SOLAR INVERTERS

Battery World’s Absolute’s philosophy is to supply and install systems that are going to provide high efficiency and remain reliable over many years into the future. There is a significant difference in the Type of Inverters available and our experts will help you on choosing the appropriate Model you’re your solar PV panel system.

What an Inverter Does

Unless you have the most basic system providing a low voltage DC power source, an inverter will be necessary to provide an alternating current at a higher voltage.
An inverter uses a DC (direct current) power supply and creates an AC (Alternating Current) supply, usually at a voltage similar to that of your normal mains supply. In other words, it enables you to run household appliances from a low voltage DC supply such as a car battery or a more sophisticated solar power system.

 

How does an inverter work?

An inverter will use transistors to continually switch the direction of the voltage or in an oscillator to produce a sinewave.
This switching or oscillation will happen 50 or 60 times a second, according the frequency required.
We have more detailed information on the workings of inverters here: How Inverters Work

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A square wave displayed on a graph of voltage against time will be produced by a simple switching of the voltage direction.

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A sinewave displayed on a graph of voltage against time will be produced by an oscillator
Some smaller inverters are described as producing a modified sinewave and will have a waveform somewhere between the two shown. Our experience is that they produce something much nearer to a square wave.

The sinewave is the waveform produced by the power stations that produce the mains power, and this is the ideal.

The inverter circuitry is likely to produce this waveform, at the voltage of the DC supply (your battery) but at a low current. This will then be amplified by further transistors to make a much higher current available.

All that needs to be done now is to increase the voltage to that required (usually the same as your normal mains supply).
This part is relatively simple as a transformer can be used. The transformer will consist of two coils of wire (named primary and secondary), wound together round an iron former. If the secondary coil has double the number of turns as the primary, an AC voltage applied to the primary coil will cause double the voltage to be produced in the secondary.
Transformers only work with alternating currents.

 

Choosing an Inverter for your Solar Power System

 

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.

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Many AC appliances will work perfectly well with a modified sine wave form wave.
Some appliances such as computers, televisions, radios or music theatres 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 spikiness 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 sinewave 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 its 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 (e.g. 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. 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 synchronizing with an existing mains electricity supply (synchronizing its sinewave 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 its input circuitry.

String Inverters

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

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.
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 kilowatts. 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 overheating and shutting down

 

How Much of your DC Power Comes Out as AC Power?

 

By efficiency, we are really saying, what percentage of the power that goes into the inverter comes out as usable AC current (nothing is ever 100% efficient, there will always be some losses in the system). This efficiency figure will vary according to how much power is being used at the time, with the efficiency generally being greater when more power is used.
Efficiency may vary from something just over 50% when a trickle of power is being used, to something over 90% when the output is approaching the inverters rated output. An inverter will use some power from your batteries even when you are not drawing any AC power from it. This results in the low efficiencies at low power levels.
A 3Kw inverter may typically draw around 20 watts from your batteries when no AC current is being used. It would then follow that if you are using 20 watts of AC power, the inverter will be drawing 40 watts from the batteries and the efficiency will only be 50%.
A small 200W inverter may on the other hand only draw 25 watts from the battery to give an AC output of 20 watts, resulting in an efficiency of 80%.

Larger inverters will generally have a facility that could be named a “Sleep Mode” to increase overall efficiency. This involves a sensor within the inverter sensing if AC power is required. If not, it will effectively switch the inverter off, continuing to sense if power is required. This can usually be adjusted to ensure that simply switching a small light on is sufficient to “turn the inverter on”.
This does of course mean that appliances cannot be left in “stand-by” mode, and it may be found that some appliances with timers (e.g. washing machine) reach a point in their cycle where they do not draw enough power to keep the inverter “switched on”, unless something else, e.g. a light, is on at the same time.

Another important factor involves the wave form and inductive loads (i.e. an appliance where an electrical coil is involved, which will include anything with a motor). Any waveform that is not a true sine wave (i.e. is a square, or modified square wave) will be less efficient when powering inductive loads – the appliance may use 20% more power than it would if using a pure sine wave. Together with reducing efficiency, this extra power usage may damage, or shorten the life of the appliance, due to overheating.

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