The concept of a (1) normal power back-up system differs somewhat from a (2) grid interfacing power back-up and we refer to the latter as an off-grid system.
Furthermore, there is a third option for Photovoltaic solar solutions, referred to as a (3) Grid-Tie system, in which there is NO Backup power available. It is purely a system which generates and supplies power when the Sun is up and photovoltaic energy harvesting is possible, only really suited for reducing electricity bills.
Very briefly, the differences are as follows:
* Power Backup – This system usually employs a normal power dc ac inverter or UPS type inverter. Whenever the main grid power (Eskom) is available, it will feed the load via Eskom power. At this time the battery (energy storage) is being charged by either Eskom and/or via Solar panels. This system does not have the intelligence to give preference to whatever alternative energy is available at that time.
* Note that it is also possible to build up a small system on which Eskom, or the solar panels only, charge the solar batteries, and the load is actually always connected through the power inverter, and also works from the power dc ac inverter- however, this is not the system discussed here. These are the most common systems employed, as they usually utilize a "lower cost" Inverter/Home Ups/UPS, or inverter-charger with an automatic transfer switch.
* Remember these systems are purely for backup power i.e. a homeowner or office need power for a certain period (8 hours) whenever ESKOM falls away. It is possible to use very good (and preferable) inverters. The focus is not so much on energy saving or optimisation, but to focus on a backup power facility.
Hybrid power backup system – these systems are more intelligent. Here a more expensive type power inverter system is generally used - such as those supplied by Victron or Outback, etc. The concept is as follows:
* System is a Hybrid power backup system, the input energy sources are usually a Solar Panel array and Eskom, Solar Panels and a Generator, or even Solar Panel, Wind Generator and Eskom, as examples.
* The System is designed to offer a power backup facility for when Eskom would not be available
* The power backup period and autonomy is therefore determined by the size of the battery pack
* When the "safe"(full state) storage level of the battery is reached, all additional energy from the alternative energy source (Solar Panels) is then used as the main energy source to the load. However, when the load's requirements exceed what is available from the alternative energy source, only then is the additional power required, drawn from Eskom (or generator).
* This kind of systems can successfully be used for a system from as small as 750 Watt, up to practically around 24kW. (bigger systems are possible, but for a household this is typically a maximum)
* The ratio of Solar Panel array versus Inverter, size is determined by optimal appliance draws and, for battery bank sizing, the storage time required. For instance, it could be logical to have a 9kW inverter, with only 1kW (6kW.h) of solar panels and 12 kW Worth of batteries.
* The power Inverter size is determined by peak load employed
* Single, and/or 3 phase systems, are possible
* Battery backup autonomy can vary from 3 hours backup-up, right up to 72 hours.
These systems are the most common and practical systems for a household or office who want to convert to alternative energy and have the option of using their photovoltaic solar panel (PV) system as an energy source, which can be expanded with time, but will also have Eskom as a convenient backup. However, a well-designed system allows you a backup when it’s cloudy and ESKOM has failed.
Grid Tie system – these systems are very intelligent, usually bigger systems, and a Battery power backup does not form part of the greater system.
* These systems are usually not practical for homes/dwelling but more for offices - which have only day-time occupancy.
* These systems are Grid-tied solar solutions in such a way that whenever photovoltaic solar energy is available, it will use that power as first priority, but when no photovoltaic solar energy is available, it is then drawn from Eskom.
* The photovoltaic solar energy on these systems is usually linked to the rating of the inverter itself. E.G. a 3KW inverter will have a 3kW solar array. Power from the alternative source is thus only available when the sun is shining, or with certain wind turbine systems, when the wind is blowing.
* Inverters used here are typically SMA, Victron and for Big systems of 100KW upwards - Hitachi/ Siemens.
* The understanding of the differences between the various systems is very important in order to understand the respective cost implication associated with each system.