Off grid Inverter

 

An off-grid inverter is a power converter meant for electrification of isolated sites which, are not coupled with local electricity network. Delivering stable solar power supply to such sites is a challenge since the complexity of the system is much bigger than in case of a common grid-connected system. The off-grid solar inverters are used to power up not only energy-autonomous self-sustaining households, but can be used electrify appliances and tools in Recreational Vehicles (RV’s) or on boats.

Most commonly, the off-grid inverters are:

  1. Capable of generating their own AC sine wave: unlike the grid-tie inverters, which only synchronize their AC connection with the signal coming from the grid, the off-grid inverter must be able to generate an appropriate sine wave (in France it’s 220Vac L-N, at 50Hz frequency). The inverter, instead of having a grid interactive connection to feed the power to AC grid, would have a separate output, through which the renewable power from the battery would be fed to loads.
  2. Capable of handling the battery: like the grid-tie machines the off-grid inverters are DC/AC converters but while the grid tie inverters convert the DC power coming from the solar array into AC, the off-grid units use the DC power stored in the batteries. The proper handling of batteries in terms of currents and depth of discharge is crucial for its optimal service.
  3. Capable of being integrated with auxiliary power supplies. The off-grid units, with a use of an external transfer switch can work together with diesel generators or even, which is a bit counterintuitive, the electrical grid! The transfer switch would alternate between power supplies to connect the appliances to the off grid unit if the latter is capable to take power from the batteries, or to an auxiliary power supply (genset / grid) when the battery is charging.

The off grid inverters will not:

  1. Inject power to electrical grid. This is the feature reserved for grid-tie and hybrid inverters. The off grid inverters since they’re not foreseen for being coupled with the grid, do not comply with various grid codes which are required for grid connection. The grid codes define, among other things, how the anti islanding protection should be triggered, which is a feature the off-grid inverter simply does not support.
  2. Handle the solar array. The off-grid installations usually require a separate device called solar charge controller to be installed next to the off-grid inverter. The solar charge controller is a DC/DC converter which adapts the voltage output of solar array to a voltage input of battery terminals and handles battery charging currents. Since the off grid inverter does not handle the power coming from the PV array, a direct consumption of power produced by the array is not possible.
  3. Use the solar energy to power up the loads when the battery is discharged. While most hybrid inverters are capable of providing the pv production to the loads and charging the battery at the same time, the off grid unit is not capable of doing that. When the battery is empty or charging, the output of the inverter is simply disabled, and is only enabled once the battery is fully charged.
  4. Output power which higher than its rated output power, or which is higher than what the batteries can supply. The condition, when the power drawn from the inverter is higher than what the inverter is built to supply, is called an overload. Most commonly the overload can be caused by appliances which require high currents for operation, or high inrush currents (for example: peak current at a startup of a motor required to rapidly achieve certain RPM). In the table below there is a couple of examples of common appliances and tools, with their peak power requirements.
Load Operating power Start-up coefficient Max peak power
Electric oven 2500W 1 2500W
Television 300W 1 300W
Portable grinder 900W 2,5 2250
Circular saw 1100W 2,5 2750W
Freezer 300W 3,5 1050W
Single phase compressor 1500W 3,5 5250W
Washing machine 3000W 3,5 10500W

 

While an off-grid inverter cannot be replaced by an on grid unit, it could be replaced by a hybrid inverter. The use of a hybrid inverter immediately brings one advantage to the table – allows to limit the number of the necessary devices. The hybrid inverters are capable of charging the solar power into the batteries just like a Charge Controller would, they can convert DC to AC and AC to DC just like a multi-mode off-grid inverter would, they have the dedicated output of the loads as well as built in AC input for connection to electrical grid or to a diesel generator eliminating the need for the transfer switch.

Most importantly the hybrid solar inverters can electrify the loads using the solar production which is only supplemented with power coming the batteries. A typical off-grid inverter would only use the energy stored in the batteries. This means that before being supplied to loads, every single kWh of energy produced by the solar would have to go through several electrical conversion steps (DC/DC in charge controller, DC to AC in the off-grid inverter) as well as conversion related to storage which requires electrical energy to be transform in its chemical form. The impact of the conversion steps on the system’s efficiency is non negligible and therefore positive improvement of efficiency which hybrid inverters make possible is quite substantial.

Besides, using a hybrid also may help some other elements of the off grid system to achieve their best results. The lifespan of the battery is expressed as a number of the charge-discharge cycles the battery can support in function of a parameter called depth of discharge (DOD). It’s easy to understand: if the DOD is 20% it means that the battery discharges 20% of its capacity before it can be charged again. The volume of energy which passes through the battery during charge and discharge (so called energy throughput) is insignificant and therefore does not cause a lot of wear and tear. This would change if the DOD is set to 80%, the volume of energy passing through the battery during 1 cycle is 4 times higher and therefore wears out the

Below is a table showing the advantages of the Off-grid inverter.

 

Off-Grid
Self-Consumption ++
Electrification of isolated sites +++
Back-Up +++
Yield +
Battery Management +
Ease of Installation +
Cost Savings +
Price ++