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Energi Mart
 
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Sizing your load shedding system CORRECTLY

18 June 2015, 12:30

The sudden uproar of solar and battery “experts” driven by the need to mitigate the risk and inconvenience of load shedding represents a risk to the unwitting consumer, with all the technical jargon flying over the heads of most, including the seller or installer. It’s not as simple as plugging everything together and incumbents will know there is a lot of consideration that needs to go into sizing a load shedding and solar solution for its application.

In lieu of what I have seen happening in the market with grossly undersized systems being proposed to the markets that will not deliver on what is promised purely based on the general lack of knowledge on the subject, I will give you some advice from my own experience in system sizing and important considerations. I will cover, battery sizing and operation, solar considerations and sizing, inverter selection and ideal operation.

A few basic principles that you need to know:

·         (Power = Voltage x Amperage) This equation is absolutely critical to understanding your need and what your system can provide you with. Power expressed in Watts, Voltage in Volts and Amperage in Amps.  

·         (AC vs DC) Incidentally not the world famous band, but rather the type of current flow that we use in everyday applications. Alternative Current (AC) is what we receive from Eskom, meaning the current changes direction 50 times each second at 50Hz. When you plug an appliance into the wall you will be using 230 Volts of AC power. Direct Current (DC) as the name suggests, only flows in one direction and is what you get when using a battery or the type of current that your solar power will generate.

·         The inverter is the device that you use to convert DC current from your batteries and solar to the AC current that you require for your appliances. Bi-directional inverters conventionally have chargers or rectifiers built-in which allows you to charge your batteries and in turn converts AC from your Eskom supply to DC for charging those batteries.

·         Grid-tie inverters and MPPT’s are used in conjunction with solar PV panels, where the grid-tie inverter converts the DC current generated by the PV panels directly into AC current in line with the standard required by the Eskom power network, allowing you to provide excess power to the utility grid. The Grid-tie system however does not provide you with load shedding backup as it is required by law to shut down when there is a power failure for line operator safety reasons. It is also heavily regulated and consumers need to become acquainted with their local regulations. The MPPT or Maximum Power Point Tracker is a charge controller that will optimise the solar yield and charge your battery bank. It will not convert DC to AC and is functional only to use solar with battery backup. This system will provide you with both solar power and load shedding backup.

Batteries:

The proverbial Achillies heel of any system, the sizing of your battery backup is literally the make or break for a good return on investment. It all starts at the manufacturer that will test a battery and give the battery specification at 25°C operating temperature and at a given hourly rate or C-Rate. This is absolutely critical to sizing your battery. The most well know battery size in the South African market is the 105 Amp hour 12 volt deep cycle battery and this battery is mostly given at a 20-hour rate or C-20. This means this battery will deliver a TOTAL of 105 Amp hours over 20 hours or only 5.25 Amps per hour for 20 hours when you divide the 105 by the 20 hours it is given at. So in essence when you use the equation above you will get Power = 12 Volt x 5.25 Amps = 63 Watts or 0.063 Kilowatt per hour for 20 hours.

The problem is that we do not have 20-hour load shedding periods, but only 2 – 4 hours per load shedding, which means we will not be using the battery at its 20 hour rate, but rather its 2 – 4 hour rate. This rate is hardly given by the manufacturer, seller or installer, but is critical to correct sizing. The same 105Ah 12V battery will have a reduced capacity at higher discharge rates and depending on manufacturer, may be in the vicinity of only 75Ah at its 3-hour rate. This in turn means the total of 75Ah divided by its hourly rate of 3 hours gives us a discharge current of 25 Amps and this multiplied by the voltage of 12 Volts will give us 300 Watts per hour for the duration of 3 hours.

You can see that four of these batteries will then give you 1200 Watts or 1.2 Kilowatts per hour for 3 hours and eight of them will give you 2.4 Kilowatts per hour for 4 hours and so forth. I have seen plenty of sellers and installers offering consumers 4x105Ah batteries using the 20-hour rate and selling these solutions as a 3 Kilowatt load shedding backup which is far from the truth. If you draw 3 Kilowatts from 4 of these batteries you will simply kill them in a matter of a couple of months and they will not last your entire load shedding period.

This is not even all the considerations as you need to look at your operating temperature, charge and discharge rates and especially the Depth of Discharge (DOD) which in turn will all influence your battery life.

In the next part I will cover the solar aspect, but be aware of what you buy, there are plenty “fly-by-nights” that will not size your system correctly, give you a cheap solution and never be able to carry the warranty. 

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