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(with or without solar)

NB! The examples, prices, installation, household appliance energy consumption and battery life cycles used in this example are based on averages for 2018. The actual usage or costs can increase or decrease significantly depending on a varied field of factors including but not limited to Energy Demand, Quality, Manufacturers, House Hold Appliance Energy Usage Ratings & Installation Difficulty Factors. All figures are rounded to the closest decimal point for simplicity.

A Back-Up system or UPS (Uninterrupted Power Supply) consists of an inverter and batteries that are used during power failures or load shedding
. They automatically detect power failures and transfer the load to the batteries in milliseconds. Thus ensuring no delays or interruptions while transferring between grid and battery power. The info below will help with selecting the correct battery and inverter combination to ensure sufficient power and longevity. (Solar panels can also be added to the UPS if needed, but to keep it simple we won't be addressing that now)


INVERTERS - There are basically two types of inverters used for UPS systems. MODIFIED Sine Wave and PURE Sine Wave:


MODIFIED Sine Wave Inverters electronically imitate PURE Sine Wave inverters to satisfy the minimum needs of electrical devices at a lower cost. Their voltage range will peak together with battery voltages making it unstable and causing erratic behavior in some devices. They are less efficient than their PURE sine wave counterparts meaning they use more power from stored batteries to power themselves than PURE sine wave inverters. A 70% efficiency means that 70% of it's power will go towards powering your devices and 30% is used to power the inverter it self. When using a MODIFIED SINE WAVE inverter to power equipment with electrical motors it will not be able to handle the inrush current required to start the motor unless the inverter is at least 2 or 3 times the size in KW as the electrical motor. Modified Sine Wave inverters should not be used for household appliances with electrical motors that contain variable speed controllers as they will not work. In general electrical motors do not like being powered by modified sine wave inverters (almost all kitchen appliances have electrical motors). MEDICAL EQUIPMENT should never be used with modified sine wave inverters. Their only true benefit is that they are cheap.


PURE Sine Wave Inverters have very little harmonic distortion, is totally clean power exactly the same as generated by power plants and have no adverse effects on electrical appliances. It will not cause erratic motor behavior or weird sounds from monitors, fluorescent lights, microwaves or fridges. They are generally much more efficient than MODIFIED SINE WAVE inverters thus producing more power for longer. Their only disadvantage is that they are more expensive.



BATTERIES - two basic types can be used, FLOODED CELL and LITHIUM Ion. LITHIUM Ion batteries are seldom used in UPS systems. UPS systems are usually only used for black outs and not to be discharged to low levels every day (unlike solar and off grid systems). For this reason more cost effective batteries are normally used.


FLOODED CELL BATTERIES are available in GEL, AGM & LED ACID. There are some benefits and disadvantages to each but we will not be discussing that now. These are then classified as DEEP CYCLE, HIGH CYCLE & DEEP DISCHARGE. DEEP CYCLE means that it can generally be discharged to much lower levels than any of its counter parts. These are normally not used for UPS systems but rather Off-Grid Solar power solutions. DEEP DISCHARGE means that a massive amount of energy can be discharged from the battery in a very short amount of time. These should never be used for any power back-up solution. HIGH CYCLE batteries are normally used for UPS solutions as they do not require to be heavily discharged as often and are more cost effective. These batteries are cost effective at around R2,500 - R3,000 per 100AH. General rules for any flooded cell battery:

- Never fully discharge the battery. Doing so will drastically shorten its life expectancy.

- Optimum battery temperatures should be between 20° and 30° Celsius.

- Charge rates are usually at C0.1 meaning that it will take 5 hours to replenish a half drained (50%) 100A battery.

- The mid quality Deep Cycle battery in the example below has a 5000 (14 year if discharged every day) cycle design life rated at 10% DoD (Depth Of Discharge). This means that it will last for 5000 charge and discharge cycles only if no more that 10% is discharged from the battery with each cycle. Discharging the same battery to 50% DoD each time yields as little as 1000 cycles. Discharging to 80% DoD (20% remaining) yields as little as 500 cycles and discharging to 100% DoD (0% remaining) yields 300 cycles and my cause the battery to fail outright. life cycles of lower quality batteries in blue below is clearly far less. These batteries are used for UPS and back-Up power systems since they don't discharge every day. Remember that these batteries are rated in charge and discharge cycles not days.




LITHIUM BATTERIES are very lightweight, when compared to FLOODED CELL and last much longer at high discharge rates. They are however much more expensive. In general they yield 7000 (20 year) cycles at 70% DoD. In other words, if 70% of the battery's charge is depleted each time they will yield 7000 cycles. But with a cost of around R10,000 for a 1KW (77AH) battery they are a bit pricey. In reality they are only cost effective in larger solar and back-up power solutions of 10KW or more.




I) Calculate - What you would like to power during outages. Check the appliance's labels for power consumption in Watts (per hour). Add all these together eg. TV (200W), Lights (60W), Computer (300W). 200+60+300 = 560W. (this will be the minimum inverter size).

II) Calculate - How long the appliances should be powered for in hours. Lets use 4 hours.

III) Multiply the above in step I with the amount in step II then multiply by 2 to ensure that the batteries will not be depleted below the 50% mark.

(560W x 4Hours = 2240W) (2240W x 2 = 4480W)

Inverter = 560W. As a rule power supplies and inverters must never be permanently operated at their maximum capacity so we will use a 1KW inverter.

Batteries = 4480W (4x 100AH/12V) (Minimum Recommended)


2x 100AH/12V (2400W) batteries, load shedding 4 hours 3 times per week = 1 Year (300 cycles)

4x100AH/12V (4800W) batteries, load shedding 4 hours 3 times per week = 3.1 years (500 cycles)

8x 100AH/12V (9600W) batteries, load shedding 4 hours 3 times per week = 15 years (4000 cycles)


2x 100AH/12V (2400W) batteries, load shedding 4 hours 3 times per week = 0.5 Year (100 cycles, may fail sooner)

4x100AH/12V (4800W) batteries, load shedding 4 hours 3 times per week = 1.7 years (270 cycles)

8x 100AH/12V (9600W) batteries, load shedding 4 hours 3 times per week = 6 years (1000 cycles)




1x 1KW PURE SINE WAVE high quality inverter = R10,000

4x 100AH/12V entry to mid quality batteries = R10,000


LABOUR & ACCESSORIES - Depends heavily on installation type:

Installed with single dedicated 3 pin wall outlet - R2650.00
Standard integration into current electrical system with own DB board - R7500.00 (2KW + Inverters only)