Mobile & Electronics

Power Supply Load Calculator

Total load from a list of devices and the supply rating to look for, with a margin you choose rather than one we impose.

25 inputs Free, no sign-up

Your figures

Leave blank for this page's suggestion of 20%, which the result names as a suggestion. Set it to zero to see the bare total.

Device 1

Device 2

Device 3

Device 4

Device 5

Device 6

Device 7

Device 8

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

Sizing a supply means adding up what everything draws and then leaving room. The adding up is arithmetic; how much room to leave is a judgement, and one this page will not make for you beyond suggesting a starting figure.

A supply run at its limit runs hot, is less efficient and ages faster. A supply run at half its rating lasts. Where between those you land is a trade between cost and margin that depends on what you are powering.

How to use this calculator

  1. List each device with its wattage and how many of them.
  2. Leave unused rows blank — they are ignored.
  3. Set a margin, or leave it for the page's 20% suggestion. Zero shows the bare total.
  4. Compare the suggested figure against the supplies available, and round up.

The formula

Total load  = sum of (Watts x Quantity)
Suggested   = Total load x (1 + Margin)

The total assumes everything draws its rated power at the same time. That is the safe assumption for sizing a supply — and a pessimistic one for predicting a bill, which is a different question with a different page.

SymbolMeaningUnit
Watts Each device's rated power W
Quantity How many of it
Margin Headroom you choose %

Worked example

A router, a switch and a hard drive, with a 20% margin:

Router       12 W x 1 = 12 W
Switch        8 W x 1 =  8 W
Hard drive    6 W x 1 =  6 W
              Total   = 26 W
+ 20% margin           = 31.2 W   -> a 35 W or 40 W supply

Notes and limits

  • The margin is yours. 20% is a starting point this page suggests, not a manufacturer's requirement, and the result says so.
  • Motors and compressors draw several times their rated power for a moment when they start. A supply has to cover that surge as well as the steady total, and the rating plate gives it.
  • Rating plates give maximums. Everything drawing its rated power simultaneously is unusual, which is why the total is conservative.
  • Voltage has to match as well as wattage. A supply with ample watts at the wrong voltage is not a supply for that device.
  • This is arithmetic, not electrical design. Wiring, protection and earthing are work for a qualified electrician.

What this calculation assumes

  • Everything draws its rated power at the same time, which is the safe assumption for sizing.
  • The margin is your choice; the default is a suggestion the result names as one.
  • Start-up surge is not included and has to be checked separately.

Frequently asked questions

What size power supply do I need?

Add up every device's wattage times its quantity, then add a margin. Twenty per cent is a reasonable starting point, more if anything has a motor or if the supply will run continuously.

How much headroom should I leave?

Enough that the supply is not at its limit. Twenty per cent is a common starting point; 30-50% is kinder to the supply and to its lifespan. There is no universal requirement, which is why the figure here is yours to set.

Do I need to allow for start-up surge?

If anything in the list has a motor or a compressor, yes - the surge can be several times the running figure and lasts long enough to matter. The rating plate usually gives it, and this total does not include it.

Is a bigger supply wasteful?

Not meaningfully. A supply draws what the load needs plus its own small losses, so an oversized one does not consume its rating. Very lightly loaded supplies are slightly less efficient, but the effect is small compared with running one at its limit.

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