Convert Watts To BTU: Calculator + Conversion Chart (1 W = 3.412 BTU)

Watts are a basic unit of power. BTU (British Thermal Unit) is a unit of heat. We need to convert watts to BTU when it comes to different HVAC devices (including air conditioners, furnaces, and so forth).

Watt is a SI unit of power equivalent to J/s (joule per second).

BTU or British Thermal Unit is a part of US customary units. 1 BTU is defined as the amount of heat required to raise the temperature of 1 lb of water by 1°F. In practice, when we talk about BTU, we’re also referencing BTU/hr or BTU per hour (unit of power, not heat).

When we have two units representing the same quantity (ie. both watts and BTU/hr are power units), we can convert them or use them interchangeably if we follow the conversion rules.

How to convert watts to BTUs?


Just use the ‘Watt To BTU’ calculator you will find below. Basically, we only need to know how many BTUs are in 1 watt. Here’s the answer:

1 Watt = 3.412 BTU

Based on this simple conversion, we can design a simple-to-use watt to BTU calculator. You can use freely use it for simple unit conversion in HVAC or for any other purpose:

Watts To BTU Conversion Calculator


Using the watt to BTU calculator, you can simply input (or slide the slider) the number of watts and you will get how many BTUs is that.

We will solve a few examples of how to convert watts to BTU for an air conditioner, a furnace, and a space heater below.

But first, let’s address the question most people have when it comes to watts to BTUs conversion. It goes something like this:

“How many BTU is (NUMBER OF WATTS) watts?”

Basically, we want to know how many BTU is 500W, 1,000W, 2,000W, and so on. You can answer all of these questions pretty easily using the watts to BTUs unit conversion calculator above.

On top of that, we have calculated the most common BTU to watts and made a summary of them in the conversion table here:

Watts-To-BTU Conversion Chart For 500W To 20,000W

Watts: BTU:
How many BTU is 500 watts: 1,705 BTU
How many BTU is 1,000 watts: 3,412 BTU
How many BTU is 1500 watts: 5,118 BTU
How many BTU is 2000 watts: 6,824 BTU
How many BTU is 3000 watts: 10,236 BTU
How many BTU is 4000 watts: 13,648 BTU
How many BTU is 5000 watts: 17,060 BTU
How many BTU is 10,000 watts: 34,120 BTU
How many BTU is 15,000 watts: 51,180 BTU
How many BTU is 20,000 watts: 68,240 BTU

Let’s solve a few examples to demonstrate how to calculate the number of BTU from watts by hand and using a calculator:

Example 1: How Many BTU Does 3,500 Watt Portable Air Conditioner Produce?

Portable air conditioners are one of the most popular kinds of AC units. In most cases, especially across the US, you will see that the cooling capacity of portable AC units is expressed in BTUs. You will find 8,000 BTU, 10,000 BTU, 12,000 BTU, 15,000 BTU portable air conditioners, for example.

In Europe and Asia, however, they often use watts (or kilowatts; you can check kW to BTU conversion calculator here) to specify how powerful a portable AC is. If a European AC manufacturer is selling portable AC in the US, they have to convert watts to BTUs, as per regulations.

Example: Let’s say that we have a 3,500-watt portable air conditioner. How many BTUs is that?

Answer: We can calculate by hand or using the calculator above.

Converting 3,500 watts to BTUs by calculator:

how to convert watts to btu for air conditioners

Converting 3,500 watts to BTUs by hand:

We can start by knowing that one watt is equal to 3.412 BTU. That means that we can calculate how many BTU is 3,500 watts like this;

3,500 watts * 3.412 BTU/watt = 11,942 BTU

In practice, you won’t find an 11,942 BTU portable AC. All manufacturers will round that up to 12,000 BTU. That means we can say that 3,500-watt portable AC is basically a 12,000 BTU portable AC.

You can check a list of the best portable air conditioners here.

Example 2: Many BTU Is A 20,000 Watt Furnace?

The heating output of furnaces is most commonly expressed in BTUs. In the case of electric furnaces, however, the output can be expressed in watts. In that case, it’s useful to convert watts to BTUs to get a grip on what size of the furnace are we talking about.

Let’s say we have a 20,000-watt electric furnace. How much is that in BTUs?

Well, let’s calculate:

20,000 watt * 3.412 BTU/watt = 68,240 BTU

You can see that using the watt to BTU conversion calculator above gives us the same result:

convert watts to btu for an electric furnace


Most manufacturers would round that up as either a 68,000 BTU or 70,000 BTU electric furnace.

Example 3: How Many BTUs Does A 1500 Watt Heater Put Out?

Most electric space heaters are 1500-watt heaters. They are used to heat up space. In most cases, it makes more sense to get an idea of how many BTUs does a 1500-watt heater put out.

That’s how we can actually get a more realistic idea of how powerful a heater we have (because in the US, we use BTUs more commonly than watts for heating output).

Using the conversion calculator above, we can see that a 1500-watt heater puts out 5118 BTU of heating output:

how many btu does a 1500 watt heater put out

You can calculate that yourself by hand like this:

1,500 watt * 3.412 BTU/watt = 5,118 BTU

In short, how many BTUs is a 1500 watt heater? 1500 watt space heater can produce  5,118 BTUs.

Knowing the wattage of an electric heater is very useful when trying to estimate how much does it cost to run an electric heater (electricity costs). However, to get a feeling of how much warmth the heater can produce, BTU output is a more useful specification.

We use BTU to watt conversion quite a lot in our rankings. Example: You can check a list of the best quietest space heaters here. You will notice that you can calculate both how much noise can a heater make both in watts (how many watts per dB) and BTUs (how many BTUs per dB).

If you have any questions about how to convert watts to BTUs, you can use the comment section below and we’ll try to help you out as best we can.

1 thought on “Convert Watts To BTU: Calculator + Conversion Chart (1 W = 3.412 BTU)”

  1. I had a good laugh. I had a hvac contractor come out and give me an estimate for a 3 car garage in Colorado. He estimated the load at over 100 btu per sq ft for 600 sf.
    So they wanted $10,000 do the work. The actual load is about 36,000 btu. This in Denver CO.
    You really have to watch these contractors


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