What does 4000 VA mean?

Hi jolooote,

I prefer to not overload electrical devices. They last longer and don't let out the magic blue smoke. So it behooved me to find out exactly what 4000 VA means.

I.E. what you DON'T know can hurt you. I live on a limited budget.

jolooote wrote:
If u'r not an electrical engineer designing something...don't worry about it. lol

If u'r not an electrical engineer designing something...don't worry about it. lol

CJW8 wrote:
I have traveled a large part of the world and only in the USA are generators and inverters badged with WATTS. Everywhere else they are badged in VA. I believe it is purely a marketing ploy to attach a bigger number.

That could be for small generators for home a RV-ing use. Large gensets (for prime power or standby power) are always rated in KVA. Kinda like in years past, home compressors had hugely inflated HP figures on them until the regs. changed.

VA (or KVA) will tell you how much current can be produced or handled by a generator, transformer, etc. or wire gauge. If you look at an AC motor nameplate for ex. (which have a PF less than 1.0), the full load amps will be indicated which can be used to determine if a transformer, or gen. will be adequate (notwithstanding starting inrush current for a single motor). Power companies are highly interested in knowing what a commercial or industrial customer's power factor is because it requires them to use heavier gauge service wire and larger KVA transformers than power consumption in watts would indicate. Sometimes they install a meter that measures PF and a customer will be charged for having a low PF.

Taking a look at compact fluorescent lamps, a 13 watt-er for ex. has a current indicated on the lamp of 175 ma. But 120V x 0.175A = 21 watts (meaning they have a rather low PF). So where do they get the 13 watts from and do the new smart meters charge customers based on current or is it true KW like with the old dial meters?

Then there are the terms "apparent" and "imaginary" power...

P-T, Thanks for the wave! Hopefully our 4KW (HOW many VA?) RV GenSet won't get altitude sickness before I do...

Drew, Amazing insights as usual! Let me ask this:

1. Could you help the OP on This Thread figure out a Soft Start for his A/C? I found a couple in Schneider/SquareD but the specs didn't make sense.

2. Sometime I'd like to chat about a project at home. Could PM you. OK?

Hi j-d,

That also applies to generators. And they get altitude sickness, too!

j-d wrote:

I'll buy a bigger UPS than I thought I needed.

Hi everyone,

Thank you. I think I finally understand.

This was all in aid of helping me understand the rating on my new Sola Basic autoformer which is rated at 4000 VA for output. It can draw up to 4320 watts.

I'm head over heels with how well the SB is working. It is fast enough at switching to voltage support to take care of the start up surge on my roof air conditioner. If voltage is 117 it boosts to 121 for a few seconds as the air calls for power.

VA rating like GVW rating is a maximum rating. A generator rated 4000VA can not supply more than 4000W of power. And it can only do it if the load is purely resistive or power factor of the load is 1.0. Any other loads, inductive or capacitive will cause loss of part of VA that will not do any work. Any capacitance or inductance of the load will only affect transient states in DC circuits but in AC circuits in plays major role. Capacitance of the load in AC circuits causes current peaks to occur before voltage peaks and inductance of the load causes voltage peaks to occur before current peaks. This phase shift has to be eliminated for most efficient operation-thus power factor correction.
The converter with poor power factor correction will cost you extra every time you use it, just like a car with poor mileage.

wa8yxm wrote:

pianotuna wrote:
Hi all,

What exactly does 4000 VA mean in an autoformer? In a generator? In an inverter?

Thanks in advance.

It means that the Voltage times the Amps (Which by the way is NOT the same thing as Watts in this case) must not exceed 4000

For example 120 Volts times 40 amps = 4800 VA which is 800 two much

120n volts times 30 amps is 3600 VA, and thus ok

For the record 3600 VA times a power factor of 0.9 = 3240 Watts

Power factors in DC systems are always 1, but in AC systems involving Motors, Transformers or other inductive loads may be lower. Only restive loads are PF 1.0 (light bulbs, Space/water/Fridge heaters)

NOTE: The above applies to Autoformers, Transformers, Inverters, Generators and, in fact all things A/C.

pianotuna wrote:
Hi all,

What exactly does 4000 VA mean in an autoformer? In a generator? In an inverter?

Thanks in advance.

It means that the Voltage times the Amps (Which by the way is NOT the same thing as Watts in this case) must not exceed 4000

For example 120 Volts times 40 amps = 4800 VA which is 800 two much

120n volts times 30 amps is 3600 VA, and thus ok

For the record 3600 VA times a power factor of 0.9 = 3240 Watts

Power factors in DC systems are always 1, but in AC systems involving Motors, Transformers or other inductive loads may be lower. Only resistive loads are PF 1.0 (light bulbs, Space/water/Fridge heaters)

j-d wrote:
This thread is interesting but it gave me a headache. Still, I take away the following:

VA doesn't mean something with rated wattage will run on advertised/rated VA.

VA is advertised instead of the more useful Wattage spec because VA is a bigger number and bigger sells better.

I'll buy a bigger UPS than I thought I needed.

If GOD wanted me messing with Electrons, HE'd've made them big enough to see.

Wattage (actual power) is never higher than VA (apparent power).

Generators and transformers and so forth are rated in VA because they are limited by the apparent power they can supply, not the actual power. In other words, a generator can only supply so much current at its specified voltage, regardless of whether the current is in phase with the voltage or not. Similarly, wires and circuit breakers or fuses must be sized for the actual current, and hence based on apparent power rather than actual power.

Advertising is in watts because that's what more people are familiar with, I guess. In the case of the UPS you described, the larger VA number simply means that it's capable of producing its full rated (actual) power for a load with a poor power factor. That is indeed an improvement over one that can only do when powering something with a nice power factor.