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101 renewable - off-grid power inverters

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Last Updated
7th of January, 2020

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off-grid plant

Stand-alone PV power plants are not directly connected to the grid, and they consist of PV panels as power generators plus a storage system that will guarantee electric power supply when light is poor. Since the current delivered by the PV generators is DC (direct current) we will need an AC inverter (it receives the DC power from PV panels, and delivers AC power to the dwelling electrical consumers, the critical load).  The 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 is over-dimensioned in such, during the high irradiation hours acts as a power supply to the critical load, and at the same time it recharges the battery bank.  Off-grid inverters are designed to handle at the same time battery banks and external backup power generators (most of them petrol-based). Some of the off-grid inverters are built as hybrid inverters. Hybrid inverters have multiple functionalities, they work off-grid and grid-tie, and they have the capacity to connect to utility power grid through phase coupling and unload energy produced by the PV plant to the utility grid, or to juice in energy from the utility grid and recharge the battery bank when the battery bank is on a low and the PV plant does not generate enough energy to help in. In case of a utility grid outage in order to maintain the critical load active, hybrid inverters work as an islanding buffer for the PV plant and the backup power associated.

Common functions build into off-grid inverters:~

>_Charges Batteries While Supplying.
>_Cleans and Regulates Power Output.
>_Correlating with an External (mostly 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, the battery bank and even to the backup power generator set.

Here is a list for 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. The modified square wave is used in the budget type of installations (remote cabins and observation posts). They do not run optimal some types of loads and they may harm some of the appliances.

Total harmonic distortion (THD):~

It 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:~

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 in use off-grid inverters are designed for 60Hz at 120V or 120/240V electrical load voltage. So-called export inverters are designed for 50Hz at 240V or 240V/380V electrical load voltage.

Adjustable Frequency:~

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

Dual MPPT:~

It 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 lengths 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. They are handled separately by each of the MPPT.

Peak Surge:~

Some loads (like motors) require significantly more power during startup than they need to run. To start those loads, inverters will briefly run at a higher than continuous power rating. Surge ratings include the maximum amperage and a time period when the inverter can run at a high power level without sustaining damage or turning off to protect itself. A good number is 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. Higher efficiency means that the inverter power waste is less when converting DC into AC. Common values are in the range 90% up to 95% ( Xantrex - XW4024 works with 95% efficiency).

Power Factor Corrected (P.F.C.):~

The power factor is the report between True Power and Apparent Power. The best value for the power factor is one. At this value the reactive power associated with consumer load is null. For us the reactive power is the culprit, it brings electric power losses and thermal exchange, heat dissipation. Heat destroys the components inside Inverter/Chargers, and in the same time the battery bank charging takes more time becouse of power losses. To compensate reactive components of power and reduce the noise associated with those, we use Inverters/Chargers equipped with Power Factor Correction. P.F.C. will improve the power factor by taking out the noise (harmonics) from the output sinus power wave. Usually is done with a trafo and a block of power electronic components mounted in a bridge formation and following the output.


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

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