Mobile & Electronics

Electronics Power Budget Calculator

A power budget for a desk, a rack or an inverter - every load added up, ranked, with the headroom you choose.

25 inputs Free, no sign-up

Your figures

Leave blank for this page's suggestion of 25%, which the result names as a suggestion. More for anything with a motor; zero for the bare total.

Load 1

Load 2

Load 3

Load 4

Load 5

Load 6

Load 7

Load 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

A power budget is the sum of what everything draws plus room to be wrong. It is the first number you need before choosing an inverter, a UPS, a solar array or a rack circuit, and it is also where people discover that the total is not what they assumed.

The loads are ranked heaviest first, because the ranking tells you where a change makes a difference — and usually one or two items dominate everything else on the list.

How to use this calculator

  1. List every load with its wattage and quantity. Rating plates, adapters and labels are where the figures live.
  2. Leave unused rows blank.
  3. Choose the headroom. 25% is this page's suggestion; increase it for anything with a motor, or set it to zero for the bare total.
  4. Use the suggested figure as the minimum rating for whatever will supply it.

The formula

Each load   = Watts x Quantity
Total       = sum of all loads
Suggested   = Total x (1 + Headroom)

This assumes everything runs at once at its rated power. For sizing a supply that is the right assumption — a supply has to cope with the worst case. For predicting a bill it is pessimistic, because devices neither run constantly nor draw their full rating, and the multi-device cost calculator is the page for that instead.

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

Worked example

A desk with a computer, two monitors and a router, at 25% headroom:

Computer   150 W x 1 = 150 W
Monitors    28 W x 2 =  56 W
Router      12 W x 1 =  12 W
             Total   = 218 W
+ 25% headroom        = 272.5 W

So an inverter or UPS rated at 300 W or more, and a circuit comfortably above that.

Notes and limits

  • The headroom is yours. 25% is a starting point, not a rule, and the result says which of the two produced the figure.
  • Motors and compressors surge hard at start-up — several times the running figure. A supply must cover that, and this total does not include it.
  • For an inverter, the wattage sizes the inverter and the battery sizes the runtime. They are two separate calculations; the required capacity calculator handles the second.
  • Eight rows is the limit. Group similar items with a quantity rather than listing each one.
  • This is arithmetic for planning, not electrical design. Circuits and protection are work for a qualified electrician.

What this calculation assumes

  • Every load runs at once at its rated power, which is the right assumption for sizing.
  • The headroom is your choice; the default is a suggestion the result names as one.
  • Start-up surge is not included and must be checked for motors and compressors.

Frequently asked questions

How do I work out a power budget?

Add up every load's wattage times its quantity, then add headroom for the fact that estimates are wrong and supplies should not run at their limit. Twenty to thirty per cent is a common starting point.

What size inverter do I need for my house?

The inverter is sized on the total wattage you want to run at once - this figure. How long it runs is a separate question decided by the battery, which the required battery capacity calculator covers.

Should I include everything or only what runs at once?

For sizing a supply, include everything that could run simultaneously. That is the case the supply has to survive. For estimating a bill, use actual running hours instead, on the multi-device cost calculator.

How much headroom is enough?

Enough that the supply is not at its limit, since one run flat out is hot, less efficient and shorter-lived. Twenty-five per cent is a reasonable starting point; more if anything has a motor or if you expect to add loads later.

Related calculators