Add 20% energy losses for battery inefficiency and losses. 74.4amp hours +20% = 89.28amp hours used for 5 days without sun.

We said that we do not want to discharge the battery more than 20% of its capacity so we divide the amp hours by 20%.

89.28amp hours / 20% =446.4 amp hours.

You therefore need 446.4 amp hours of batteries to run your load of 892.8 watt hours for 5 days without sun.
 
BATTERIES SIZE REQUIRED = 892.8 / 200 = 5 x 200 Ah batteries will provide 5 days backup at a discharge rate of 892.8 watt hours per day in a 12V system.