An off-grid solar PV system is a solar PV system that, as its name suggests, works off the grid. So in other words, it is not connected to the grid.

The main use of off-grid solar PV systems is in places where the grid isn’t available like the rural parts of India. They are also very useful in applications like street lights, street signages, traffic lights where the grid might be close by but it is more convenient to have a standalone or off-grid solar PV system.

The figure above shows what an off-grid solar PV system looks like.

  • Solar PV modules generate DC electricity.
  • The solar charge controller takes the DC electricity generated by the solar PV modules and stores it in the batteries. Typically, the solar charge controllers also do one other job which is called Maximum Power Point Tracking, or MPPT in short. More information on maximum power point tracking can be obtained in the solar charge controller FAQs section.
  • The batteries convert the electricity (or electric energy) into chemical energy and store it in them. Normally, batteries used along with inverters are called into action (or come into play) only when there is grid failure. However, in case of off-grid solar PV applications, these batteries are called into action every single day. Also, they are discharged quite a bit, by as much as 80%, which is the maximum that you can discharge them without impacting their lifetime adversely. This is done so that the batteries are in a fully discharged condition when the sun rises again so that all the electricity generated by the solar PV modules can be stored in them again.

That is why the batteries used in solar applications are deep discharge batteries, or batteries that can withstand daily deep discharges.

  • The inverter converts the chemical energy stored in the batteries into electricity (or electric energy). The input to the inverter (energy stored in the batteries) is DC whereas its output is AC. This off-grid solar inverter is very much like a normal inverter, but is called a solar inverter because it is used in a solar application.
  • The loads are household appliances like fluorescent lamp, tube light, fan, refrigerator, TV, computer, pumps, among others.

The main advantage of an off-grid solar PV system is that it can provide electricity to remote places where the grid is not available, especially in a sun-rich country like India where most places get good sunshine for 8 to 10 hours a day and for at least 300 days in a year.

The main disadvantages of off-grid solar PV systems are:

  • Cost: Off-grid solar PV systems are quite costly. Solar PV modules are quite costly and contribute a god percentage of the overall system cost. Additionally, off-grid solar PV systems also require batteries, and that too deep cycle batteries, which are quite costly too. That increases the system cost as well.
  • Maintenance: Since off-grid solar PV systems need batteries, it automatically means that they need to be maintained properly. This mainly involves adding distilled water to the batteries (or “topping up” as it is popularly called) if they are of the flooded lead acid type, which they are in many cases. Although flooded lead acid batteries have advanced quite a bit in the recent years and don’t require frequent topping up, it still needs to be done and can be cumbersome if the place of installation is not easily reachable.
  • Recurring expenses: Off-grid solar PV systems need batteries, and even if you maintain them well, they will wear out in anywhere from 300 to 1000 cycles. (In most cases, these are typical cycle numbers. Batteries that last longer are prohibitively expensive in which case it is what the first disadvantage talks about.) So off-grid solar PV systems necessarily have recurring expenses associated with them.

There are many types of off-grid solar PV systems:
  1. Solar power packs: They can vary from small to large size. The small ones have only a DC output and are used to charge mobile phones and run other DC applications. The larger ones have an inverter as well and are used to AC appliances.
  2. Solar home lighting systems: These can vary from small to large sizes. The small ones can be used to power 1 lamp while the larger ones can be used to power multiple lamps. Typically, solar home lighting systems have only a DC output. So the lights that they power should necessarily be DC powered.
  3. Solar lanterns: These can have the solar PV module and battery integrated into the product, or have the battery integrated into the product but have a separate solar PV module in case of higher capacity lanterns. Typically, solar lanterns allow the users to select the luminosity level; lanterns run for fewer hours on higher luminosity levels and for longer hours on lower luminosity levels. Solar lanterns are always DC powered.
  4. Solar street lights:Solar street lights comprise of pole, luminaire, solar PV module, solar charge controller, and battery. The luminaire is always DC powered. It almost always has an IP65 environmental rating since it is exposed to the elements. In many cases, the charge controller is embedded inside it. In some cases,even the battery is embedded inside it. If that is the case, then the batteries are invariably Li-ion batteries since they have to be compact.
  5. Solar signages (or signboards): Solar signages or signboards are very much like solar street lights and have the same components as compared to solar street lights.
  6. Solar pumps: Solar pumps have solar PV modules and a three-phase inverter which drives the pumps. Solar pumps do not have batteries since there is no need to store the energy. Whenever the solar PV modules produce electricity, it is used to pump water from the well to the ground level (actually in storage tanks that are slightly above the ground level). And since solar pumping systems do not have batteries, they don’t have solar charge controllers as well.