Inverter Size Calculator

Learning how to calculate inverter size for your needs can be a tricky task, especially if you’re unfamiliar with how an inverter works or how much power you need to produce. Inverters are useful pieces of equipment, but you’re likely to have questions about the necessary equipment in order to make an accurate estimate or find the correct answer to your concerns. Some common questions include:

• Can I run a freezer on an inverter?
• What can I run off a 300w inverter?
• What inverter do I need to run a microwave?

There are more, of course. But whether you need a big inverter or a small inverter, you can figure out the appropriate size by taking a look through our inverter size calculator.

First, how much power does a power inverter use? An inverter needs to supply two needs: Peak or surge power, and the typical or usual power.

• Surge is the maximum power that the inverter can supply, usually for only a short time (usually no longer than a second unless specified in the inverter’s specifications). Some appliances, particularly those with electric motors, need a much higher start up surge than they do when running. Pumps, compressors and air conditioners are the most common example and another common one is freezers and refrigerators (compressors). You want to select an inverter with a continuous rating that will handle the surge rating of your appliance so you don’t prematurely burn out the inverter. Don’t rely on the inverter’s surge to start your equipment because inverters don’t like to operate in their surge mode unless the manufacturer claims to have a longer surge time than normal.
• Typical is what the inverter has to supply on a steady basis. This is the continuous rating. This is usually much lower than the surge. For example, this would be what a refrigerator pulls after the first few seconds it takes for the motor to start up, or what it takes to run the microwave – or what all loads combined will total up to. (See our note about appliance power and/or name tag ratings at the end of this section.)

How Big of an Inverter Do I Need?

Finding the proper inverter size for your needs is as simple as adding together the necessary wattages of the items that you’re looking to power. Whether you’re looking for what size inverter is best for your house or something as simple as an inverter for power your TV, the proper size will be a measurement based on the typical power and surge power necessary. If your equipment has a startup requirement, you’ll need to supply additional surge power in order to not wear out your inverter.

Inverter watts to amps calculator: Finally, it may be necessary to find the required amps for your inverter in order to measure how much battery drain your inverter will need. This can be useful to find the right battery size for your inverter (which you can calculate using our handy guide) or for measuring the necessary volts. You can use the following formula to determine the size:

Volts * Amps = watts

or

Watts / Volts = amps

1250-watt example:

1250 / 120 Vac = 10.41 amps AC (typical number found on equipment)

or

1250 / 12 Vdc = 104.1 amps DC (battery drain per hour)

Here is an example:

First, you need to determine what items you need to power during a power failure and for how long. Here is a brief example (watt requirements vary):

• Lights – About 200 watts
• Refrigerator – About 1000 watts
• Radio – About 50 watts
• Heater – About 1000 watts

Total wattage needed is 2250 watts. The fridge and heater have a startup power requirement so let’s allow 2x the continuous wattage for startup requirements. 2250 * 2 = 4500 watts

To get a total watt estimate for all of the items you plan on powering with your inverter, check this convenient estimator. This useful measurement tool can save you time and give you an accurate measurement.

Second, select an inverter. For this example, you will need a power inverter capable of handling 4500 watts. The continuous power requirement is actually 2250 but when sizing an inverter, you have to plan for the start up so the inverter can handle it.

Third, you need to decide how long you want to run 2250 watts. Let’s say you would like to power these items for an eight-hour period. Well, this can be tricky because heaters and fridges run intermittently. Let’s assume all of the appliances will run 40% of the given time period, which is 3.2 hours of actual run time. We need to convert the AC watts to DC amp hours because that’s how batteries are rated.

To convert AC watts to DC amps per hour, you divide the watts by the DC voltage (usually 12v or 24volts). Let’s use 12 volts since it is the most common.

2250 watts / 12 vdc = 187.50 DC amps per hour

187.50 is now your power requirement per hour

You have now determined that 187.50 is your power requirement per hour, and now you need to multiply that by total hours of run time which is 3.2 in our example.

187.50 DC amps per hour 3.2 hours = 600 DC amps

Because you are using an inverter, you want to calculate the loss for converting the power which is usually around 5%.

(600 DC amps * 5%) + 600 DC amps = 630 DC amps per hour (This is how much power you need in an eight-hour period running your appliances 40% of the time.)

Fourth, now that you know your total power requirement is 630 DC amps, we can select a battery source. Most typical deep cycle batteries are six volts or 12 volts. I will give you two examples using each voltage.

12-volt battery example: If you select a 12-volt battery rated at 100 DC amps, you will need six or seven batteries in parallel (I will explain parallel vs. series later).

630 DC amps / 100 DC amp battery = 6.3 batteries

Six-volt battery example: If you select a six-volt battery rated at 200 DC amps, you will need six batteries in series and parallel. 3.15 * 2 = 6.3 batteries No, I didn’t make a mistake. When you use six-volt batteries, you have to connect them in series to reach 12 volts. Then you connect each series pair of six volts in parallel to create your 12-volt battery bank.

What is the Difference Between Running Batteries in Parallel vs. in Series?

When you connect batteries in parallel, you are increasing amps. When you connect batteries in series, you increase voltage. In the battery world, it is better to limit your parallel strings. It is better for your power system. In this example, I would recommend using six-volt batteries because of the number of batteries this example requires.

How do we charge these batteries? You will need a charger to charge the batteries when you have access to city power. Most deep cycle batteries need a “smart” charger so the charger doesn’t damage the batteries. In this example, you will need at least a 40-amp charger, if not bigger. The bigger the charger, the faster the charge. Make sure your charger is for 12-volt batteries because the system we just identified is a 12-volt system.

You will also need cables. For this example, a 4 AWT (0000) cable is required to handle 4500 watts of startup power. That is huge cable. You may also want to consider an inline fuse. A 500 amp for this example is perfect. To figure out the size of fuse, you divide your AC watts (startup) by DC voltage.

4500 watts / 12 vdc = 375 amps

You would need a 375-amp fuse or bigger. I recommend a 500 amp just in case you were to max out the 5000-watt inverter. This is just a brief example. There are many different ways to set up your system. You can use solar panels, wind, etc.

So, when asking yourself: What size inverter do I need? Remember that there are multiple factors that affect the necessary wattage. While the process for calculating this may seem simple enough, you have to consider the surge power in addition to power loss and other details that involve your electronic systems. From there, you’ll need to calculate your battery size, whether it would be ideal to run your batteries in parallel or series, what charger to use and how to connect them. The process of selecting the right equipment can feel daunting, but we’ve made our first steps.