What size inverter do I need?


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An inverter needs to supply two electrical needs:

  1. Start-up, peak or surge power
  2. Typical, usual or continuous power
  • Peak or surge power - this is the maximum electrical power that the inverter can supply, usually for a short time only (this is typically no longer than a second, unless stated in the inverter’s specifications). Some household appliances, particularly those with motors inside them that use electricity to run various components like cooling fans etc., need a much higher start-up surge of electrical power than needed once the appliances are running. The most common examples of these kinds of appliances are pumps, compressors, air conditioners, freezers and refrigerators. In these instances, you want to select an inverter with a continuous rating that will handle the surge ratings of your appliance so you don’t prematurely burn out the inverter. It is important not to rely on the inverter’s surge to start up your appliances/equipment because inverters can’t operate in surge-mode indefinitely, unless the manufacturer claims to have a longer surge-mode time than normal.
  • Typical or usual power – this is the electrical power the inverter has to supply on a steady basis to keep appliances running. This is also referred to as the ‘continuous rating’ which is much lower than the surge-mode and lasts longer. For example, this would be the level of electricity a refrigerator uses after the first few seconds it takes for its motor to start up, or the electrical power it takes to run the microwave – or what all electrical loads combined will add up to. It is normally the wattage rating given in the appliance/products specifications manual. If an appliance like a fridge is listed as using 400 watts in the specs, the surge electrical power required to start-up of that specific fridge could be as high as 800 watts.

Now watt?

If you are considering purchasing a 1kW inverter with a 100ah.12 volt battery, and have calculated that you will need 300 watts of power for a TV and a decoder to power your entertainment system during loadshedding but are wondering how long the battery will last, use the simplified formula below to work out how long the inverter will last.

*please note this formula doesn't take into account battery aging or factors like surge power requirement, but it will give you a good estimate on run times

(Battery voltage * its AH capacity) multiply that by the battery’s efficiency rating which for a 12 Volt batteries commonly around 75%, then divide the answer by the number of watts you expect to be using. This calculation should give you the number of hours that the battery should last for before it discharges to levels which can be dangerous and damage it.

Example Calculation Based on needing 300 watts:

12 * 100 = 1200

1200 * 0.75 = 900

900 / 300 watts = 3 Hours

Based on the calculation above the 1kW inverter system would offer you 3 hours of backup power using a 300 watt load. As stated, this amount of time could vary based on the battery and exact electric load needed, but you can expect that it would last roughly 3 hours.

If you are worried that you may need a bigger back-up electrical power supply, or you want to tough it out through longer periods of power outages, a larger battery system is recommend. For example, the 2kW system with a parallel run of a 200Ah battery capacity, it would in the same situation last around 6 hours, which is definitely enough for the higher stages of loadshedding (longer power outages) and gives you some extra electrical power in case you need to go slightly over 300 watts.

If you have any extra questions please email us on info@razorbillgoods.co.za.

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