Sizing an Off-Grid System

Introduction

Sizing of an off-grid system requires modelling of the expected output from the renewable energy system, and of the load that the system must meet. 

The three key variables are as follows:

• kW output of renewable generator(s);
• kWh of battery storage;
• kW output of back-up generator.

In addition the battery voltage must be compatible with the charger.

Renewable energy generation is by nature intermittent, and the battery storage system and / or generator must be sufficiently large to cover the times when the generation is unable to meet the load.  Typically the generator output (kW) is sized to cover peak load, and thus it serves as a full power back-up. 

The renewable energy system and battery storage can either be sized to reduce the contribution of the back-up generator below a certain minimum (e.g. 10% of annual load), or it can be optimised to minimise the lifetime cost per kWh.

Modelling the Renewable Output

There are established models which can be used to estimate the hourly output of a solar PV or wind generator in a given location.  For solar, the model should include shading analysis to establish the impact of any shading. For wind, the model must be very localised, taking into account average wind speed, prevailing wind directions, and seasonal variations.

Note that wind and sun are generally complementary (high winds and low sun in winter) and so, depending on location, it may be advisable to combine both.

Powering down

If the renewable energy system is ‘over-sized’ it will need to be powered down in favourable conditions (wind or sun) as there may be nowhere for the full output to go (e.g. if the battery is already full). There is a balance between how often the system has to power down due to ‘over-sizing’ in favourable conditions and how often the generator has to be used due to ‘under-sizing’ in unfavourable conditions.

Modelling of the loads

Modelling of the loads requires a systematic exercise of listing all of the loads (dc and ac), including running currents and start-up currents.

An ‘hourly profile’ should then be built up for each load, to give an average consumption by hour. Consideration should also be given to the peak load if all key loads are run together.

For residential off-grid systems, see Residential Load Considerations.