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Sizing of OFF-GRID systems with 12/24/48 VDC voltage


When designing a solar OFF-GRID system, you will have to deal with the calculation of the backup battery size. The main goal of the calculation is to store enough energy to ensure the system fully functions even on days with clouds, snow and rain. Below is a brief overview of the steps and risks that need to be considered in such a situation.

Step 1: Calculate the Energy Performance of the System

For correct sizing of the whole system and selection of a charger it is necessary to find out:

  • the maximum power consumption of connected devices
  • daily system consumption

Examples:
Camera MOBOTIX M26 has a power consumption of < 5 W.
PLC with LTE interface IPLOG-G3-05-RE8.1 also has a power consumption of < 5 W.

The maximum power consumption of connected devices = 10 W
Daily system consumption = 10 x 24 = 240 Wh

Step 2: The Backup Time

Depending on the installation location and purpose of the system, you must decide what the maximum system backup time will be. This has a major impact on the size of the backup battery. Always consider the local atmospheric conditions and temperature drops. Even with the highest quality lead-acid batteries, the temperature drops to about 50% when the temperature drops to -20 °C. For most systems, a backup time of two to five days will be acceptable.
In our example we will consider Backup time = 3 days

Step 3: The Backup Battery Size Calculation

Battery manufacturers report their capacity in Ah (Ampere hours). Therefore, you will need to recalculate the Wh to Ah and add another reserve to ensure that the battery only discharges up to 50%. A reserve ensures long-term reliable battery operation in cyclic operation.

Total Wh in 3 days=  240 Wh x 3 dny = 720 Wh
Battery reserve capacitz for max. 50% discharge = 720 Wh x 2 = 1440 Wh
Conversion from Wh to Ah: 1440 Wh / 12 V = 120 Ah

The illustration below shows the life of Victron Energy's top-of-the-range batteries at different discharge depths.

In OFF-GRID systems, it is even more important to emphasize energy savings compared to conventional 230V systems. Any additional Watt of consumption has influence on the increased battery capacity. The advantage in such a situation is the PLC, which can help with a running program to reduce system consumption:

  • Measuring Station - the PLC only powers the sensors during the measurement,
  • Dohledová PoE camera - the PLC turns on PoE power to the camera on demand (SMS, Android / iOS applications) or automatically when an alarm comes from the PIR. See our MPPT chargers with MODBUS interface.

Step 4: Dimensioning of the PV Modules

The energy gain of photovoltaic panels significantly influences the season and geographical location of the installation. Here are two examples of the same system in the locations Česká Skalice (CZ) and Porto (PT).

Pannels: 4x Victron Energy 80Wp/12V
Battery: 60Ah / 12V
Discharge limit:50%
Daily system consumption: 240 Wh

Battery charge level in Česká Skalice (CZ)

Battery charge level in Porto (PT)

Step 5: Select 12, 24 or 48 VDC System

The battery charging current should not exceed 0.2C (20A for 100Ah battery) when using lead-acid batteries. Therefore, in systems with higher power consumption, it is more advantageous to connect the panels and accumulators in series and operate at a higher voltage of 24 or 48 VDC.

If you have any questions, please contact our technical support. The design of the technical solution and the preparation of the quotation of the OFF-GRID monitoring / control system are completely free of charge.