Watts to kVA Calculator
Size a UPS, inverter or generator from a load you already know — turn the real watts your equipment draws into the VA rating a machine has to carry, and the commonly-sold size above it.
Last checked against the code
Watts to kVA Calculator
Updates as you type- Next commonly-sold sizeRatings vary by brand — confirm against the actual product
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- Spare capacity at that size
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VA = W ÷ PF · kVA = W ÷ (1,000 × PF)
How this is calculated
This is the sizing direction. You know what your equipment draws — measured at a meter, added up from nameplates, or taken from a load schedule — and you need to know what rating to buy. Machines that back up or invert power are sold by apparent power, so the load has to be translated into those units before a product list is any use:
VA = W ÷ PF · kVA = W ÷ (1,000 × PF)
Dividing rather than multiplying is the whole difference from reading a specification, and it means the answer is always larger than the load below unity. That direction is worth internalising, because the intuitive mistake here — matching an 800 W load to an 800 VA unit — fails in the dangerous direction rather than the safe one.
The power factor in the formula is the confusing part, and it is not what most readers first assume. It is not your load's power factor. It is the design ratio of the machine you are about to buy: the number you get by dividing a candidate unit's watt rating by its VA rating. A 1500VA/1350W unit is a 0.9 machine, and sizing against it means dividing your load by 0.9. Choose a different product and the required rating changes even though nothing about your equipment has moved.
What comes out is a threshold, not a purchase. Nobody sells 888.9 VA. Consumer and small-business ranges cluster around a familiar set of values — 500, 650, 750, 1000, 1500, 2200 and 3000 VA — and the requirement has to go up to the first of those it clears. Those clusters are a market convention rather than a published series, so a particular brand may step differently.
Three things this deliberately excludes. It adds no design headroom for equipment you have not bought yet. It says nothing about duration, which the cells decide and the datasheet publishes separately. And it takes no account of starting inrush, which governs the choice outright once a motor is involved.
Worked example
One full calculation with real numbers, so you can follow along and check the tool by hand.
An 800 W setup, sized against a 0.9 unit. The division is 800 ÷ 0.9 = 888.9 VA. Looking up the commonly-sold ladder from there, 750 VA is below the requirement and out; the answer is the1,000 VA size, leaving 111 VA of spare rating, or about 12.5%.
Twelve percent is thinner than it sounds. Runtime falls away sharply as a unit approaches its rating, so a battery that manages fifteen minutes at half load may give four or five here. If the 800 W is genuinely everything and always will be, the 1,000 VA unit is a reasonable buy. If a second monitor or an external drive is likely, the next rung is the better call — and that is a judgement about your circumstances rather than something the arithmetic can settle.
The same 800 W, sized against a budget unit. Suppose the shortlist is a standby model rated at 0.6. Now the division is 800 ÷ 0.6 =1,333.3 VA, which clears 1,000 and lands on the1,500 VA size. Identical equipment, identical watts, and a machine one full rung larger — because a lower-rated output stage has to be bigger to hand over the same real power.
That comparison is the practical reason to check a product's two figures before shortlisting on price. The 1,500 VA budget unit and the 1,000 VA better-rated one do the same job for you, and which is actually cheaper is no longer obvious from the headline numbers.
One caveat on the ladder itself. These sizes are what product lines converge on, not a standard anyone publishes — the same caveat that applies to the generator ratings elsewhere on this site. A brand may offer 1,440 VA or 1,600 VA where this list says 1,500, and rack-mount ranges step differently from desk-side ones. Use the rung as the size to search for, then confirm against the real product.
Visual comparison
| Rating | Real load supportedW |
|---|---|
| 500 VA | 450 |
| 650 VA | 585 |
| 750 VA | 675 |
| 1,000 VA | 900 |
| 1,500 VA | 1350 |
| 2,200 VA | 1980 |
| 3,000 VA | 2700 |
Source: Commonly-sold ratings, a market list rather than a standard series, evaluated at 0.9.
Reference tables
Most people arrive here with a setup rather than a number. This table starts from the setup: find the bundle closest to what you are protecting, and read across to the rating it needs and the size that satisfies it at a 0.9 design ratio.
The right-hand note is the column to read carefully, because it names the variable that will move your total away from the row. PoE switches are the extreme case — the switch itself draws very little and every camera or access point hanging off it adds up to 30 W. Two installations described identically can differ by hundreds of watts on that alone.
Notice how many different setups land on the same rung. Everything from a router at 30 W to a home lab at 250 W buys the smallest size, because the ladder simply has no finer steps down there. That is worth knowing before agonising over a precise figure for a small load: below about 450 W the arithmetic rarely changes the answer.
Nothing with a motor or a heating element appears here on purpose. Those loads draw a large multiple of their running figure at the moment they start, and that surge — not the steady total these rows describe — is what sizes the machine that has to swallow it.
| Setup | Typical loadW | Rating neededVA at 0.9 | Size to buy | What moves it |
|---|---|---|---|---|
| Router, modem and ONT | 30 | 33.3 | 500 VA | The cheapest thing to protect and the one that keeps working when everything else is dark. |
| Laptop, monitor and network gear | 120 | 133.3 | 500 VA | The laptop has its own battery, so the unit is really protecting the screen and the link. |
| Desktop workstation and one monitor | 300 | 333.3 | 500 VA | The volume case. A gaming machine under load can be double this; an office PC half. |
| Workstation, two monitors and a NAS | 460 | 511.1 | 650 VA | A four-bay NAS spinning adds around 60 W, and rather more for the few seconds its disks spin up. |
| Home lab: NAS, mini PC and PoE switch | 250 | 277.8 | 500 VA | PoE is the variable — every powered camera or access point on the switch adds to this. |
| AV rack: receiver, projector and streamer | 650 | 722.2 | 750 VA | A receiver driven hard draws far more than at conversational volume. Size for the way it gets used. |
| Small rack: firewall, switch and two 1U servers | 800 | 888.9 | 1,000 VA | Server draw varies enormously with what it is doing. Measure at the PDU rather than trusting a class figure. |
| Two-workstation office with shared storage | 1,100 | 1,222.2 | 1,500 VA | At this point the unit needs its own circuit rather than sharing a general-purpose one. |
| Small comms room with 8 PoE access points | 1,400 | 1,555.6 | 2,200 VA | PoE+ budgets up to 30 W per port, so the switch alone can account for most of this. |
Common mistakes
Buying a VA rating equal to your wattage
An 800 W load does not fit an 800 VA unit unless that unit happens to be rated at unity. At 0.9 it needs 889 VA; at 0.6 it needs 1,333 VA. Matching the two numbers is the most common way to end up one size short of a machine that works.
Sizing from idle draw instead of working draw
Measure or estimate with everything switched on and busy. A workstation can triple its consumption between an idle desktop and a full render, and a NAS pulls noticeably more while its disks are actually being read.
Leaving something off the list
The monitor, the network switch, the external drive, the phone charger nobody thinks about. Undersized units are far more often the result of an incomplete inventory than of bad arithmetic, and the omissions are always the small things.
Reading the answer as a runtime
Sizing to a rating is not sizing to a duration, and the second one gets worse fast: minutes collapse non-linearly as you approach the nameplate. A machine good for twenty minutes at half load can manage four at full, so a unit chosen with no margin is short on both counts at once.
Sizing a generator this way
For anything with a motor on it, the steady total is only half the specification. Starting inrush on a compressor or a well pump is a multiple of its normal draw, lasting seconds, and it is what decides the machine you buy — size to the running figure alone and the set stalls the first time that motor kicks in.
This tool provides planning estimates. Always verify final values against your local code and a licensed electrician.
Frequently asked questions
What size UPS do I need for an 800 W load?
Assuming a unit rated at 0.9, you need at least 889 VA, which means buying the 1,000 VA size. If you are looking at a budget standby unit rated at 0.6, the same load needs 1,333 VA and you are buying the 1,500 VA size instead. The load has not changed — the machine you are sizing against has, and it moves you a whole rung up the ladder.
Which power factor do I put in, mine or the unit’s?
The unit’s. This is the one genuinely counter-intuitive thing about sizing in this direction. You are not describing your own equipment here; you are describing the design ratio of the machine you are about to buy, because that ratio is what determines how much of its VA rating it can hand you as real watts. If you have a specific product in mind, divide its watt rating by its VA rating and enter that.
Should I add headroom on top of the calculated figure?
Rounding up to a commonly-sold size usually supplies some by itself, but not reliably — how much depends entirely on where your figure falls between rungs. Twenty to twenty-five percent above your measured load is a reasonable target, both for equipment you add later and because runtime at 90% of rating is dramatically worse than at 70%. If rounding up leaves you only a few percent clear, take the next size.
Are these standard sizes?
Not in the way that breaker ratings are standard. There is no published series for UPS and inverter ratings — these are the values that consumer and small-business product lines happen to cluster around, and one manufacturer’s 1500 VA rung is another’s 1440 VA or 1600 VA. Rack-mount and online ranges step differently again. Treat the ladder as a guide to what you will find on a shelf and confirm against the actual product.
Can I use this to size a generator instead?
For the steady running load, yes — the arithmetic is the same and a set’s kVA rating is the figure this produces. What it will not do is account for motor starting. Inrush on a compressor or a well pump can be three to five times its running draw for a second or two, and that transient rather than the running total is what decides the size of any set backing up equipment with motors on it.
What about the circuit the unit plugs into?
Worth checking once you have a size, particularly at the top of the range. A unit near 3,000 VA drawing its full rating is close to the working limit of a 15 A receptacle before you add the current it pulls to recharge its batteries after an outage. Take the VA figure to the kVA to amps converter with your supply voltage to see what the circuit is actually being asked for.