With the high cost of unreliable 3rd world national utility power being greater than $0.35 in some locations, the capital expense of a solar system can be recuperated in three years or less. The solar controllers and inverters are very reliable and the lifespan of a PV panel is 25 years. The cost of these items has dropped dramatically in the past 5-7 years.
There are basically three types of PV systems:
A Grid Tied PV system is directly connected to the national utility power and will only produce electrical power if the national utility is available and within voltage tolerance. If the national utility has a brownout or outage, the PV will shut down within 100 milliseconds and will not turn on again until the national utility is within specifications for 5 minutes. These restrictions are dictated by Federal law and are universally accepted around the world. There is not standby or emergency power available during the utility outage or event. The Grid Tied system is normally used to reduce the electrical utility bill. Care must be taken to communicate with the national utility of the design of the system and its capacity. VERY few 3rd world utilities had BUY/SELL contracts for the exchange or sell of electrical power. The “avoided cost” of electricity purchased from the national utility can be significant.
A Grid Tied with Battery Backup system supplies power to the electrical loads from the utility source when it is available and also charges batteries from the PV Panels. If there is a utility event or outage, the system will electronically switch to battery/inverter mode with the PV panels still charging the batteries. This system will greatly reduce standby generator runtime if implemented correctly and properly sized.
An Off Grid system will supply electrical power from the battery/inverter system with the PV panels charging the batteries. Generally, there is not a national utility in most cases. A diesel generator set may be used as a standby electrical source of the batteries in the battery/inverter system get TOO low in voltage.