Receiver Power Consumption Rating vs Output Power Is Not Watt You Think!

The Power Consumption on the back panel is NOT always indicative of MAX Power!

Ever wonder how your favorite manufacturer rates power consumption on their AV receivers and how it relates to maximum available output power to your speakers? I can't tell you how many comments I've read on our forums or Youtube community alleging dishonest wattage claims based on the back panel power consumption of AV receivers. This article explores this topic to determine the truth. We give several product examples including power calculations which were vetted by the brands for accuracy.

Typical Discussion:

Happy Consumer: I just bought a new receiver (enter favorite brand here) rated at 120wpc x 7CH and it kicks ASS!

Skeptic: No way that receiver can deliver 120watts All Channels Driven (ACD) when the power consumption is only 500 watts. At best it can do (.6)500/7 = 43wpc ACD.

Happy Consumer: No way man, this thing is powerful. I saw a review on Audioholics where its bench test results exceeded the power consumption ratings (like 90wpc x 7). This thing is magic.

Skeptic: Not possible. That would mean it can deliver 630 watts which is greater than the 500 watt back panel rating.

Pursuing the Truth: The truth is, they are both sort of right, but not likely for the reasons they think. Skeptic incorrectly believes 500 watt power consumption rating is a max power rating while Happy Consumer thinks the manufacturer somehow violated the first law of thermodynamics by delivering more power with ACD then the power consumption rating on the back panel.

Receiver Back Panel Power Consumption Ratings vs Output Power YouTube Discussion

I emailed our friends at Sound United to inquire about how they rate the back panel power consumption of their products and the answer was very revealing.

What does the back panel power consumption rating mean?

• Unless it says "max power" don't assume it's a max power rating with all channels driven.
• ·According to Sound United, they follow IEC 62368-1 electrical safety standard for rating power, specifically Annex B for operating conditions and Annex E for test conditions.
• Amplifier to be tested at 1/8th unclipped de-rated power @ 1kHz & rated load impedance.
• Sound United follows IEC 62368-1, the electrical safety standard for audio, video and similar equipment for their AV receivers.

• Follow operating condition under Annex B: Normal operating condition tests, abnormal operating condition tests and single fault condition tests, and the amplifier output conditions defined under Annex E: Test conditions for equipment containing audio amplifiers.

• Measured input current/power under normal operating conditions shall NOT exceed the rated current/power by > 10%. Note: ACD testing is NOT considered a "normal operating" condition.

Before continuing this discussion, it's important to note how amplifier efficiency works when determining power produced vs. power consumed. Illustrated below are efficiency curves of a typical Class AB linear amplifier and a switching Class D amplifier. Most AV receivers still employ linear Class AB amplification such as Denon, Marantz, Sony and Yamaha, while some models of Pioneer and Onkyo utilize Class D amplification.

Efficiency vs Amplifier Output Power Class AB vs. Class D - courtesy of Elliot Sound Productions

As you can see, Class AB amplifiers are least efficient (< 20% efficient when driven at 20% rated power level) when driven at low power levels, whereas Class D amplifiers reach peak efficiency at a much lower power level (90% efficient when driven at 40% rated power). If we were to compare two 100 watt/ch 7ch amplifiers driven at 1/8th  power (12.5%), the power consumption differences between a Class AB vs. a Class D topology would look like this:

Class AB: 0.125 * 100 = 12.5 watts / .20 (eff) = 62.5 watts x 7 = 437.5 watts

Class D: 0.125*100 = 12.5 watts / .80 (eff) = 15.6 watts x 7 = 109.4 watts

437.5 watts - 109.4 watts = 328 watts greater power consumption for the Class AB amplifier vs the Class D amplifier for the same power output!

As you can see, a Class D amplifier topology is MUCH more energy efficient, especially at lower power levels where the AV receiver operates most of the time.

Let's look at some real world examples of products on the market, to verify the back panel power consumption ratings.

Marantz SR8015 Back Panel - Note the 780 watts power consumption

Marantz SR8015 Example:

The SR8015 rated power 140wpc x 11 channels but the back panel power consumption is only 780 watts.

Marantz de-rates 2CH power rating to drive ALL 11 channels for the IEC 62368 safety test. In this case, 90.4 watts for ALL 11CH driven.

1/8th power (90.4 watts) = 11.3wpc / 0.17 eff = 66.5 watts

66.5 watts x 11 = 731 watts + 35 watts from HDMI, DAC = 766.5 watts < 780 watts back panel rating

Note: Our bench tests of the SR8015 produced 100wpc x 7 ACD which would yield a power consumption of 700/0.55 (eff) 1272 watts + 35 watts = 1307 watts > 780 watts back panel power rating.

Someone on an audio forum is about to type "But wait, 1307 watts > 10% above 780 watts (858 watts). Yes they are correct, however, this is NOT a violation of the safety standard since the ACD test is NOT considered to be a "normal operating" condition as previously stated.

Denon AVR-5805 Back Panel. Note 13A power consumption

Older Receivers per UL1492: Denon AVR-5805:

The AVR-5805 was a 10CH powerhouse receiver topping the scales at 90lbs with a 170wpc x 10 (8ohm rating) power rating. The back panel power consumption is not rated in watts. Instead, it states 13 amps. Denon engineering confirmed to me that this rating was derived using UL1492 testing methods.

The power de-rating for the UL1492 test was calculated based on 70% of the 6 ohm stereo power rating (200 watts/ch) = 140 watts.

1/8th power (140watts) = 17.5wpc/0.17 eff = 102.9 watts

102.9 watts x 10 + 35 = 1064 watts

We need to convert watts to Amps since the back panel states 13A power consumption.

P = V*I => I = P/V but we also must factor in Power Factor when going from watts to VA which is about 75% for linear transformer power supplies.

1064/120/0.75 = 11.82A + 1A (AC Outlet) = 12.82A < 13A rear panel rating

Efficiency, Temperature Rise and Power Ratings

The efficiency (AC consumption) to-actual watts RMS output relationship is variable and inconsistent from brand to brand, and across time periods. For example, a vintage Kenwood KA-7002 integrated stereo amplifier from 1973 shows 275 watt AC power consumption at full power, which is 50 wpc RMS, ACD, 20-20kHz. Let’s call that a 2.75:1 consumption-to-output ratio (275/100 = 2.75.)

Parasound NewClassic 2250 v.2 2CH Power Amplifier

The Parasound NewClassic 2250 v.2 stereo power amplifier from 2020 shows an AC power consumption of 1000 watts (maximum) and is spec’d at 250 wpc @ 8 ohms, 400wpc @ 4 ohms, ACD, 20-20kHz. That will be a 1.6:1 consumption-to-output ratio (1000/550 = 1.8 for 8 ohms and 1000/800 = 1.3 for 4 ohm loads.) A 45-year newer design than the Kenwood and obviously more efficient, even allowing for the fact that the Kenwood includes a pre-amp section and the Parasound is strictly a power amplifier.

The point is, the back panel “power consumption” figure is not always indicative of the device’s actual maximum wattage power output. Many inexperienced fanboy rooters like to think it is, but it often is not. The actual mathematical relationship between “AC power consumption” and actual maximum wattage output of the device is dependent on the amplifier’s efficiency, the duration of the output, the frequency and distortion parameters, and how the manufacturer rates back panel power consumption. It will vary, no question, so don't ASSume.

In my conversation with Sound United engineering, they pointed out that back panel power consumption is usually specified per a safety standard with the most important criteria being a temperature rise test. Safety standards approval agencies will look for the most demanding conditions for each product. To satisfy the temperature rise test, limiting the output in multi-channel driving conditions automatically lowers the back panel rating. As a result, if the power consumption of two Class AB power amplifiers with the same rated output power and same number of channels have different power consumption ratings on the back panel, it's natural to think the lower power rated receiver has less maximum output due to derating to meet a specific temperature safety standard test. It's quite possible the lower power rated receiver has less heat sink area or less ability to dissipate power during long term test conditions.

Editorial Note About Safety Testing:
Sound United engineering informed me they used the UL1492 test method about ten years ago and now they are testing to the IEC-62368 safety standard for their latest products. In between the periods of these two standards, the IEC-60065 safety standard was used. As for the back panel rating, IEC-60065 is almost same as IEC-62368.  IEC-62368 is a newer standard and many safety items (requirement for fire and so on) has been updated. 

Editorial Note About Temperature Rise Test:
This test executes in the normal operating condition (1/8th power of ACD) required by the safety standard certification. And the test is necessary at least 4 hours until the temperature reaches saturation. The ACD is measured to determine the test condition. Because it depends on the results of the ACD, it is equivalent to adjusting the normal operating environment.

Under the ACD condition, the absolute test limit values of the temperature rising test are set based on safety regulation standards for the DUT in the abnormal operating condition.

Each manufacturer decides the time and the output level of the ACD using a shutdown mechanism or cooling system based on their respective design policies or their know-how.

Conclusion

It is our hope that this exercise has now cleared up the back panel power rating confusion debate that has raged on with audiophiles for many years on the  audio forums. As you can clearly see, there is often little to no correlation between back panel power ratings vs. actual max power consumption of AV receivers when the amplifiers are driven at full power. The Denon and Marantz examples illustrate this very clearly. Upon closer inspection, I confirmed with Yamaha that they rate back panel power consumption similarly. It also appears Pioneer does the same, even with their Class D receivers such as the 140wpc x 11 rated Pioneer SC-LX904 with back panel power consumption of only 340 watts meanwhile their Multi-ch Simultaneous Drive (8ohms, 1 kHz, THD 1 %) is rated at 880 watts. Assuming 90% Class D efficiency, that's actually a max power consumption of 977 watts > 340 watts!  This receiver is no slouch despite what an inexperienced enthusiast may misjudge based on the back panel power rating. 880 watts / 7 would mean this baby can deliver 125wpc with 7 channels driven or 80wpc with all 11 channels driven!

The back panel power rating on this particular Pioneer model is interesting. Although I wasn't able to confirm with Pioneer, I assume that the 340 watt rating is based on 1/8th power rating ACD. Since it is a Class D topology, low power drive is still very efficient (around 70%) compared to < 20% of Class AB. In this case, 1/8th power of 140 watts is 17.5 watts.

1/8th power (17.5 watts) = 17.5wpc / 0.70 eff = 25 watts

25 watts x 11 = 275 watts + 35 watts from HDMI, DAC (guess) = 310 watts < 340 watts back panel rating.

While exploring this topic, it became quite evident to the author that the industry really needs state power consumption ratings more clearly to the consumer. In addition, I'd like to see the industry move towards Class D amplification in products with high channel densities such as Atmos/DTS:X AV receivers. NAD is one example of an AV receiver manufacturer making the switch to Class D amplification in their AV receivers. As a result, I noticed one of their models, the T778, appears to rate max power consumption as indicated by the 1000 watt back panel rating. If you assume 90% efficiency of Class D, then it's easy to see their 85wpc x 9 ACD is achievable based on this rating. Meanwhile, their older receivers such as the T758 employing linear AB amplifier designs appear to be rated  per UL1492 in VA at a de-rated power level (5A consumption) with all channels driven just like the Denon AVR-5805 example.

Note: I was unable to confirm if NAD T758 meets the IEC 62368 4 hour safety test certification at this power level.

Editorial Note about Back Panel Power Rating:

It's not illegal per safety standard to specify a higher back panel power rating than what the product will consume when driven at the 1/8th power test with ACD. It appears the manufacturer can specify their power consumption rating as they choose as long as the number specified on the back panel isn't exceeded by 10% under normal operating condition. IE. A receiver back panel power consumption rated at 1000 watts cannot exceed 1100 watts under normal operating conditions.

NAD T758 (left pic) - note 5A power consumption rating ; NAD T778 (right pic) - note the 1000 Watt Power consumption rating

We've reached a point now where there are several SOTA Class D amplifier solutions that offer load invariant designs that measure every bit as good as the best Class AB topologies. An argument can be made that SMPS power supplies can be quite costly to implement correctly in order to be free of noise and radiated emissions.  However, an AV receiver manufacturer can very easily make the Class D amplifier switch while still employing their time-tested and proven linear power supplies in current products.

It is our hope that this article serves as a nudge to move the needle forward towards more energy efficient amplifier deployment in AV receivers. Tell us what you think in the related forum thread below.