What Size Inverter Do I Need – Inverter Size Calculator Online

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To answer what size inverter do I need, you must know nominal load, surge power and continuous load of your appliances. After compensating the harmonic distortion losses of an inverter from your power demand, you get the right inverter size.

Here’s my guide to power inverters.

Disclaimer: Actual results may differ from the estimation.

Here’s my blog on best 1000 watts inverters.

My guide to best 2000 watts inverters here.

Read my post on best 3000 watts inverters here.

Best solar panel angle calculator here.

How to calculate solar panel battery and inverter here.

You are thinking to setup your backup system or going off grid, inverter selection and installation lies at the heart of this process. Because it is inverter which makes DC power available for your AC equipment.

To select best inverter or solar inverter for your backup or off grid system for any application, here is little homework plan to ease your search. Here’s my step by guide on what size inverter do I need:

Regardless of the fact, you need this inverter for home, car, or office; you must have an idea of load which will be powered through this inverter. Making a list of all items would be a good idea. Here, a sample list with assumed loads is shown.

DeviceUnitLoad (Nominal power)
Tube Light (40W)6240 W
Fan (60W)3180 W
Water Pump (250W)1250 W
TV (60W)160 W
Refrigerator (150W)1150 W
Total880 watts

You must adjust it according to your appliances.

Read my guide on inverter vs. converter here.

Many devices at home or RV withdraw some characteristic nominal power from start to end of the operation. Nominal AC power is mentioned on the label of the product. Anyhow, some appliances which have motors or compressors, withdraw heavy current upon startup (5 to 7 times of nominal surge power).

This startup current is for few seconds in case of motor and many seconds in case of compressor. Peak capacity of inverter is usually for a second unless otherwise mentioned. So startup power need of motors and compressors can’t be satisfied by peak capacity of inverter and must be accounted separately.

So, here, surge power load for fan, water pump, and refrigerator is calculated.

DeviceUnitLoad (Nominal power)Nominal Power Load Multiplied ByStartup Load (Surge Power)
Tube Light (40W)6240 WNot applicable (No motor/Compressor)Not applicable
Fan (60 W)3180 W5180*5 = 900 W
Water Pump (250W)1250 W5250*5 = 1250 W
Refrigerator (150W)1150 W5150*5 = 750 W
TV (60W)160 WNot applicable (No motor/Compressor)Not applicable
Total880 Watts2900 Watts

Now total load becomes: 900W + 1250W + 750W + 240W + 60W = 3200W

Note: Here, nominal load is taken from table 1.

Read my blog on solar inverters here.

All the power, an inverter receives from battery, doesn’t go to output terminals of inverter to power loads. Switches in inverter arrays have switching losses. Moreover, there are Ohmic losses from battery to inverter and inverter to load.

Overall efficiency turns out to be 80-90% for an inverter. While calculating size of an inverter, these losses must also be accounted for in this manner. Here, 80% efficiency is assumed.

Required power capacity of an inverter = 3200W / 0.8 = 4000W

So you must choose an inverter of rating above 4000 watts.

You may get one 5000 watts inverter or two 2000 watts inverters to distribute the load. Here’s my list of best 2000 watts power inverters.

Read why solar cells need an inverter here.

Once you are done with determining power rating of inverter, you must also know what input voltage it will accept, and what output voltage and frequency it will yield. For example, here in USA utility voltage is 120V at 60 Hz nominally. While Australian grid gives 220V at 50Hz. As appliances are also designed for some nominal voltage and frequency so inverter must be chosen as per needs of load.

As far as input side voltage is concerned, it depends on battery availability. If one has 12V battery available then that inverter must be chosen which accepts 12V DC. Anyhow, input voltage can be adjusted to some extent by placing batteries in series and parallel connections.

Read my blog on inverter vs. generator here.

Size of inverter for your home depends on the load. Suppose you want to attach accumulative load of 1000W with inverter. 200W of this 1000W is motor. Then your total load will be (1000-200) + (200*5) = 1800W.

Now, assume that inverter’s efficiency factor is 85%. So you must select an inverter with rating 1800W / 0.85 = 2117W or above.

Moreover, modified sine wave inverter is good to run almost all your domestic appliances but not radio set and few sensitive laptops. But putting in little extra money and buying continuous load pure sine wave inverter with built in charge controller would extend life cycles of your load items.

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Typically, a camper or RV needs 2000W to 4000W continuous power. But one must know one’s own camper to have a good estimate of inverter size for the vehicle. For example, you have microwave, TV set, cell phone chargers etc. in your camper and the total load turns out to be 1300W (after counting surge loads also).

Considering 80% efficient inverter, your inverter must be that of 1300 / 0.8 = 1625W or above. Ideally, camper inverter should have a transfer switch so load could be shifted between shore power and inverter power.

Pure sine wave inverter with built in inverter charger is better for campers to go noise free, and to ensure that every electronic device could be powered during your trip.

Pros and cons of solar tubes here.

Inverter Size In A Nutshell

Inverters have become important part of modern day electrical systems and questions like what size inverter do I need is becoming more common. Before buying an inverter, one must know the type of load (so startup current could be estimated), and watt ratings of the load. Inefficiency margin of inverter should also be accounted for while making calculations.

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