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Off-grid system sizing

Off-grid Sizing tool (PV array, Wind Turbine and Generator)

To get the best from an off-grid renewable energy system requires careful consideration of your specific requirements, location, available space for installation and budget. The first stage in designing your system is to evaluate your total electricity energy demand.

List each of the appliances you want to use with your off-grid system, the wattage of each appliance and for how long you expect to use them daily or weekly. You must include all AC appliances such as a TV, Washing Machine or Laptop and all DC appliances such as lighting regardless of whether they are being used simultaneously or not.

Use our Off-grid Sizing Calculator (requires MS Excel) or follow the instructions on this printable version to add all the appliances and calculate the peak power consumption and average daily amp hours.

Sizing the inverter and/or inverter charger is based on the peak power consumption (i), the total watts of the more demanding appliances being used at any one time. Although most inverters have an overload function allowing for short surges in load the peak demand of simultaneous automatic startup motors such as fridge and freezer must be accounted for in this total. The inverter's continuous rating should be higher than the total peak power consumption (i) to prevent overload and power outage.

The total amp hours per day (ii) helps to size the battery bank along with other factors such as type of battery, number of renewable sources in the system and climate. The battery bank needs to be considered carefully; charge and discharge times, budget, where they are to be stored as all batteries are heavy and open-valve or flooded lead-acid batteries can emit gases and need to be stored in a vented separate location to the plant equipment. A larger battery bank has a longer discharge and thus longer times between charges but has a greater charging time which may not be achievable with the renewable source alone and at a higher initial cost. Too small a battery bank may not harness the potential of the system as more energy is being produced than can be stored and if not being charged enough on lower yield days (no sun or wind) they will have more frequent deep discharges reducing the lifespan of the batteries.

The next stage to consider is the sizing of the solar pv array and / or wind turbine if one would be suitable in your location. Many off-grid systems combine wind generation with solar as they complement each other during the different seasons. It is recommended you incorporate a generator as a backup for most off-grid systems where a consistant supply is required. A generator can also help reduce overall costs as it can be used for peak loads such as washing machines, reducing your peak consumption and average amp hours and thus lowering the costs of inverter and battery bank. Using the energy wisely when being generated will help minimise the size of the batteries such as using the washing machine when it is sunny with a solar pv system.

By now you can see how much of a balancing act it is designing the best off-grid system. We recommend you use our Off-grid Sizing Calculator and return the results to us. With this initial specification, our experience and understanding of the technology and by discussing your requirements we can work with you to ensure the best solution for your off-grid system.

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