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Invertor Ratings Explained

Watt (W)
Watt is the measure of how much power a device uses when turned on or can supply. If a device uses 100 watts, it is simply the voltage times the ampere (rate of current).If the device takes 10 Amps at 12 Volt DC, it uses 120 watts power. That is 10A x 12 V = 120 W.

Watt Hour (WH)
A watt hour (or Kilo Watt hour – kWh) is simply how many watt times, how many hours the device is used. If the device uses 100 watts for 10 hours, it is 1000 watt hour or 1 kWh. The electricity tariff is based on kWh.

Ampere (A)
It is the measure of electrical current at the moment. Amps are important to determine the wire size for connecting the inverter to the battery. Low gauge wire will heat up and burn if heavy current flows through it from the battery.

Ampere Hour (Ah)
Amp-Hour usually abbreviated as Ah is the Amps x Time. Ah is the measure of battery capacity which determines the backup time of the inverter

Volt Ampere (VA)
It represents the maximum load capacity of the inverter. Commonly available inverters are 500 A, 800 VA, 1000 VA, 1500 VA etc.

Power in Inverter

Peak power and Typical or Average power
An inverter needs Peak or Surge power and Typical or Average (Usual) power. Peak power is the maximum power that an inverter can supply usually for short time.Some heavy current appliances like motor and refrigerator requires a startup peak power than they require when running. Typical power is the power that the inverter gives on a steady basis. This is usually much lower than the peak power. Typical power is useful in estimating the battery capacity. Therefore inverters must be’ sized’ for the maximum peak load and typical continuous power.

Power Rating of Inverter

Inverters are available in different ‘Size ratings’ from 50 VA up to 50000 VA. Inverters larger than 11000 VA are seldom used in household applications. The first thing you have to consider about the inverter system is its maximum peak power or surge power and steady current supply. The surge rating is usually specified at so many watts for so many seconds. This means that the inverter will handle an over load of that many watts for a short time. This ‘surge capacity’ will vary considerably between inverters and even with in the same brand. Generally 3-15 seconds surge rating is enough to cover 99% of appliances. Inverters with lowest surge rating are the high speed electronic switching types.

Current consumption
Depends on the wattage of the appliances used, current consumption of the inverter can be calculated using the formula

I = W / V Where I is the current in amps, W is the wattage of the appliance and V the 12volt (battery voltage).

Wattage of an instrument is calculated using the formula

W = V X I Where V is the 230 volt AC and I is the current consumption.
Watt rating is usually printed on the back side of the appliance near the power cord.

It indicates the ‘Size’ (capacity) of the inverter. To select the inverter size, the given formula is useful

VA = W x Inverter loss. Inverter loss is typical around 1.15. If the total load connected to the inverter is 400 watts then the minimum inverter size should be 400 x 1.15. That is 460 VA. A 500 VA is suitable for the load.

Ampere Hour (Ah)
The capacity of the battery is represented in Ah. It is the amount of current a battery can give during one hour of charge / discharge cycle. High capacity batteries (100 Ah, 150 Ah) are used to power inverters to get sufficient backup time. The formula to select the battery power (Ah) is Load in watts / Voltage of battery x Backup hour.

For example if you wants to run 400 watts load on 12 volt battery for 3 hours, then the capacity of the battery should be minimum 100 Ah.
Ah = 400 / 12 x 3 = 100 Ah. If the load increases (within the capacity of the inverter), backup time reduces.


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