# how much watts am I actually using on an amp with 500 watts RMS?

Discussion in 'Amps and Cabs [BG]' started by Whippet, Mar 14, 2019.

1. ### Whippet

Aug 30, 2014
Say I am using a 500 watt RMS amp with an 8 ohm cab. If I am not blasing the thing at concert or gig level volume but playing at home, am I still using the 500 watts (RMS) all the time? Or am I using a fraction of the watts since the volume is low?

Is there a way to figure out how many watts I am using at a particular volume and impedance.

Thanks for all information in advance.

2. ### Bob ClaytonAnti-tort heathenStaff MemberSupporting Member

Aug 14, 2001
Philly Suburbs
3. ### Whippet

Aug 30, 2014
there has to be something more simpler..... like if I am at volume 5 of 10, I am using 250 watts. LOL

thanks for the information. It is not, not simple. It is downright impossible to figure it out for me.

4. ### mmon77Supporting Member

Jul 9, 2008
Southern MN
Ugh. Right away, in the first answer in that link, they perpetuate the "clipping amps output DC" myth.

As far as the OP question, you're not using the full 500 watts if the amp isn't running wide open.

It would be difficult to measure the actual wattage being output while playing, because it changes constantly.

Last edited: Mar 14, 2019
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5. ### Rick James

Feb 24, 2007
New Jersey
Measure the voltage. Using an Ohm's Law calculator you can find the power.
Ohms Law Calculator

The problem is that the impedance load is different at every frequency, and the signal out of your bass isn't constant level, so you can't get an accurate result.
The position of the volume control has almost no relationship with the power output. It's a factor, but only one of many.

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6. ### basscookerCommercial User

Apr 11, 2010
cincy ky
Owner, Chopshopamps.com
And wide open you could be making your amp output more than the rated RMS power into the minimum impedance. As a guideline, if you have a standard "500 watt" amp you actually only have an amp rated RMS of about 225-300 watts. Do much cutting frequencies with eq? Less. Do a lot of boosting? Then you will be more likely to use more power at a lower setting on the volume knob.

Speaking of volume knobs; there is ( for the most part) no correlation that could be considered set in stone between the knob position and output power.

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7. ### Whippet

Aug 30, 2014
Thanks for the information.

My Markbass Multiamp says RMS 500 at 4 ohms and RMS 300 at 8 ohms. This just means the average right?
So theoretically it could be something like 1000 at 4 ohms or 600 at 8 ohms from time to time?

The reason why I am asking is because I want to hook up two low powered amps to one cabinet through a Mesa Head Track.

The Head Track can take up to 150 watts. Not RMS. This means that the RMS the Head Track could take would be something around 70-80 watts?

8. ### agedhorseSupporting MemberCommercial User

Feb 12, 2006
Davis, CA (USA)
Development Engineer-Mesa, Product Support-Genz Benz
St low volume it will be ok but that unit was designed for guitar.

Also, as a general note, that unit was not designed for bridged amps. I don't know offhand how the sleeve signal routing is handled, but I would not recommend using it with a bridged amp without analysis for isolation on the sleeve circuit.

9. ### Whippet

Aug 30, 2014
Is there anything out there like the Head Track which can be used with a bass amp, as in up to 500 watts RMS?

10. ### Jack

Sep 6, 2003
Newcastle, UK
You can get plug in energy use meters.

I suppose you'd either keep looking at the instant use display as you played and sort of average it in your head or measure total power used in a given time and then divide by the time to find an average? I dunno. The real answer is 'some'.

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11. ### Whippet

Aug 30, 2014
Looks simple and seems like a solution. Thanks for the information.

12. ### mmon77Supporting Member

Jul 9, 2008
Southern MN
That would measure the power consumption of the amp, not the power output. The two may or may not be proportional, but they're definitely not equal because no amp is 100% efficient.

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13. ### lz4005

Oct 22, 2013
It isn't really an average.

There are other factors in the power equation that are almost always buried in the fine print somewhere, if they're published at all. And each manufacturer does things differently.

The rating is really more like X watts at Y ohms....with Z% distortion at Q frequency or frequencies, for R seconds or milliseconds, measured at S distance from T speaker which has U sensitivity.

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14. ### Whippet

Aug 30, 2014
so the power output should be lower than the input right?

15. ### Whippet

Aug 30, 2014

If the companies only post the RMS, it just means that it can handle up to that much watts for extended time?

Kinda sounds fishy.... I mean you could literally just make up some numbers and assume that an amp is putting out 500 watts at 4 ohms and 300 watts at 8 ohms but it could just be a brief moment but in general can only handle 150 watts for an extended period of time.

Wouldnt this have a major problem with getting the desired volume out of your cabinets?

16. ### lz4005

Oct 22, 2013
The numbers listed by large manufacturers are often the result of a negotiation between the "it won't blow up before the warranty expires" numbers the lawyers want and the "we have to make it look good so it will sell more" numbers the marketing department wants.

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17. ### Whippet

Aug 30, 2014
I guess when bean counters and lawyers get involved, you get distortion without a dummy load.

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18. ### GreyvagabondSupporting Member

Aug 17, 2007
Los Angeles
Non-snarky question: why do you want to know this?

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19. ### ashtray

Feb 25, 2008
Ventura County, CA
I’ve bought power amps that were bottom tier that claimed high wattage - then in very very fine print, you read the wattage was at a certain frequency for a nano-second with some high % THD. Infamously, JC Whitney used to sell a 1,000 watt car stereo amp with EQ for \$29.99. I think their 1,000 watt rating was theorhetical based on how much you could boost each eq band, because the actual clean volume that trash put out was on par with the stock 20 watt stereos of the day.

Btw - why are you trying to run two heads in to one cab? I think the safer thing to do wpuld be to pick one amp (power section), and then run an A/B pedal in your signal chain. Run each output to the input of each amp you want to use, but on one, run a cable from the line out to the line in on the other amp. Now plug that other amp in to the cabinet.

Now you’ll have the ability to switch between which preamp section you are using, without worrying about blowing anything up.

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20. ### mmon77Supporting Member

Jul 9, 2008
Southern MN
Well to be fair, I've never measured it, but I can't see how it wouldn't be. Some power is always lost in the form of heat, and there are other losses as well.

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