What is the Difference between a Pure-Sine wave and a Modified-Sine wave with respect to Inverters?  

In this regard, a sine wave or sinusoidal wave is a voltage supply in the form of AC (Alternating Current). In South Africa, a household plug socket supplies a voltage of ±230 Volts AC. Where ‘230’ is the amplitude of the wave.

Figure 1 below represents what a sine wave looks like measured over a period of time.Figure 1: 230V Sine wave.

As depicted in figure 1, the amplitude alternates between +230V and -230V. The frequency of alternation of the supply in South Africa is 50 Hertz, meaning 50 times per second.

How is the voltage from the battery converted to AC?

The voltage supply from a battery connected to a general household inverter will be in the range of 12-24 Volts. However, this voltage is DC (Direct Current) and must be converted to AC in order to run household appliances, hence and Inverter is required.

Figure 2 (a) depicts a general DC voltage of 24 Volts. (b) Follows a square wave created from the DC voltage that switches from positive to negative at specific time intervals.  (c) The square wave is then transformed into a modified sine wave.

Figure 2: a) 24V DC, b) 24V square wave, c) 24V- 3 step modified sine wave.


The inverter then amplifies the modified sine wave to 230V allowing compatible home appliances to be powered directly from the inverter.

What is the difference between a Modified Sine Wave and a Pure Sine Wave?

As shown in figure 1, a sine wave is a smooth alternating voltage. Figure 1 in essence is a pure sine wave. But when dealing with inverters the sine waves are not generated by a motor/generator/turbine but rather a DC voltage is modified electronically to produce a sine wave as in figure 2. Therefore manufacturers will refer to the inverters output voltage as a “Modified Sine Wave”. This is more of an estimation of what a sine wave should be.

However, some manufacturers distinguish their inverters that produce much less distortion than the wave shown in figure 2 (c) by calling it a ‘Pure sine wave inverter’. I.e. more steps are used between the positive and negative alternation. The more steps used, the smoother the signal and the more ‘pure’ the wave appears to be. However the more steps involved, the more complex the inverter will be and hence more expensive.

Figure 3 shows a comparison of a sine wave , a simple modified sine wave and a pure sine wave that some complex inverters are capable of.

Figure 3: Overlay of a Sine wave, Modified Sine wave and Pure Sine wave.

For general household appliances such as a TV, PC, lights, etc… a modified sine wave is perfectly suitable.

What devices will not operate on a Modified Sine Wave?

Any appliances resistive in nature (e.g light bulb) or use a switch mode power supply (e.g motor) will run on a modified sine wave.

Devices with a built in clock that derive their time from the incoming frequencies may not function correctly. The function that provides the switching can be triggered at the steps instead of at the zero crossover point. This applies to biomedical devices that are used to monitor heart rate and oxygen cylinders.

Some cheaply made power supplies may also not function correctly. This will present itself in the charger heating up beyond its normal operating temperature.

A device called a line conditioner can be used between the output of the inverter and the non-operational device if needed. A line conditioner or power conditioner provides a “clean” AC power to sensitive electrical equipment [2]. This is an additional device that is separate from the inverter. Currently Switchman Products does not supply line conditioners.

For more information on Inverters and change over panels, please contact joshb@switchman.com

What is the Purpose of the Home/Office Inverter System? 720W / 1440W


  • Long run battery backup to run computers, TV, DSTV, lights, security and garage doors for 3 to 8 hours.
  • Suitable for home and office.
  • Plugs directly in to a wall plug – just plug your appliances in to the Inverter for uninterrupted power.
  • Automatic recharging when Eskom power is restored.
  • Add an additional battery to double the battery time.
  • Different power ratings available depending on your requirements. From 1 KVA to 2.4 KVA
  • Standard plug and play Inverters come complete with batteries.

1000­­ VA / 600 Watt Inverter with 1 battery will power a TV, DSTV, Computer a few lamps and cell phone charger for up to 4 hours.

2400 VA / 1400 Watt with 2 batteries will power 3 or 4 TV’s or 3 or 4 computers, DSTV a few lamps, cell phone chargers and an Internet router for up to 4 hours.

These units can also be used to power intercoms, gate motors and garage doors. The system can also provide a backup supply during extended outages for vital equipment such as alarm systems and security cameras. Figure 1 depicts some of the appliances that are/aren’t compatible with an Inverter.

Figure 1: Reference for some of the appliances that can/cannot be used with an Inverter. 


The unit is permanently connected to a wall plug so that while mains power is present the built-in battery charger recharges the batteries and keeps them fully charged until a power failure occurs. The equipment requiring backup is also permanently connected to the Inverter via the connector provided. In case of a power failure the backup system automatically switches over to the Inverter, which will continue to provide power to the equipment. This is extremely fast and standard equipment like TVs, DSTV decoders, fans, routers etc. are unaffected. The systems can also backup computers but it is possible that a very small percentage of computers could reset during the switchover time.

When mains power returns, the whole procedure is reversed and the unit will switch back to mains power and will automatically start re-charging the batteries. Your equipment remains connected to the system even when power is restored. The whole process is fully automated.


The amount of backup time is determined by the size of the connected battery bank and the load on the system. By removing half of the load, the backup time can be extended to double. For example, if the system is setup to power two computers for 4 hours, by removing one computer the backup time can be extended up to 8 hours. For added functionality, the system includes an LCD display that provides visual feedback of battery charge and load allowing the user to monitor usage without concern.


The recharge time of the batteries depends on how much power was drawn from the batteries during a power failure. The recharge times can take up to 8 hours from a fully discharged battery.


In order to get the best value out of the inverter all non-essential appliances should be switched off. This will reduce the load on the battery and extend its running time. Always keep the inverter plugged into the wall socket to allow for automatic recharge when the mains power returns.

For more information and pricing on Inverters please contact joshb@switchman.com or visit www.switchmanproducts.co.za