How it works
Add up the consumption of each piece of equipment—processor, video card, memory, disks, fans—and obtain the power the source needs. The total is calculated on the continuous lines of the source: it is the power delivered by its rails, not the power taken from the outlet.
A safety margin is applied to this total, because it is advisable for the source to work far from its limit: it gains efficiency, heats up less, makes less noise and absorbs the instantaneous peaks of modern video cards. With the 80 PLUS certification, it is also estimated how much is actually consumed from the electrical network, which is always more than the power delivered to the equipment.
The second part estimates the autonomy of a UPS with that consumption plus the equipment hanging from it. It takes the energy from the battery bank, discounts the inverter performance and depth of discharge, and applies the Peukert correction, which reflects something that simple division ignores: a rapidly discharged battery delivers much less energy than its label says.
Examples
420 W + 30 %550 WA device that consumes 420 W at a 30% margin calls for 546 W, so the next largest commercial size is a 550 W supply.420 W / 0,90467 WWith an 80 PLUS Gold supply (90% efficient), those 420 W delivered to the equipment translate into 467 W taken from the outlet. The difference is dissipated as heat within the source.1500 VA × 0,6900 WThe VA of a UPS is not watts. With a power factor of 0.6—typical for desktop line-interactive models—a 1500 VA UPS can only sustain 900 W.2 × 12 V 9 Ah, 500 W~8 minTwo 12 V, 9 Ah batteries in series form a 24 V, 216 Wh bank. With 500 W of charge the current is around 25 A, and at that discharge speed the real autonomy falls to about 8 minutes, less than half of the 18 that a direct division would give.Use cases
- Choose the font size before building a PC or changing the video card for a more demanding one.
- Check if the installed source can support an additional disk, a capture board or a new processor before purchasing them.
- Size the UPS for a workstation, a small rack or a NAS and know how many real minutes of backup it will provide.
- Calculate how many amp hours of battery are needed to sustain a device during the time it takes for an orderly shutdown.
- Estimate the electrical consumption of a server that works 24 hours a day to budget its operating cost.
Frequently asked questions
Are the watts of the source what my equipment consumes?
No. The power printed on a source is what it can deliver to the computer, not what it takes from the network. A 550 W source working at 80% delivers 440 W to the equipment and, at 90% efficiency, draws about 489 W from the outlet. Furthermore, a supply only consumes what the computer asks for: if you build a 300 W PC with an 850 W supply, it will still consume 300 W.
What safety margin should be left?
Between 20% and 30% for a common team. A supply's efficiency is maximum at about 50% of its load, capacitors lose capacity over the years, and modern video cards produce millisecond spikes that can double their average consumption and trip the supply's protection. If you are going to overclock or plan to add a board later, I increased the margin to 40%.
Why doesn't a 1500 VA UPS give 1500 W?
Voltamperes are apparent power and watts are active power; The relationship between the two is the power factor. Desktop UPSs typically claim 0.6, mid-range line-interactive UPSs 0.8, and double-conversion rack UPSs 0.9 or 1.0. A 1500 VA UPS with a factor of 0.6 runs out of capacity at 900 W even though it has extra volt-ampere.
What is the Peukert correction and why does it reduce autonomy so much?
The ampere hours of a lead battery are measured with a slow discharge, usually twenty hours. When discharged in minutes, the internal resistance causes it to deliver significantly less energy. The Peukert equation models this loss with an exponent that is between 1.1 and 1.3 in sealed UPS batteries: the higher it is, the worse the battery behaves in the event of strong discharges. With exponent 1 the equation is equivalent to direct division.
What discharge depth should I use?
A UPS cuts off the output before fully emptying the batteries to avoid damaging them, so 100% is rarely used. 80% is a reasonable estimate for sealed lead acid batteries with short, occasional backup; If the power outage is frequent or you want to extend their life, go down to 50%. Lithium batteries typically tolerate 80% or 90%.
What should I not plug into the UPS?
Laser printers, stoves, vacuum cleaners and any equipment with a motor or resistance: they consume enormous peaks that overload the UPS and shorten the life of the batteries. In the exit with backup, leave only the computer, the monitor, the modem, the router and the NAS; the rest goes to outlets that only have surge protection.
Are the predefined values accurate?
They are typical consumptions at full load for each type of component and are used for sizing, not for measuring. If you have the technical data sheet for your processor or video card on hand, write the exact value in the watts column: each field is editable and the total is recalculated instantly.