# How much power am I consuming from using an AC-DC converter?

I am duped on how power supplies work as I am not an electrical engineer but rather a hobbyist or a DIYer.

I am looking for cooling solutions and discovered the inverter technology.

If I understood it correctly, the compressor is run by DC, running at variable speed for whatever the system needs. If I want to assemble a phase-change system, using a DC compressor, and I have an AC power source (residential) and convert it using an AC-DC converter, I do not know how much electricity I am consuming.

This is what I am thinking, and these are just examples for simplification: suppose that I use a switched-mode power supply, rated at 10 A, and connect it to 220 V AC - the power consumed is 2,200 W. If I connect it to a load, DC, for example it outputs 12 V, and the load uses 5 A, so that is equal to 60W...

My question is, how much electricity am I consuming, and consequently have to pay for? Is it 2,200W or 60W?

• Is this to cool an Intel CPU on a PC? Is that why a 12VDC compressor? – jonk Oct 18 '16 at 18:46

My question is, how much electricity I am consuming, and consequently have to pay? Is it 2,200W or 60W?

Full load power will be somewhat less than 2.2 kW, probably about 80% of that meaning that about 440 watts are wasted in the power supply. Worst case scenario is that you will be paying for 440 watts even if the power supply feeds no output power at all. Best scenario is probably about 100 watts.

It's always best to target a design of power supply that can cope with maximum demand of the load and not go overboard or you end up paying for wasted heat.

For instance, a 100 watt power supply might waste 20 watts supply with no load connected and can quite easily supply a load demand of 60 watts.

Of course, if the current specified is for the output load then this changes things somewhat. For instance that would make the power supply a 120 watt device and would probably waste about 30 watts in heat.

Alternatively, if you are looking at (say) the CE marking label the 10 A specified may be when the power supply is being run from an 85 volt AC supply i.e. the power supply might have a universal input range such as 85 Vac to 265 Vac. You need to be clear on this.

Something between 60W + efficiency of power supply, likely between 10~20% for a decent switching supply. So 66~72 Watts.

I expect that the power supply is rated to deliver up to 10 Amps at 12 volts = 120 Watts. If you only draw 5 amps, the supply is only delivering 60 Watts, and will only draw 60 watts, plus a little for inefficiency, maybe a total of 70 watts, from the AC power source.

if you use a switching AC to DC converter with an efficiency of 90% when the load is dissipating 60 watts, then the converter will require an extra 6 watts from the mains in order to do its job.

If the power the load requires is less than 60 watts, then the efficiency will drop since the converter needs to eat VA just to keep itself alive.

• what is VA sir? – iamoni Oct 19 '16 at 11:04
• @iamoni: VA is the product of Volts and Amperes. For DC or AC into a resistive load, volt-amperes is equivalent to watts. For AC into a reactive load where current and voltage aren't in phase it's sometimes called the "apparent power", where the real power dissipated by the load will be: $$P=VA \cos\phi,$$ where $cos \phi$ is the cosine of the phase angle. – EM Fields Oct 19 '16 at 11:26