How to Determine the Suitable Size of Inverter for Home Appliances?

Electrical Technology

How Much Watts Inverter/UPS Do You Need for Home Appliances? Solved Example

A power inverter is a device that converts DC supply to AC supply. Most household appliances are designed to operate on AC supply (120V or 240V in the US and 230V in the UK and EU). That’s why we need an appropriately sized inverter to run and handle the AC load and appliances in case of an emergency or electricity failure.

As DC supply is available from different sources such as batteries, solar panels, portable generators, we can convert the DC power into AC power using inverters to run 120V or 230/240V appliances. To do this, we need to find the suitable size of inverter and batteries based on the required load in watts.

A power inverter is always rated in VA (Volt-Amps), but we assume its rating in watts based on the appliances’ wattage rating. The following example will illustrate the difference between the VA and wattage ratings of inverters based on our required wattage.

How to Calculate & Choose the Right Sized Inverter / UPS?

The following step-by-step example demonstrates how to determine and select a properly rated inverter or UPS (Uninterruptible Power Supply) for home and office appliances:

Example:

Calculate the proper rating of an inverter for the following home appliances.

3 No. of Fans: 3 x 80W = 240W
4 No. of LED Light: 4 x 15W = 60W
1 No. of LCD TV: 120W
1 No. of Laptop: 110W
1 No. of Laser Printer: = 60W
2 No. of Cell Phone Charger: 2 x 25W = 50W

Solution:

Step 1:

First of all, calculate the total required power in watts as follows.

240W + 60W + 120W + 110W + 60W + 50W = 650W

Appliance	Quantity	Wattage
Fan (80W)	3	240W
LED Light (15W)	4	60W
LCD TV (120W)	1	120W
Laptop (110W)	1	110W
Laser Printer (60W)	1	60W
Phone Charger (25W)	2	50W
Total Load		640W

Good to Know: If you have other loads, you can find the wattage rating by multiplying its voltage to the amperes mentioned on the nameplate printed on the device. For example, the wattage rating of a device designed for 120V, 1.5A would have 180W (Power = Volts x Amps). Similarly, a 230V, 0.5Amp has the wattage rating of 115W.

Step 2:

Ideally, you need a 650W inverter for 650W load points but this is not possible in real life as there are multiple factors which affect the overall efficiency of the inverter. As there is no such a 100% efficient inverter due to some losses in it as well as the role of power factor. This is where we should know about the VA rating of the inverter.

Generally, we consider 70%-80% efficiency of the inverter (if not mentioned on the nameplate or user manual from the manufacturer). To find the VA (Volt x Amp) rating of the inverter, we divide the calculated wattage rating on inverter efficiency or power factor. Consider the following two basic formulas

Power in Watts = Volts x Amp
Power in VA = Volt x Amp x Power Factor (Efficiency)
Power in VA = Wattage ÷ Power Factor (Efficiency)

By using the third formula (as applicable), we find the apparent power of the inverter as follows

Power in VA = Wattage ÷ Power Factor (Efficiency)

Power in VA = 640W ÷ 0.8 = 800 Watts.

Based on this calculation, we need a minimum of 800W inverter to handle 640W load appliances.

Step 3:

As a final touch, we add the future expansion (safety factor) as 1.25 to the calculated wattage. This way, we will be able to put some additional load on the inverter in future (if needed). In addition, it will protect the inverter from voltage spikes and power surges. To do so, simply multiply the calculated wattage by 1.25 to calculate the appropriate size of inverter rating in watts.

Right Size Inverter = 800 W x 1.25 = 1000 Watts

This is the most suitable size of inverter e.g. a 1000 Watts inverter will handle a 640W load safely and smoothly.

Good to Know: Inverter are designed for two specific operations viz

Peak Power – Surge Operation: Most new inverters are designed to handle the peak power known as surge operation for a very short time period. This is the case where motors and compressors (water pumps, air conditioners & refrigerators etc will take high current at the initial stage while starting the appliance. If the 100% peak power operation is not claimed by the manufacturer, you would have to increase the size of the inverter or simply multiply the calculated wattage rating by 2. In our example mentioned above, you will need 2000W (1000W x 2) Watts inverter instead of 1000 W inverter.
Continuous Power – Typical & Normal Operation: This is the normal operation an inverter is designed for i.e. you can connect safely and continuously the above mentioned or similar electric load to the inverter. For example, it is the load after the initial surge (during starting a motor or refrigerator) where it will operate at normal load conditions.