Amps watts volts

[nzjimmy]

First off, I love Netonix switches. I have 13 minis in my WISP and they have been running for over 4 years, with only one dead one - amazing.

Please help me understand something; my Ignitenet radios will run at both 24v and 48v. I have tried them on both and they use around 11watts no mater which voltage. It must be true that at the higher voltage (48v) they use less current - my question is; this surely can't use less battery right? I always figured 11watts is 11watts no matter which voltage, and wattage is a complete measurement. Am I wrong?

My other question is about powering the switch. I use those Meanwell security UPS at 12v, then step up to 48v using a Meanwell dc/dc convertor. I then use a Tycon 802.11bt passive injector to power the mini via ethernet. Lets say the mini was reporting 50watts on its stats page. What voltage is this at? Is it the 48v the switch is getting? And how much wattage will the 12v security UPS be delivering? As I wind up the voltage on the dc/dc converter from 48v to around 55v, it's not using less battery running time is it?

I feel a little dumb asking this as ive always just thought watts are watts, but someone smarter than me was saying I should be thinking in amps and not in watts, as amps is always amps. We got onto this topic when I was trying to get the right size cable for these systems, and was getting confused about what size wire for AC amps vs DC amps.

I'm either overthinking or under-smart haha! Can anyone clear this up without confusing me more?

Thank you

[Dave]

well now...that's a lot of questions.....but you are right...watts is watts...as in power used is power used, and basically you are using twice the current when using 24 volts versus 48 volts for same wattage load...

the mini is reporting its estimated total unit usage, which includes the POE loads & the actual boards power usage....

So, your question about does it use less battery power if you are running at say 24v versus 48v..the ohms law answer is NO...but the real world answer is different..when you for example step up from a 12 volt source to 24 volts, or 12 volts to 48 volts, you have to consider the efficiencies of the dc-dc convertors...and no simple answer for that because it depends on the actual convertors.

When it comes to deciding the cable AWG size for any system, you do indeed need to focus more on the actual current, because this question again defaults back to ohms law where the power in the cable will heat up more if more current flows through the cable....

anyways....not a simple question.....but hopefully i answered you with some insight...

[nzjimmy]

Thanks very much for your reply, Dave.

Maybe if I tell you exactly what i'm trying to work out, you will be able to help me finally understand.

I have a 12v|60 watt psu powering my WISP Mini. It gets stepped up to 48v by a dc\dc converter then powers it via a passive 802.3bt injector - for the sake of simplicity, let's remove the losses from stepping up.

questions;

1. when the switch's gui shows a total of 60watts being used by all devices [and switch itself], is the 12v psu getting 60watts drawn from it?

2. when the switch's gui shows a total of 60watts being used by all devices, what voltage is this 60 watts being measured at? Surely it can't be a combination of 24v and 48v (which is what it's providing to the radios).

3. in switch's gui, ignitenet radio uses 11watts @ 24v. Changing it to 48v and still uses 11watts. Does this prove the port's watts reading is measured at the native 48v input?

Should I be thinking in amps or watts? It seems like amps are always amps, but watts vary depending on volts. This is why I ask question #2 - what use is seeing the wattage a device is using if I don't know what volts this is at ... if comparing power consumption surely you need to use a complete measurement - and this is amps and not watts?

Finally, is there any advantage of pushing the dc\dc up to say 55v rather than running it at 48v? And if a radio can run at both 24v or 48v, will it use less battery at 48v? cable losses aside.

Thanks again Dave, I really appreciate you explaining this to me. It's weird to realise you've been thinking the wrong way about someting for a long time