What makes a stand alone PV system different from a grid connected system is that it's not connected to the grid.

The PV modules are responsible of producing enough energy to meet the load requirement.

Excess energy is stored in a battery bank which in it turn provides electricity to the connected loads when the sun is not available.

The brighter the sunlight, the more the voltage the PV modules produce, then, a charge controller is used to prevent overcharging or excessive discharge of the battery bank.

The brighter the sun, the more the voltage but, what if a string of cloudy days happens? Or, what if you draw more power than what the battery bank can support? Off course the battery bank will get empty, the system will go down and you will run out of power. To prevent this, the battery bank is over sized. In the same goal, off grid solar PV systems are often supported by a generator set or a wind turbine to enable continuous charging of the battery bank. It is also advisable to reduce load demand: all the connected appliances must be the most energy efficient.

Power from the PV modules up to the battery bank is DC. If AC loads has to be connected to the system, an inverter is then necessary. The inverter is responsible of converting DC power from the battery bank into AC power usable by most of appliances.

Four major issues arise when designing a system:

• The load (power) required to be supplied by the system is not constant over the period of one day;
• Daily energy usage varies over the year;
• Energy available from the PV array may vary from time to time during the day;
• Energy available from the PV array will vary from day to day during the year.

Since the system is based on photovoltaic modules, then a comparison should be undertaken between the available energy from the sun and the actual energy demands. The worst month is when the ratio between solar energy available and energy demand is smallest.

Basic system design process follows 4 steps:

Step 1. Determination of the energy usage that the system must supply.

Step 2. Determination of the energy input required from the PV array

Step 3. Determination of the battery storage required.

Step 4. Selection of the remaining system components.

Additionally, off grid PV systems design is influenced by budget and site constraints.

Again here, the importance of energy efficiency is revealed; making small changes on the way you use energy could make a difference to the size of the system you need and therefore help fit in the budget.

Solar modules produce more power when they are pointed directly to the sun. Designer should inspect the site and find out to comply with this requirement. Additionally, suitable areas should be found where controller, battery bank and inverter will be located.

Furthermore when designing a system, it must comply with local electrical standards requirements.