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101 renewable - off grid power inverters

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Last Updated
30th of September, 2018

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off-grid plant

Stand-alone PV power plants are not connected to the grid and consist of PV panels as power generators and a storage system which will guarantee electric energy supply when lighting is poor or is dark. Since the current delivered by the PV, generators  is DC (direct current)  we will need an  AC inverter.  This Off Grid Inverter becomes the core of the B.O.S. (Base of System) and in most of the installations works as a charger for the battery bank. In an Off Grid configuration, the PV array it is over-dimensioned in such, during the high irradiation hours both the power supply to the load , as well as the recharge of the storing batteries, a be guaranteed.  Off grid, inverters are designed to handle battery banks and external conventional generators (petrol base) at the same time. Some of the off-grid inverters are Hybrid Inverters. They have a multiple functionalities, they work off grid, grid tie and as backup energy generator and they have the capacity to connect to AC through phase coupling and feed in energy from the PV plant when the production exceeds consumption and the battery bank it is charged, or to juice energy from the AC grid and charge the battery bank when the battery bank is on low and the PV plant does not generate enough energy to help in process. In the case of a grid outage they work as a power generator and they do supply the critical load. They are called Smart Grid Inverters.

Common functions build into Off Grid Inverters:~

>_Charges Batteries While Supplying.
>_Cleans and Regulates Power Output.
>_Correlating with an External Petrol Generator Set.
>_Efficiently Handles Surge Requirements of Electric Motors and Pumps.

Choosing an Off Grid Inverter requests some of the Inverter specific values to be correlated to the PV array output, to the battery bank and even to the petrol based power generator set.

Here is a list of Off Grid Inverter parameters:~

#Waveform (sine wave vs. modified square wave).

Off-grid inverters are sold with two waveform options: sine wave and modified square wave (sometimes called modified sine wave). Sine wave output has low total harmonic  and is the most common used for residential systems. Modified square wave is used in the budget type of installations (remote cabins and observation posts). They do not run some types of loads optimal and they may harm those appliances.

#Total harmonic distortion (THD).

Is the measure of how closely the waveform matches a perfect sine wave. A THD of 0% is a perfect sine wave, and the larger the percentage, the further it deviates from a sinusoidal waveform. Sine wave inverters typically show a THD of 5% or less, while the THD of modified square wave inverters may range from 10% to 40%.

#Rated Continuous Output Power.

An off-grid inverter must supply enough power to meet the needs of all the appliances running simultaneously. Most of the time those appliances are considered as a critical load.

#Nominal Battery Voltage(s).

Each inverter has a nominal battery voltage where it can be connected to. Common off-grid inverter battery voltage options are 12, 24, or 48 volts.  Exeltech’s line of inverters, for example, includes models that can connect to 32, 66, or even 108 volts battery banks.

#Output Voltage.

The United States based Off Grid Inverters are designed for 60Hz at 120V or 120/240V appliance (electrical load) voltage. So-called export inverters are designed for 50Hz at 240V or 240V/380V.

#Adjustable Frequency.

The Conext SW Inverter/Charger is capable of operating from a 50Hz and 60Hz power source by extending the AC qualification frequency range.

#Dual MPPT.

Is one of the new smart inverter attributes. When is it useful? Think about 2 strings or two different arrays with different azimuth or different tilt. Or think about arrays with different length and from here different Voc values, or a different type of panels on each array.NEC section 690.9 allows connecting two PV strings to a single input of an inverter without a combiner fuse in each string.

#Peak Surge.

Some loads (like motors) require significantly more power during startup than they need to run. To start these loads, inverters will briefly or run at higher than their continuous power rating. Surge ratings include the maximum amperage and a time period that the inverter can run at that high power level without sustaining damage or turning off to protect itself. A good number are in a range of 50Amps to 120Amps for 1sec.

#Inverter Peak Efficiency.

Efficiency is measured as the ratio of the inverter’s AC power output to the DC power input from the batteries. Higher efficiency means that the inverter wastes less power while converting DC into AC. Common values are in the range 90% up to 95% ( Xantrex - XW4024 works with 95% efficiency).

#Power Factor Corrected (PFC).

Is the type of inverter charger which uses less energy and less AC current from a generator than a standard inverter charger and all the battery bank charging process goes faster.


Some off-grid inverters include the capability to connect several units to operate as a single, larger unit. Stacking options allow 120 V inverters to work together to power 240 V loads. The Conext SW Inverter/Charger is also Xanbus-enabled which allows network compatibility and communication with other Xanbus enabled devices.

About the author of this article:
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