How Much Amplifier Power Do I Need?

It’s a common question: “How much amplifier power do I need?” We get asked all the time about power requirements to drive anything from large tower speakers to small satellites or non-amplified sound bar speakers. There’s a simple solution to the question—but it may not be what you might at first think.

How Big is Your Room?

Before we can answer the question of how much amplifier power you need, we have to ask what the room and venue will look like. Are you attempting to power a small home theater? Are you looking to run some powerful outdoor speakers, or a whole-home audio system from a multi-channel amplifier? The answer to the question will determine, at least partially, how much power you need to drive your speakers. A bigger room will invariably require more power, and when you go outdoors, you almost never have enough (provided your speakers are sufficiently rated. Which brings us to our next point.

What Do Your Speakers Tell You?

Every pair of speakers I know of gives you a rating for their power handling. This is a range of power that the speakers can handle. Find that range…and then ignore it. Just kidding.

Sort of.

You see, while speakers will give their power ratings in a range, those ranges are typically very wide—and they don’t take into account the quality of the audio going into them at those power ranges. A massively distorted signal can blow a speaker at lower levels much faster than a clean signal will at high levels. The bottom line is that you want to have an amplifier that can drive the speakers hard enough that you don’t need to be maxing out every part of the signal chain in order to get the music loud enough for the speakers. That’s where you start to run into messy audio and risk damaging your drivers.

Some good specifications to understand are the “Nominal Impedance” and “Continuous Power Handling” ratings (sometimes it’s called the “IEC rated power”). With those two numbers you can figure out the ballpark on where you want your amplifier power to fall. In general, for a typical home theater you want to use speakers that have 4-ohm nominal impedance or greater. Most speakers today are 8-ohm nominal, but you can find exceptions. At 8-ohms, you want to have an amplifier that provides at least twice the power of the “continuous Power Handling” spec. That will keep you from sending lots of clipped audio to the speaker and damaging it over time. It sounds counterintuitive, but it’s not—you need more available power than what your speakers are rated for, not less.

The reason for this is because your speakers and your amplifier need to be able to handle the peaks found in most music and soundtracks. It’s those peaks that defy the “Nominal” numbers and which demand some extra power and additional power handling.

What’s Your Application?

This section is a bit tricky. It takes the above into consideration but adds one additional factor: What are you listening to? If you’re looking to play back fairly mellow tunes, then you can get away with a wide range of amplifier power that falls within the specs of your loudspeakers. If, however, you’re looking at more dynamic music, then you may want to ensure you have two to three times the nominal rated power for your speakers so that you can handle the peaks associated with more dynamic tracks.

Calculating Amplifier Requirements—Oh No, Math!

When you’re doing the math, the amplifier power rating must match the impedance rating for your speakers. If not, then you need to do some conversions.

Don’t worry, this won’t hurt…much.

If your loudspeakers are rated to 8-ohms, chances are you’re just fine as most amplifiers are rated to 8-ohms (in addition to other ratings they may have). If your speakers are rated to 4-ohms, however, then your amplifier or AV receiver needs to be able to support that speaker at its 4-ohm rating. Let’s take a quick look at how this works. If we want an amp rated to 1.5x the continuous power rating of a pair of 8-ohm speakers, then the math looks like this:

1.5 x 100W = 150W at 8-ohms

If you have a large room and listen to dynamic classical music on 4-ohm speakers, then your equation might look something like this:

2.5 x 100W = 250W at 4-ohms

That may seem like a lot of power, but you have to remember that an amplifier will have a much lower power output rating into 8-ohms than it will into 4-ohms. As you drop the impedance (the speaker load) the power output (or at least the power output potential) goes up.

Always match your impedance when factoring your calculations and you won’t go wrong. This is important because if you under-power a speaker you might damage it from turning up the volume too far (causing clipping and distortion.) If you over-power a speaker, well that is pretty bad as well.

While the above gets you in the ballpark, there is a lot more to calculating amplifier power requirements. You also need to know a few more specs, like the sensitivity rating of the loudspeaker, peak headroom, and the average distance to the listener. Of course, to really factor everything you also need to know what volume (SPL—Sound Pressure Level) you want to hit as well.

Note for Propeller Heads: The equation for calculating required amplifier power looks something like this:

Power in dB@1W = SPL@listening position – Loudspeaker Sensitivity + 20 * Log (Distance to Listener/Reference Distance) + Headroom

To calculate the required power (in watts) , you take 10 to the power of the above solution divided by 10. Sound confusing? It is, which is why we use calculators to help us. If you grab an amplifier calculator, then you merely need to plug in the critical numbers to arrive at the required power for your application. And in the case of home theater, most of this is unnecessary and overkill.

What’s in a Watt?

Watts are simply the unit of measure for power in amplifiers, but it’s amazing how power and wattage are used in various applications. Take home audio, for example. While it may take only 25 watts to comfortably drive a pair of desktop speakers for your computer to acceptable levels, you may require 150 watts of power to achieve theater levels in your living room. If you’re at a live stadium-level concert, you may be listening to upwards of 250,000 watts to hit those huge peaks. That’s a lot of power, but we’re also talking a lot of output for an incredibly large venue. How much amplifier power you need depends a lot on all these factors.

Summing It Up

Don’t sweat it. The bottom line is that you have a lot of resources to calculate the right amount of amplifier power. Most entry-level amplifiers and AV receivers will provide enough power for smaller rooms and speakers with higher sensitivity ratings (speakers which don’t require lots of power to play loudly). For larger rooms or outdoor applications, a dedicated amplifier or high-end surround receiver with dedicated Zone 2/3 amplifiers may be needed to get enough gain and headroom. You don’t want to waste money on an amplifier or surround receiver that’s overkill for your room, but you also don’t want to damage your speakers by driving your amps at max volume to compensate for inadequate power. Check the specs and ratings, and when in doubt give the pros at Sanway Audio a call when it’s time to to match up amplifier and speakers.