What Size Inverter Do I Need

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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.

Inverter Size Calculator

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:

Load Estimation

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.

Device	Unit	Load (Nominal power)
Tube Light (40W)	6	240 W
Fan (60W)	3	180 W
Water Pump (250W)	1	250 W
TV (60W)	1	60 W
Refrigerator (150W)	1	150 W
Total			880 watts

You must adjust it according to your appliances.

Read my guide on inverter vs. converter here.

Accounting For Surge Or Startup Load

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.

Device	Unit	Load (Nominal power)	Nominal Power Load Multiplied By	Startup Load (Surge Power)
Tube Light (40W)	6	240 W	Not applicable (No motor/Compressor)	Not applicable
Fan (60 W)	3	180 W	5	180*5 = 900 W
Water Pump (250W)	1	250 W	5	250*5 = 1250 W
Refrigerator (150W)	1	150 W	5	150*5 = 750 W
TV (60W)	1	60 W	Not applicable (No motor/Compressor)	Not applicable
Total			880 Watts		2900 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.

Compensating for Inverter Losses

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.

Determining Input, Output Voltage Characteristics Of The Inverter

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.

What Size Inverter Do I Need For Home

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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What Size Inverter Do I Need For Camper

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 FAQ

What size inverter do I need to run a TV and satellite receiver?

Power withdrawn by a TV or satellite receiver depends on employed technology and size. For example, modern day 32 inch LCD or LED TV needs 40 to 80 ac watts for operation. While 42 inch LCD or LED needs 60 to 140 AC watts. For accurate information, you must see catalogue provided by your device manufacturer.

Similarly, a typical satellite receiver might demand anything between 30 and 120 watts during its operation mode (refer to catalogue of your receiver). Let’s assume TV and satellite receiver withdraw 60W each. Efficiency of inverter due to cooling fan and switching losses is 80%. So you need an inverter rated (60W + 60W) / 0.8 = 150W or above.

For TV and satellite receiver pure sine inverter is highly recommended because of presence of sensitive electronics.

What size inverter do I need to run a fridge?

Before decide the inverter sizing for a refrigerator, you must know the power need of your device. Power need of an inverter depends on many factors like size, environment – it is placed in, age, technology etc. Typical wattage need of a refrigerator unit might range between 120W and 400W. Let’s assume, your refrigerator needs 150W continuous power. Compressor in the refrigerator needs 5 to 7 times more power during startup for quite long time. This need can’t be compensated by peak power rating of inverter (see time factor). For 80% pure sine wave inverter, you will need inverter rated above (150 * 5) / 0.8 = 937W.

What size inverter do I need for a microwave Oven?

For microwave oven, both modified sine wave and pure sine wave inverters are safe to use. Size of inverter depends on microwave power rating. In standby mode, it needs few watts. Wattage rating mentioned on nameplate of the device is usually cooking power when microwave has got to a specific temperature. To rise to that temperature, usually double than mentioned power is needed. So let’s suppose, microwave label mentions that 500 watts are needed for this appliance. After accounting for losses and starting demand, inverter of (2 *500) / 0.8 = 1250W is needed just for this specific microwave.

What size inverter do I need to run a laptop and computer?

Typically, a laptop needs 50 to 80 watts. A desktop computer needs 100 to 250 watts for its operation. Suppose, one needs to run one laptop and one computer simultaneously on the inverter rated 60W and 150W respectively. If inverter losses account for 20% of processed power, then needed inverter must be rated (60W + 150W) / 0.8 = 262.5W or above. As no inverter of 262W is available so one with 300W or 500W would do well for the purpose. Moreover, modified sine wave inverters perform well with laptops and computers because all such appliances have their specialized chargers which take AC and provide DC power. Anyhow, pure sine wave inverters will do even better.

What size inverter do I need for my CPAP machine?

Continuous Positive Airway Pressure (CPAP) machines are usually recommended to run off grid to avoid effects of power shortage. And inverter is a basic requirement while going off grid. Mostly manufacturers recommend use of pure sine wave inverters for CPAP machines. Rating of inverter, to be attached with CPAP machine depends on size of machine, user settings, and model. For example, CPAP machine rated at 100W can operate with an inverter of 100 / 0.8 = 125W or larger inverter. Here, 0.8 is efficiency factor.

What size power inverter do I need to run a coffee maker?

Coffee maker produces heat from electricity and thus needs lot of power. Exact power withdrawn by a coffee maker depends on manufacturing technology, and size. A typical coffee maker with capacity of 4 to 5 cups withdraws 800W to 1300W on average. Coffee maker is an inductive load. It means it will need almost three times of its nominal power during startup. So if your coffee maker is that of 1000W, your inverter has 80% efficiency, you must buy an inverter of minimum (3 * 1000) / 0.8 = 3750W. Pure sine wave inverter is recommended for inductive loads.

What size inverter do I need to run a fan?

With the advancement in electric drive technology, motor based machines including fans have become more efficient. Power need of a fan depends upon its size, technology, settings etc. how much power does your fan needs, will be mentioned on its nameplate. Moreover, inrush current of a fan is 5 to 7 times of nominal load because fan is motor based load. For noise free operation, pure sine wave inverter is highly recommended in this case. Let’s suppose that your fan needs 60W for continuous operation and inverter is 80% efficient. Then your inverter must be rated above (5 * 60) / 0.8 = 375W.

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.