Fuel Generators, un-interruptible power supplies (UPS) and hybrid grid tied photovoltaic systems (PV) are the main systems of choice, however each system has its own features and capabilities. A long backup inverter-charger system, basically a UPS with a larger battery storage capacity, is well suited for longer periods of power interruptions, in particular for the residential and small business sector. Unlike fuel generators that cause noise, air pollution and need refilling and maintenance, these systems could be installed indoors in confined spaces, as long as they meet the correct ventilation requirements.
Fully automated UPS/Inverter System
The system is fully automated which reduces human interference, thus reducing the
risk of electrical contact or manual change-over of switches and cables. The
permanently connected system is connected to the mains and constantly charges the
batteries through the charger. During a power failure, the sinewave inverter
automatically converts the DC battery power to AC power to run the permanently
connected essential equipment. The automatic change-over occurs within 15
milliseconds, which is fast enough to supply backup power without affecting standard
equipment. A standard UPS system has an even quicker automatic change-over
speed but the battery backup capability of approximately 5 -10 minutes, which is ideal
for sensitive equipment such as servers and medical equipment.
As soon as the main supply is restored, the automatic change-over reverts back to its
normal power supply to the equipment and recharges the batteries.
Systems can be a stand-alone system or can be wired into the main distribution board. Smaller stand-alone systems could be a mobile unit that includes a battery cabinet on wheels with the inverter-charger on top for the Do-It-Yourself (DIY) market and connected with ease to a standard wall socket. This can power a TV or personal computer, however the hardwired option done by a registered electrical contractor has the ability to supply more equipment from a single correctly sized system. It is best practice to install the hard-wired system in an unoccupied room of the house such as the garage or basement, and store the battery sets in a specifically designed battery cabinet with adequate ventilation vents and which is lockable.
Away from explosive or flammable material. Various battery system configurations exist – from wall mounted to staggered shelf systems to reduce space
The sizing of the system depends on the load and backup duration required. Inverters are available in various power ratings, starting at a couple of 100 Watts to thousands of Watts. An installed equipment load, start-up requirement and backup duration need to be determined and the inverter needs to be sized accordingly. Standard equipment such as TVs, decoders, sound systems, lights, a kettle, security systems, garage and gate motors can be connected to an inverter-charger of 5kW or lower.
However large systems, 5kW and above, are needed for equipment that uses large amounts of power such as your geyser, stove, washing machine, dishwasher, tumble dryer and air conditioning systems – and you need to add up the power ratings in watts for all of these to determine the total equipment load in Watts. Electrical equipment is rated at a continuous load rating, however, some equipment that has electrical motors have a start-up peak rating of 3 to 10 times the continuous power rating.
For this reason, inverters are rated in continuous power and peak power, of which the latter can only be supplied for a short period, usually when the equipment starts up. Both of these ratings need to be considered during the sizing process and it is advisable to oversize the continuous load rating by a minimum of 20%.
Equipment with a resistive load (electrical element) such as a kettle does not have a start-up load, therefore a 2 200 Watt inverter will be able to supply the kettle. In case of an inductive load (electrical motor) the start-up load could be 3 to 10 times more than the continuous load. A typical inverter of 2 000 Watts continuous power and 5 000 Watts peak power is needed to supply an electrical motor with a continuous output of 1 600 Watts and 4 800 Watts start-up power (3 times more than continuous load). This type of equipment increases the size and the costs of your back-up system and home and business owners needs to decide if it’s worth the extra cost.
The battery size is determined by the backup duration needed to supply the installed equipment load and peak power requirements. A worst-case scenario calculation of maximum equipment load over the total backup period and allowable battery depth of discharge will give a good indication of sizing. If some of the load is removed or is intermittent, the backup period will increase accordingly.
The recharge period depends on the depth of discharge (DOD) during a power failure, the charger size and rate of charging. If the system is sized correctly, it will be able to supply backup for a second backup period during the day if adequate recharging time is available, typically 4-6 hours.
The battery type and size depends on the equipment load and duration as well as the required input voltage to the inverter-charger. Smaller inverter-chargers typically require a 12V input, medium sized inverter-chargers at 24V and the larger inverter-chargers at 48V. If a typical 12V battery is selected, 1 battery is needed for the 12V inverter selection, 2 batteries in series for the 24V inverter selection and 4 batteries in series for the 48V inverter selection.
Deep cycle batteries are preferred for a backup application due to their ability to deliver electricity for a long time, even multiple days, because they are designed for constant discharge and charge cycles with a depth of discharge (DOD) of between 60-80%. For the smaller systems the Lead Acid deep cycle batteries are suitable – this includes technologies such as Absorbent Glass Mat (AGM) and Gel.
These battery types are more affordable although the life expectancy is between 3 to 5 years. These are suited for a backup system that don’t go through continuous charge and discharge cycles, this could lead to a longer than specified life span. Larger systems require a more sophisticated battery system that is typically used for renewable energy systems to allow for usage cycles.
Designers of these systems predominately use Lithium type batteries which ensure greater DOD, resulting in more useable capacity, more charge cycles and a life span closer to 10 years. Battery suppliers will specify a disposal methodology for end of life units, since some of the batteries are recycled to salvage the precious metals.
The backup system could be modular, however the battery supplier will specify if new batteries could be installed in series with the older ones. If allowed, a time limitation will be set and typically range between 1 to 2 years.
The minimum maintenance is required for these systems and depends on the quality and type of batteries chosen. By operating the battery according to manufacturer charge and discharge requirements and following the Institute of Electrical and Electronics Engineers (IEEE) recommendations for battery testing, it should be possible to maximize the life of the battery system.
When designing a system, especially larger non-DIY systems, the designer will need the following information:
- Load required in Watts (both peak and continuous power)
- Type of equipment used
- Duration of backup
- Frequency of occurrence
Installation type: stand-alone or hard-wired Various product types exist and designing these larger systems becomes complicated. Therefore it is advisable to make use of a competent designer.
Sinetech. 2019. Backup Systems for Long Power Failures. [Online] Available: http://www.sinetech.co.za/long-backup-systems.html. [18 February 2019].
Omnipower. 2019. Battery backup power system. [Online] Available: http://www.omnitech.co.za/ .[18 February 2019].
Utility Products. 2019. Maintaining Backup Battery Systems: A checklist approach. [Online] Available:
https://www.utilityproducts.com/articles/print/volume-19/issue-10/feature-stories/maintaining-backup-battery-systems-a-checklist-approach.html . [18 February 2019].
SPW. 2014. How to choose the right battery for your solar project. [Online] Available: https://www.solarpowerworldonline.com/2014/02/choose-rightbattery-solar-project/. [18 February 2019].
Solar Training Centre SA. 2017. Introduction to Solar. Potchefstroom: SUNCYbernetics (PTY) Ltd.
Lang, Z. 2012. Battery Recyling as a Business. [Online] Available: https://batteryuniversity.com/learn/article/battery_recycling_as_a_business . [18 February 2019].
Compiled by Eskom Corporate Affairs January 2019 www.eskom.co.za/idm