Solar autonomy (“off-grid”), the practical guide!

Before acquiring any equipment needed for a solar (hybrid) or off-grid energy system, it is crucial to master the fundamentals of designing and sizing energy storage systems. As illustrated below, the first step is to develop a charging profile via our calculator, to estimate the amount of energy you will consume on site on a daily basis. 

Example (click on this link to access the online excel calculator) :

Step 1 – Evaluation of consumption in kWh:

 The most crucial element in the design of an off-grid solar system is the estimation of the energy required on a daily basis in kWh. For grid-connected sites, accurate load profile data can be obtained by using meters to directly measure loads. For off-grid or stand-alone systems, always start by using our off-grid load calculator for summer and winter needs. The load table will also help calculate peak loads, power factors, and maximum demand needed to size the appropriate system. Be careful to distinguish between the notions of kW (power) and kWh (energy)!

Step 2 – Battery Sizing:

Battery capacity is measured in Ah or Wh. THE Nickel-Iron batteries are sized in Ah (to obtain the capacity in KwH, you must multiply the capacity in Ah x the voltage, for example 200Ahx48V = 9.6 kWh of nominal energy), while the capacity of the batteries lithium is measured in kWh. All loss factors must be considered to ensure the battery size is sufficient to meet the loads, including the maximum allowable depth of discharge (DoD), which will also have an impact on the lifespan. Also consider battery type and chemistry, battery voltage range, minimum run-time days (continuous days without sunlight), and maximum battery charge rate (C rating), as explained further in detail later.

Step 3 – Sizing the solar installation

 It is necessary to have a correctly sized solar installation to charge the battery while powering the loads. To ensure the solar installation is large enough, consider local conditions, including average solar irradiance throughout the year (peak sunlight hours), shading issues, orientation and panel tilt angle, cable losses, and thermal degradation (loss factors). The PVGIS solar design tool can help estimate solar generation throughout the year, based on the orientation and location of the panels.

Step 4 – Selection of inverter-charger

Once steps 1 to 3 are done, you then need to choose a suitable inverter charger, as well as an MPPT solar charge controller to match the solar installation based on the length of the panels and strings, which will determine the voltage Chains. Use a chain tension calculator to estimate the maximum and minimum chain tensions, which will help you determine the choice of the most suitable MPPT charger (I used the example of the Victron MPPT calculator in this case). Then, the battery inverter-charger can be selected to meet your continuous and peak load needs.

How to select the right battery inverter?

LBattery inverters for off-grid applications have numerous specifications to take into account before choosing and sizing a suitable battery inverter. Several types of systems are available, including grid-interactive inverter-chargers, hybrid inverters, complete systems with integrated battery storage (known as BESS), and AC Coupling battery systems. Below I outline some key concepts to consider when selecting a suitable inverter, by analyzing the datasheet of a battery inverter that we use frequently, le Victron Multiplus-II. Here are the criteria that must be taken into account:

 

– Continuous output power of the inverter, and peak power (kVA & kW)

– Charging capacity of the inverter-charger to the batteries (in A)

– Transfer capacity 

– Battery compatibility (depending on technology)

– Type of architecture (DC or AC Coupling?)

– Telemetry and local and/or remote system monitoring

 

  

1. Inverter output power – maximum continuous and peak values ​​(kW)

 

Battery inverters (hybrid or off-grid) come in a wide range of sizes, determined by continuous power output measured in kW or kVA. The power of the inverter depends on its topology or design, the type of energy conversion circuit, the presence or not of a transformer, the cooling system, and the operating temperature. Below are two main types of hybrid and off-grid inverters available.

 

 

Loff-grid battery inverters employ Heavy-duty toroidal transformers, which are more expensive, but provide high peak and surge power and can handle high inductive loads. These inverters typically contain actively fanned cooling systems to maintain performance in high temperatures. As explained below, most of these inverters have integrated chargers and are also network interactive (this is the case of the Victron Multiplus-II)

 

LHybrid inverters and AC coupled battery systems use transformerless inverters with 'switching transistors' (example: Fronius GEN24, GROWATT or DEYE hybrid inverters, etc...) These compact and lightweight inverters have lower peak power and surge ratings, but are more economical, being cheaper and easier to manufacture. They are usually also fully weather-rated, meaning they can be safely installed in more exposed locations, although direct exposure to sunlight should always be avoided. However, they are not designed for year-round off-grid operation, but can very well take on occasional back-up functions.

 

   

 

It is crucial to understand the maximum power the inverter can deliver continuously, as well as the peak power it can handle for short periods of time, which is often necessary when starting inductive loads like motors. Don't be forced to choose whether to start the hair dryer or the toaster! 

 

 

Ppower in KVA or kW? What do we choose?

 

It is important to note whether the power output of the inverter is stated in kW or kVA. Kilowatts are generally the most accurate measurement. This can be confusing when sizing an inverter for your needs. The general conversion ratio used to convert from kVA to kW is shown below:

 

kVA x 0.8 = kW

 

For example, a 5kVA Victron inverter is approximately equivalent to a 4kW inverter. Another example is an inverter with a continuous power output of 3000VA (3kVA) which typically generates only 2400 Watts continuously, or about 80% of the stated 'apparent' power.

 

 

 

2. Inverter charging capacity (generally expressed in A):

 

This is the capacity of the inverter to charge the battery from a so-called “shore” source (a generator). A higher charge rate means the battery can be recharged more quickly, which can be beneficial in areas with limited periods of sunlight or when you are forced to run the generator. Example with the Multiplus-II battery inverter:

 

 

3. Size of photovoltaic installation (kW)
 

The size (understand the power) of the photovoltaic installation must be compatible with the capacity of the inverter. An oversized or undersized inverter compared to the photovoltaic installation can result in a loss of efficiency and performance. SIf for example you take a Multiplus-3000VA and add two Victron RS 450/100 chargers of 5 kWp each, you will only be able to use the power of the Multiplus, i.e. 3KVA out of a total potential power of 10KVA (if your panels produce in ideal conditions!).

 

 

4. Passing/transfer power (A):

 

Passing power refers to the inverter's ability to transfer power from the grid or a generating source to the loads without passing through the batteries. This is important for maintaining power during outages or when the battery is discharged. This notion is especially important on hybrid configurations, and is less so on an isolated site (in fact, with 32A * 230V = 6000W, which already corresponds to a fairly comfortable generator).

 

 

 

5.Battery Compatibility – System Voltage and Battery Type
 
It is crucial to ensure that the battery inverter is compatible with the system voltage and the type of battery used, whether lithium, nickel-iron, or other batteries. For example, a Multiplus-II 48V will have to be equipped with a 48V battery pack, obviously, etc.

 

6. System architecture type: DC or AC Coupling?

 

Inverters can be coupled with alternating current (AC) or direct current (DC), each configuration having its advantages and disadvantages. The choice will depend on your specific system and your needs.

 

En DC Coupling architecture, the most widespread in off-grid, an “MPPT” battery charger controls the solar panels during the day in order to maximize solar production, which will be reinjected into the batteries. The efficiency is excellent (92-96%), however, if the self-consumption of energy takes place during production, the DC/DC/AC conversion (since the MPPT supplies direct current, itself corrugated by the battery inverter to power your 230V loads), will cause additional losses:

 

 

 

En AC Coupling diagram, less common and often found on higher power systems, a solar inverter (Fronius) is interconnected to the output of the Multiplus battery inverter. The latter is responsible for controlling it according to instantaneous power requirements, so that the energy produced by the Fronius is directed as a priority to consumers, WITHOUT passing through the batteries:

 

 

 

 

This type of architecture has two notable advantages:

 

- Better performance if most of your consumption is done during the day (you will literally consume “as the sun goes by”).

- Cumulative power of the Fronius output AND the battery inverter. In other words, if your Fronius produces 3KWP of instantaneous power, and you have a 3KVA Multiplus, you can theoretically reach 6KVA!

 

However, this type of system has two disadvantages:

 

- The Fronius must be controlled using frequency modulatione by the Multiplus, which can shift digital clocks, and be problematic for certain sensitive equipment (household appliances, etc.). We speak of the phenomenon of “flickering”.

- Battery charging performance is mediocre, because there is a DC/AC/DC conversion and the losses are greater.

 

So what would be the best architecture for a robust off-grid system? 

Finally, we can have a mixed architecture, combining AC and DC Coupling. This has the advantage of being robust and redundant (if the Fronius is in after-sales service, you have the solar production of the Victron MPPT charger, and vice versa)! 

Example of a mixed off-grid system, combining DC & AC Coupling (via a Fronius): 

 

 

7. Telemetry and local management (supervision):  

 

The ability to monitor and control your system locally and remotely is a considerable advantage, allowing optimal management of energy consumption, solar production and energy storage. This also makes it possible to have precise monitoring day after day of the various system variables (solar production, consumption in kWh, power peaks, state of charge of the battery), and to possibly identify anomalies. It is therefore an essential tool to integrate into an off-grid system.