Skip to main content ↑ ↓
Topic: Inverters, Watts and Volt-Amperes (Read 639 times) previous topic - next topic

Inverters, Watts and Volt-Amperes

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.
David and Carolyn Osborn
1995 U320C SE 40' Build 4726 Feb 1995
FMCA 147762
Motorcade 17186

Re: Inverters, Watts and Volt-Amperes

Reply #1 –
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

For further evidence, note the technical specifications of a Victron MultiPlus 3kVA inverter/charger:
Technical Support Manager and official poker of electronics at Laurel Technologies / Bay Marine Supply. Distribution and expert support of Victron Energy, Sterling Power, Blue Sea, Bussman, and Battle Born products.

Re: Inverters, Watts and Volt-Amperes

Reply #2 –
Quote from: Justin Cook –

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.


3,000 (80%) = 2400
3,000 (0.80) = 2400
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Inverters, Watts and Volt-Amperes

Reply #3 –
Power factor is calculated in percent.
Volts x amps x pf = watts
 
120 x 20 = 2400 VA x .80 = 1920 watts

FWIW ..... ?
 
Justin & Cathy Byrd
1995 U280 "Old Faithful"
36' Build #4673
C8.3 Cummins
Allison MD3060R 6 speed - retarder
Powertech 10KW  4cyl Kubota

Re: Inverters, Watts and Volt-Amperes

Reply #4 –
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.





"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Inverters, Watts and Volt-Amperes

Reply #5 –
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.


Technical Support Manager and official poker of electronics at Laurel Technologies / Bay Marine Supply. Distribution and expert support of Victron Energy, Sterling Power, Blue Sea, Bussman, and Battle Born products.

Re: Inverters, Watts and Volt-Amperes

Reply #6 –
Quote from: Justin Cook –
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.


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!
David and Carolyn Osborn
1995 U320C SE 40' Build 4726 Feb 1995
FMCA 147762
Motorcade 17186

Re: Inverters, Watts and Volt-Amperes

Reply #7 –
Quote from: D.J. Osborn –
I appreciate Justin's polite correction of my incorrect statement and 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!!
Technical Support Manager and official poker of electronics at Laurel Technologies / Bay Marine Supply. Distribution and expert support of Victron Energy, Sterling Power, Blue Sea, Bussman, and Battle Born products.

Re: Inverters, Watts and Volt-Amperes

Reply #8 –
Quote from: Caflashbob –
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.

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.



'99 U320 40 WTFE
Build #5462,
1500 Watts Solar 600 amp Victron lithium
2012 Jeep Grand Cherokee Overland Hemi
Instagram bobfnbw
Retired

Re: Inverters, Watts and Volt-Amperes

Reply #9 –
Quote from: Justin Cook –
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!!
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.
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Inverters, Watts and Volt-Amperes

Reply #10 –
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.""
Justin & Cathy Byrd
1995 U280 "Old Faithful"
36' Build #4673
C8.3 Cummins
Allison MD3060R 6 speed - retarder
Powertech 10KW  4cyl Kubota

Re: Inverters, Watts and Volt-Amperes

Reply #11 –
Quote from: FourTravelers –
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.  ::)
Technical Support Manager and official poker of electronics at Laurel Technologies / Bay Marine Supply. Distribution and expert support of Victron Energy, Sterling Power, Blue Sea, Bussman, and Battle Born products.