What Size Power Inverter Do I Need?

An inverter, simply stated, is a device that converts DC (Direct Current) electric power to usable AC (Alternating Current) power. With the help of an inverter, one can hook up power from a battery to common household appliances if the traditional AC sources fail or are not available.

Inverters have proved to be quite useful as emergency backup in case of power outages and as portable power sources on the go. These are extremely flexible and versatile devices, and can be used in a number of places. For example, you could use an inverter in your home in case power supply fails due to natural disasters, you could use an inverter as portable power supply for a camping trip and even use it to have your coffee maker running on a sailing trip.

However, with the wide range of power inverters available in the market today, it is necessary to identify the correct inverter that best suits your needs. If, for instance, you want the inverter to just charge your laptop and cellphone, it makes no sense to buy an overpriced and expensive inverter that is meant for running refrigerators and heating systems.

In this article, we provide an overview of what ratings are applicable to an inverter, what they mean and how to choose the correct size for your power needs.

A power inverter is usually rated in Watts. A typical inverter will have two different wattage ratings- continuous and surge. The continuous wattage rating determines the amount of power it can deliver continuously for a long period of time while the surge rating specifies the amount of overload it can sustain for a shorter time span.

For example, an inverter rated 1750 W continuous and 5000 W surge can serve a load of 1750 W for several hours and sustain 5000 W surge during the short start-up period (which typically does not last longer than a few seconds). Inverters are also usually capable of sustaining overloads for short periods of time (the inverter cited in the example above can run 2000 W loads for about 15-20 minutes), but this should be used only in case of emergencies. Overloading inverters for longer periods will affect its longevity, cause excessive heating and damage equipment.

Commercial inverters are available in two types- true sine wave and modified sine wave inverters. The modified sine variety is better suited for light-duty applications. Cellphone and laptop chargers need DC power to run. Hence, if the inverter has a direct DC outlet, DC load can be directly plugged in (check the DC ampere rating requirement on your appliance and make sure that the amperage matches on your inverter outlet).

All electric devices have a Watt rating associated with them. Watt is the unit for electric power and is expressed mathematically as the product of voltage and current.

Watt  (Power)= Volt (Voltage) x Ampere (Current)

All appliances have their Watt requirements listed on their labels. Inverters convert the 12 V DC from batteries to 120 V AC. If you check the current (A) rating on the equipment label, you can find out its Wattage rating as well. The table below lists some common appliances and the amount of power they need to operate (and start up).

Please note that the power ratings listed below are indicative and actual power rating on the device may vary depending on the brand and type you are using.

The first step towards deciding the size of inverter you need is determining your power needs. Identify the devices that you want to power from your inverter and add up the wattage requirements of individual appliances. It is advisable to buy an inverter which can handle additional 10-20% load than you actually need.

For example, if you need to run 1000W load by your inverter, it is advisable to buy an inverter which has a continuous wattage rating of at least 1200 W. This will ensure that the inverter is not unduly overloaded or heated up and lasts for a long time.

It is important to remember that inverters are not generators. They do not produce energy by themselves and need a reliable battery system to power your appliances. How long the appliances run depends on the capacity of the batteries that are providing juice to the inverter. In order to keep your appliances running for an appreciable amount of time, a reliable battery backup is of utmost importance.

The batteries need to be charged at regular intervals. If you are hooking your inverter to the batteries on your car or truck, you need to keep your vehicle running for the period that your inverter needs to be working. You can also use Lithium ion (Li-ion) or VRLA (Valve Regulated Lead Acid) batteries instead. 

Also note that inverter sizing has to take into account the maximum amount of load that you intend to run by your inverter. For example, if you run a washing machine for 20 minutes and just a laptop for the remaining amount of time, your inverter still has to be rated according to the washing machine’s watt requirements (~2000 W) even though the laptop needs only about 100 W to run. The average hourly power consumption comes handy while deciding the size of batteries that you need for an application.

With the large number of commercial power inverters available in the market, choosing the right one for your requirements might be a difficult decision. However, once you have your power needs identified (Ask yourself whether you really need to do the laundry when power is out?

Otherwise you can select a much cheaper inverter with a lower continuous Watt rating), the choice should become easier. A true sine wave inverter will probably give you the cleanest and most efficient power supply, but a modified sine inverter can be cheaper and better suited for light power requirements. The batteries you choose should be capable of driving your inverter as well.