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Choosing an Inverter for your Solar Power System

Graph of square wave AC voltage against time

The Main Inverter Types


1. Square wave or modified sine wave.
2. Sine wave (sometimes described as "Pure Sine wave")
3. Grid-Tied

Square Wave or Modified Sine wave

The square wave form will be as shown above right and the modified sine wave form will have had some attempt to round the corners off though will still have some sharp corners or spikes.

Compare this to the Sine wave form below right.

Graph of Pure Sinewave Voltage Against Time

Many AC appliances will work perfectly well a modified sine wave form wave.
Some appliances such as computers, televisions, radios or music centres have in built power supplies that reduce the voltage, rectify it to produce a DC current, and smooth it to give a steady DC voltage.
This process will often smooth out any spikyness that was in the original AC supply.
However, any inductive load (one where the power passes through a coil, as in a power supply transformer or a motor) causes the voltage and current to be out of phase (their appropriate graphs do not line up). Modified signwave inverters do not cope with this so well, causing the appliance to use more power than it would otherwise. This extra power consumption will cause the motor or transformer to run hotter than it would otherwise and may reduce it's life.
It will also mean that the inverter will need a slightly higher power rating to power the same appliance.

There is also the possibility that your television picture may not be as good as it should and anything with a timer (eg bread maker) may not run at the correct speed.

There may also be a noise problem. Any equipment that may give a quiet hum when connected to the mains supply, is likely to give a more annoying buzz. My own experience has shown this to be true with a ceiling fan, particularly when running on the lower speeds.
These potential problems will need to be balanced against the price difference (modified sine wave converters will be significantly cheaper than pure sine wave) taking into account the appliances you expect to be using.

Grid-Tied

A Grid-Tied inverter is capable of synchronising with an existing mains electricity supply (synchronising its sine wave output so that it is at the peak voltage point at the same time as the mains supply). This type of inverter can be used (where your electricity utility company allows it and with a modified meter if required) to enable you to push your spare electricity into the grid system. In some cases your normal electricity meter will simply run backwards when you are supplying power.
A grid tied inverter designed to be used without a battery (and therefore no charge controller), may have MPPT technology built into it's input circuitry.

String Inverters

Inverter designed to accept high input voltages (upto 600 volts in commercial systems) may be called String Inverters, refering to the series connected panels, used to produce the higher voltages, being connect as a string.

DC Input Voltage

You may already have the rest of your system setup and you are already committed to using a particular voltage. You may however still be able to choose.
The lower the input voltage you are using, the higher the current you will need to use. If you compare a 12 volt and a 24 volt inverter of the same power rating, the 12 volt item will need to draw twice the current. To carry that current, the cables from your battery to the inverter will need to be 4 times the size.
A higher voltage system is likely to be more efficient although you will find that most inverters on the market are either 12 or 24 volts. A 48 volt inverter will be more difficult to find and may therefore be more expensive.

AC Output Power

Any inverter will have a quoted output power which will be the maximum power level they can provide continuously, measured in watts or killowatts. Inverters will normally however cope with higher levels of power for a short period, enabling them to deal with a short power surge that many appliances will draw at turn on. Practically all electrical appliances will draw extra current for a split second at switch on, including low energy light bulbs.
The power output characteristics will vary between different inverters but they may be able to produce 10% over the rated figure for 5 minutes, 50% over for 5 seconds, more for 1 second.
Continuous output power capabilities of any inverter may be affected by the battery supplying the DC input voltage. The battery will need to be large enough to be able to supply the high current needed for a large inverter without the battery voltage dropping too low (causing the inverter to shut down).
Continuous output power capabilities may also be affected by the ambient temperature. An inverter that is producing high power will produce heat that is normally dissipated with the help of a fan. If you are experiencing high air temperatures, your inverter may not be able to cope with continuous high outputs without over heating and shutting down.

To check your maximum power requirement, try our Power Requirement Calculator