How to choose the right UPS battery backup

May 7, 2020

What is an Uninterruptible Power Supply?

UPS (Uninterruptible Power Supply) systems, also known as UPS battery backup systems, provide backup power to your electrical devices/ appliances during a power outage, and protect your connected equipment from power surges. It consists of rechargeable batteries, an inverter and a battery charger all stored in one box. Some UPS systems are designed to protect sensitive electronics and IT equipment from very short power cuts. In this article we focus on UPS systems that can supply you with battery backup power for several hours, effectively alleviating the impact of load shedding in South Africa. An electrician can connect the UPS directly to your main power distribution board so that it automatically switches on during load shedding or unplanned power outages.

An alternative way to use a backup system is to connect it directly to your equipment during a power outage, such as your fridge, TV, router or laptop, etc. Most UPS's easily connect to a standard wall socket and are designed to instantly provide your home or workplace with electricity when your power goes off.

A UPS system includes the following components: a charger to charge your DC batteries from your mains, a set of batteries to store electricity and an inverter to convert the electricity stored in your batteries into 220V AC. Some UPS systems include an automatic transfer switch, to automatically power your devices from the batteries when the mains power is interrupted. When AC power is obtainable, the charger recharges the batteries. When AC power is disconnected, the inverter supplies electricity from the batteries.

Choose the right type of inverter: pure sine wave vs. modified sine wave, inverter charger and power rating

The sine wave type refers to the "shape" of the Alternating Current (AC) produced by the inverter: "pure sine wave" is a sinusoidal shape, whereas "modified sine wave" is a blocky shape. The single-phase current used in South Africa is a pure sine wave alternating current, with a phase of 50Hz (50 oscillations per second) and a voltage of 220V-240V.

Always check if the inverter is a "pure sine wave inverter" or a "modified sine wave inverter".

Pure sinewave inverters (PSW)

Pure sine wave inverters are suitable to safely run a fridge, a freezer, a pump or any other device/ appliance that contains an AC motor. They can power any AC appliance sold in South Africa without any risk of damage.

Modified sinewave inverters (MSW)

Modified sine wave inverters are more cost-effective and suitable for most electrical devices that do not contain an AC motor (laptops and lights particularly).


Inverter chargers have a built-in battery charger, and some include an ATS (automatic transfer switch) feature. They are a combination of an inverter, a battery charger and an automatic transfer switch in one complete system. Inverter chargers can be pure or modified sinewave.

Power rating: constant (continuous) load vs peak load

An inverter rated at 500W (constant)/ 800W (peak) can continuously power a load of maximum 500W, with a peak load that never exceeds 800W. If the load is constantly over 500W or a peak exceeds 800W, the inverter will switch off and might get damaged. Beware that many appliances require a peak power of 2 to 3 times their rated power consumption when switched on: a fan rated at 100W might necessitate up to 300W peak.

Batteries for UPS power back up systems

OmniPower 100Ah Lead Acid battery
OmniPower Lead Acid battery, 100Ah 
battery comparison chart
Battery Comparison Chart; 2020 indicative prices, incl. VAT

Deep cycle lead acid batteries: flooded, gel and AGM

Lead acid batteries are widely used in South Africa. Many are designed to provide short, intense bursts of power (car batteries). Deep cycle batteries are designed to provide power over several hours before recharging, hundreds of times. Deep cycle lead acid batteries are mostly of 3 types: flooded, get and AGM.  

Gel and AGM batteries

The electrolyte is contained in a gel or in an absorbent glass mat (AGM), keeping it in place. These batteries are sealed and can be moved or put in a vertical position. They should never be discharged by more than 50%, as this will cause permanent damage and greatly reduce their life span. Gel and AGM lead acid batteries drained at 50% typically take 5 to 10 hours to charge.  Used correctly, gel and AGM batteries can last 600 to 800 cycles of discharge/charge before their charge capacity is degraded by more than 20%.

SMART POWER Deep Cycle Gel Battery, 100Ah, 12V

SMART POWER Deep Cycle Gel Battery, 100Ah, 12V

Flooded lead acid batteries

Flooded lead acid batteries have become much less popular in recent years as they are quite cumbersome to use: the electrolyte is not sealed, and the evaporated water must be replaced: you need to add distilled water at least every month to each battery.

These batteries cannot be moved much and should always be in a horizontal position. They require their own separate battery room due to the potential chemical hazards they pose. The performance of flooded batteries is comparable to that of AGM and gel batteries.

Lithium batteries: Lithium-Ion and Lithium-Iron-Phosphate

Lithium batteries prices have come down in recent years and they now effectively compete with lead acid batteries. They tend to cost more, but last longer, require no maintenance, and are very compact.

Lithium-Ion batteries

Lithium-Ion batteries are the most compact. They require no maintenance have hundreds of charge/discharge cycles (anything between 400 and 1000) and can be drained 100%, without any damage or loss of performance. These batteries take as little as 2 to 3 hours to recharge to 100%. Lithium-ion batteries have been previously known to be more volatile due to reported cases of fire incidents, and due to their much higher specific energy combined with a greater sensitivity to being over-charged. However, substantial progress has been made over the years making them safer, more stable, and much more comparable safety-wise to other commonly used batteries.

lithium-ion battery

"Lithium-ion battery of Varta", by Claus Ableiter, licensed under C.C by 4.0

Lithium-Iron-Phosphate Batteries (LFP batteries)

LFP batteries are used extensively in solar applications. They are less compact than Lithium-Iron batteries and should not be discharged more than 80%. Their main benefit is that they typically last 2000+ cycles without losing more than 20% of their rated capacity, making them extremely durable. LFP batteries are also very safe to use and store.

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