Solar PV Inverters: Pure Sine Wave vs Modified Sine Wave

An inverter converts power stored in batteries or generated by solar panels from DC (Direct Current) to AC (Alternating Current) power – which is needed for most household electrical appliances.

The constant DC voltage is changed to a sine wave curve that alternates both above and below 0 volts. In the first DC to AC inverters, this sine wave curve was crudely replicated by making the voltage go straight up and down, creating a blocky signal called a modified sine wave. More advanced modified sine waves make multiple steps, trying to come close to a pure sine wave.

Pure Sine wave inverter vs Modified Sine wave

DC Voltage, Modified Sine Wave, and Pure Sine Wave Graph

Here is an image of a modified sine wave (left) vs a Pure Sine wave (right), as shown on an oscilloscope.

Pure Sine wave inverter vs Modified Sine wave

Modified Sine Wave and Pure Sine Waves on Oscilloscope

Modified Sine Wave

A modified sine wave inverter is suitable for basic systems without delicate electronics or sensitive audio equipment (the choppy wave outputted by the inverter can produce hum and other unwanted noise and artifacts). Brused motors are generally fine to use with a modified sine wave DC/AC inverter, however, some eletronic devices such as digital displays may misbehave, and battery rechargers might not work at all. Other equipment may appear to work fine, but will run hotter than with a pure sine wave – and thus their lifespan is reduced.

Please note! A modified square wave inverter will not work efficiently (or possibly not work at all) with a large number of modern electronics:

  • Any device with an electrical thyristor (photoscopiers, Laser printers etc)
  • Anything with a silicon-controlled rectifier (SCR), like those used in some washing machine controls
  • Some laptop computers
  • Some fluorescent lights with electronic ballasts
  • Some cordless tool battery chargers
  • Biomass boilers with microprocessor controls
  • Digital clock radios
  • Appliances with speed/microprocessor controls (like some sewing machines)
  • X-10 home automation systems
  • Medical equipment such as oxygen concentrators

The higher total harmonic distortion produced by modified square wave inverters cause electric motors to become hotter, run less efficiently and consume up to 30% more energy. This excess heat will also reduce the lifespan of the motor. Another downside to modified square wave inverters is that they can cause audio equipment and other appliances such as microwaves to “buzz”.

You can think of a modified sine wave inverter vs a pure sine wave inverter like a vehicle with square wheels vs round wheels. Both may have the ability to transport passangers, but some will be unhappy during the journey, and some won’t be able to manage the journey at all!

Pure Sine Wave inverters are the best choice for most electronics

Gid connected Solar PV Inverters

A pure sign wave inverter is required if you are connecting your solar PV system to the National Grid to sell electricity back and recieve feed-in tariff payments. The inverter must also match and syncronise the frequency of the alternating current (AC) electricity coming from the inverter to be the same as the National Grid in able to export electricity to it.

A pure sign wave inverter is usually needed to power modern electronics such as LED TVs, low energy compact fluorescent (CFL) light bulbs and inductive loads like brushless motors. Clocks and audio equipment will also behave better on a pure sine wave, as well as cheaper electronics which are built with lesser quality power circuitry.

Best price inverter: Modified vs Pure Sine Wave Inverters

Modified sine wave inverters are less expensive than pure sine wave inverters, and for this reason are still used in simple solar PV systems. However, advances in inverter technology has reduced the cost of the more useful pure sine wave inverters, making them more affordable and thus the much preferable option.

Solar PV Inverter & Battery Banks: 12V, 24V, or 48V

Solar PV battery banks are usually made up of one more deep cycle batteries with a common voltage – either 12V, 24V or 48V.

Once the battery bank has sufficient charge to draw useful power from it, the off-grid inverter converts the 12V, 24V, or 48V direct current (DC) battery bank to a usable 230V AC voltage which can power your household appliances.

Backup Charging Solar PV Battery Bank with Mains or a Generator

Most off-grid solar PV inverters are unable to export extra power back to the UK National Grid. However, an off-grid inverter may still be able to connect to the the UK mains in order to draw power from the grid to charge your batteries if the voltage drops too low. Similarly, many off-grid homes connect a generator to the AC input of an inverter/charger to top up the batteries when needed – this is especially useful in the winter when we might not get enough sunlight to keep the batteries full.

When choosing an DC/AC inverter, you must determine the maximum electrical load you will draw (in Watts) for all the appliances that may be on at the same time.

You must also ensure that your inverter can handle any surges or peaks that might happen when electrical motors first turn because they can draw up to three or four times the rated wattage of the motor at starup. Inverters are rated in both continuous wattage and surge.

Additionally, you need to choose a voltage for your battery bank which your inverter can match. The voltage of your battery bank is either 12V, 24V, or 48V.

Some inverters have the ability to charge the battery bank from an AC source like the National Grid or a generator – some can even automatically start the generator to power the built-in AC charger in the inverter to charge up the deep cycle batteries when their voltage becomes too low, and turning it off when they are charged.

Since the solar PV inverter is often installed in a place which is out-of-the-way, such as in the loft, a remote monitor is often used so that the generation can be viewed conveniently from the main living area. Some inverters can also be monitored remotely via the web.

Some inverters can also be stacked to increase the voltage, or the current, or both. This allows multiple inverters to be configured in a master/slave configuration, automatically turning on only the inverters as needed, conserving battery power, as you are not providing power to the second inverter when it isn’t required.

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