There have been discussions here on this subject and even though it can sometimes be confusing it's not too difficult to understand. Electrical appliances are typically rated regarding the Watts they consume. For an incandescent light bulb or a resistive heating element voltage and current are in phase and the Watts and Volt-Amperes consumed are identical. A heating element using 10 Amperes at 125 Volts would consume 1,250 Watts or 1,250 Volt-Amperes. This is because Voltage and current are in phase.
Most appliances also have what's called reactance and so the result is that Voltage and current are somewhat out of phase and the term "power factor" becomes relevant.
The power factor of most modern appliances is around 80% and so they use about 125% more current than they would if the power factor were 100%. This is generally of no real significance unless one is using an inverter and operating near the inverter's rated limits. If operating well under the inverter's rated limits then there is no reason to be concerned.
Hate to say it, but the math on this is backward - VA rating must be divided by the power factor rating, not multiplied, so an inverter rated to 3000VA will be capable of supplying -depending on the quality and efficiency of the inverter- around 2400 continuous watts.
Continuous watts will always be less than VA, not more.
For a decent explanation of the VA vs Watts discussion, here's a post written by Alan a number of years ago: Converting VA to Watts (https://baymarinesupply.com/blog/converting-va-to-watts/)
For further evidence, note the technical specifications of a Victron MultiPlus 3kVA inverter/charger:
3,000 (80%) = 2400
3,000 (0.80) = 2400
Power factor is calculated in percent.
Volts x amps x pf = watts
120 x 20 = 2400 VA x .80 = 1920 watts
FWIW ..... ?
Good info. Thanks guys.
My experience is that how long any unit can put out how much of its rated power before derating is another factor to consider, at least for us.
Most rv and marine use is short term and light loads so it's rare for issues to appear.
Were we live we are surrounded by desert and high temps and heavy uses are possible for us so I test things harder than most would need to.
I suggest testing any system under a high load just to verify what it's capable of whether you currently intend to use it or not.
You might have a different need some day. But that's just me. I build a specific brand off road racing motorcycle engines and overbuild them for safety.
I assume that's why all the ratings include internal temperature numbers.
I notice our unit can show the internal parts and battery temps on its remote panel.
Rarely see them get hot in our rv use but in other uses it may be informative.
Yes, the original post had an additional paragraph that said to calculate an inverter's wattage output running an appliance with a .8PF you would multiply the VA x 1.2, which resulted in a statement that a 3000VA inverter would supply over 3000 watts continuous, which was incorrect, but that has now been removed.
VA=Watts / PF (100w / .8PF=125VA)
Watts=VA x PF (125VA x .8PF= 100 watts)
Thus why a MultiPlus 3000VA inverter is rated to 2400 watts continuous.
I appreciate Justin's polite correction of my incorrect statement and I deleted the misinformation from my post. As much as I try to stay "young and perfect" old age sometimes has a way of getting the better of me!
Man, I've confused myself about the VA vs watts thing more times than I can count, and I do this for a living, so don't feel bad!!
Depends on the battery type and amount of amps it has.
A wet cell will quickly lose voltage on a high demand load and overheat.
A AGM will take longer to sag but still will.
A Lithium battery will be hard pressed to sag and keep the same voltage until almost fully discharged.
Which is one of main benefits of LifePo4 batteries and while they are worth the money. That and they can be charged very fast where wet cell and agm take much longer. Also note that the victron multiplus PF is 1.0
Temp also plays a big role in what a battery can do. When they get hot, the amount of power available is less.
Also don't forget the amp hour rating. Just because it consumes 1000 watts doesn't mean you will use that many amps, it depends on how long your pulling that amount from the system. A hair dryer uses a lot of watts, but if you wife has short hair you will notice it a lot less than if she has a lot of hair. Same with a induction hob, or microwave. The longer you use it, the more power you use, and the longer it will take to put that power back into the battery. Another reason to go with lithium is that you can really pump amps back in with out damaging it. But with lead acid, you can't. And as it gets fuller, you have to dial back the amps or risk damage to the battery. With lithium you can continue to put in a lot of amps till almost full. Off topic I guess.
It gets a bit more confusing when not only are you pulling amps out, but at the same time putting them in with solar. That's when a good amp hour meter will come in real handy.
In the old days this was on the amateur radio General class license exam. Along with velocity factor for feed line calculations. And we used slide rules, pencil and paper.
I remember just enough to be dangerous.
Just remember that volts * amps is akin to advertising puffery.
I think you meant :
Watts is divided by power factor = VA
And
Voltamps multiplied by the power factor = watts
""Hate to say it, but the math on this is backward - VA rating must be divided by the power factor rating, not multiplied, so an inverter rated to 3000VA will be capable of supplying -depending on the quality and efficiency of the inverter- around 2400 continuous watts.""
*facepalm* Exactly correct... as I said above, I've confused myself on that more times than I can count. ::)