Inverters: What size inverter should I get?


Staff member
Jan 7, 2017
Inverters: What size inverter should I get?

No matter what inverter type you get, you must match it properly with your load current. All inverters are rated with a running wattage, and a surge wattage. In most cases, the surge wattage is usually double the running wattage. But different inverters will run at this surge for different times. Some may only run for 5 seconds, others 1.5 seconds, some as long as 1 minute. A surge wattage is when a heavy load kicks on, this is usually an electric motor (washer, dryer, ceiling fans, fridge, air conditioner being a few). When the motors first kick on, they use large amounts of current to get them started. Once running, they drop in current usage. You need to calculate what your running wattage and what your surge wattage is.

Running Wattage:
This is the amount of power consumed while a device is running. Most devices are listed with running wattage. Every electrical device has a running wattage. A phone charger may be 3W, an electric water heater may be 3000W.

Starting/Surge Wattage:
This is the amount of power consumed/required for a device to start. Most devices with a surge wattage may not list this. Ceiling fans, dishwasher, fridges, may not list this value. Any device that has compression to fight (air conditioners, fridge, freezer, etc) will have a high surge wattage. This usually lasts only a few seconds. Even a vacuum has a surge wattage, though it lasts <1second usually.

Calculating Required Wattage Size:
Ok, so now we need to know what size inverter to get. This is not always an easy answer. The reason is because most power companies give you, at most, daily consumption. My company only gives me monthly consumption and gives me an average for the month. This is not useful for an inverter.

To calculate what size inverter you need, you need to find out what your peak consumption is during the day. This is usually, for most families, either in the early morning when everyone is getting ready for the day, or in the evening when everyone is getting ready to settle in (cooking dinner, getting cleaned, washing/drying clothes perhaps, TV's/Computers are full blast, etc, etc, etc).
There are two ways you can find this information out. The long, tedious, and prone to errors way, is to go around to each device, found out what it's wattages are (both running and starting). Then, try to get as close approximation of how long each device is running at full power. If you have multiple devices you need to monitor, you can see how this can be hard. Especially with electric water heaters as you have no idea really when they start/stop.
The easiest method, and by far the most accurate, is to install an amp meter. This can either be installed on per device, or onyour housemains lines.
Installing on per device, you would need to install something similar to a Power Meter such as this: Floureon Energy Monitor Power Meter
This can be expensive if you need one for each device/powerstrip, or time consuming if you use 1 and pass it around device to device.

Another means is to install a metering device in your mains panel.This does seem a bit daunting, but it's actually quite simple. But due respect is required while you would need to be working in the mains panel, preferably turned off for safety measures. You can find a sample project here: Current monitoring with non-invasive sensor and arduino

DISCLAIMER: I am by no means an expert on any one or all of these fields/questions/topics. The results of this FAQ is a collaboration of multiple different members to come up with a common list of questions that would be asked and we have tried to answer. I was the member who was chosen to post the FAQ. If you have question that goes beyond the FAQ, please post your questions in the relevant section pertaining to your inquiry. Thank you and have a nice day