Inverter Efficiency

[MANGOFORTH]

I read somewhere that running a large (3000w) inverter with only a small load say 500w is inefficient? Can someone with more knowledge lend some advice?

Have a few 230v items that will be run constantly, with only a small draw. And some larger items with a big draw. Is it best to have 2 inverters, large and small. Or to just have one large one?

Thanks

 

[pappajohn]

Inverters use battery power when they are turned on even when they aren't actually powering anything and the bigger the inverter the more power it uses.

If you use a very large inverter to run a (say) 10 watts item the inverter itself may use nearly that much.... Doesn't sound much but that's nearly 1 amp.
So... Yes, if the high power items are seldom used then get a small one for the permanent stuff or if possible run them off 12v (phone charges etc)

 

[Yorick]

Inverters use battery power when they are turned on even when they aren't actually powering anything and the bigger the inverter the more power it uses.

If you use a very large inverter to run a (say) 10 watts item the inverter itself may use nearly that much.
So... Yes, if the high power items are seldom used then get a small one for the permanent stuff or if possible run them off 12v (phone charges etc)

Where does the extra leccy go ?

 

[HandyAndy]

The name you are looking for is quiescent current

 

[pappajohn]

Where does the extra leccy go ?

Heat

^{_{Subscribers  do not see these advertisements}}

 

[pappajohn]

The name you are looking for is quiescent current

Thank you

 

[ceejayt]

We have a 3kw inverter but we have Lithium ion batteries.

Runs our microwave and coffee machine and toaster. Doesn’t miss a beat. Very pleased with the set up.

 

[MANGOFORTH]

So does the inverter use a flat rate of power regardless or how much... Erm inverting.. ? It does?

 

[HandyAndy]

Victron inverters seem to get MORE efficient the bigger you go. Although that is in percentage terms so probably misleading.

 

[HandyAndy]

as I suspected, not the whole truth..
At rest a multi plus will use 11w just being "on" or 0% efficient
At 400w output it losses are 19w "dissipation" (aka heat) or 95% efficient.
At 5000w output it's losses are 1261w dissipation or 79% efficient.

^{_{Subscribers  do not see these advertisements}}

 

[pappajohn]

At 5000w output it's losses are 1261w dissipation

A single bar electric fire and some..... That is some serious power wasted as heat.

 

[HandyAndy]

So does the inverter use a flat rate of power regardless or how much... Erm inverting.. ? It does?

Nope, the harder you push it the more heat it generates (dissipates.)
Electric current generates heat in the conductor through which it flows. The basic formula to calculate the rate of heat generation, or power dissipation, is P = R x I² (1)where P stands for power (measured in Watts), R (Ohm) is the resistance of the conductor and I (Ampère, or Amps) the current.What is interesting about this formula is that it shows that power dissipation increases with the square of the current. A resistance of for example 2 Ohm and a current of 10 Amps results in a dissipation of 2 x 10 x 10 = 200 W. Twice that current results in 4 times more heat generated: 2 x 20 x 20 = 800 W!

 

[MANGOFORTH]

So invert big power, interter uses big power?

 

[MANGOFORTH]

Nope, the harder you push it the more heat it generates (dissipates.)
Electric current generates heat in the conductor through which it flows. The basic formula to calculate the rate of heat generation, or power dissipation, is P = R x I² (1)where P stands for power (measured in Watts), R (Ohm) is the resistance of the conductor and I (Ampère, or Amps) the current.What is interesting about this formula is that it shows that power dissipation increases with the square of the current. A resistance of for example 2 Ohm and a current of 10 Amps results in a dissipation of 2 x 10 x 10 = 200 W. Twice that current results in 4 times more heat generated: 2 x 20 x 20 = 800 W!

Oh god I'm lost

 

[HandyAndy]

The more you need to invert, the bigger allowance you need to make for losses in your battery bank yes.

^{_{Subscribers  do not see these advertisements}}

 

[MANGOFORTH]

Yeah I've got a big enough battery bank. I honestly do not understand electrics. It's the area I'm seriously lacking. I think what I'm trying to ask is, would a 3000w inverter using 500w, lose more electricity than a 1000w inverter at 500w. If the usage or power disipation changes with how hard you run it. Am I worrying about nothing or does this matter? Thanks

 

[DBK]

Oh god I'm lost

I think the conclusion is there will always be losses from an inverter but if you are only drawing a small amount of power don't worry about it providing you have the solar energy to replace the losses.

My suggestion is go with the big inverter but try and replace the steady 240v loads you mentioned with 12v if you can. You can always add a second inverter later if you find it might help.

If you can list what loads you think you are going to run it would be helpful. It doesn't matter how big a battery bank you have if the energy can't be replaced.

 

[pappajohn]

Oh god I'm lost

The power lost to heat that HandyAndy mentioned (1261watts) is enough to run our toaster at home if it could be converted back to electric.

 

[HandyAndy]

Yeah I've got a big enough battery bank. I honestly do not understand electrics. It's the area I'm seriously lacking. I think what I'm trying to ask is, would a 3000w inverter using 500w, lose more electricity than a 1000w inverter at 500w. If the usage or power disipation changes with how hard you run it. Am I worrying about nothing or does this matter? Thanks

Buy the bigger inverter and turn it off after you have finished with it, then it won't consume anything.
If you use it for light loads it will have light losses.
If you use it for heavy loads it will have heavier losses.
Horses for courses.. Buy the inverter to suit what you want to run with a decent amount of headroom instead of buying one and running it til it trips out on overload.
It's always useful to have more power than you need as long as you are not stressing out your battery bank..

 

[MANGOFORTH]

Thank you everybody. I was going to run a 230v fridge aa+ via an inverter. Only information I have is its consumption is 401kwh. I want an inverter just for this, smallest possible. Would be easier if you told me what to buy ?

^{_{Subscribers  do not see these advertisements}}

 

[HandyAndy]

How much solar power have you got?

 

[Driventemple]

Sterling Power made this video a couple of years ago to compare using a 12v Vs 240v fridge

 

[MANGOFORTH]

How much solar power have you got?

620w of solar, 500ah of batteries. Potential to increase both if necessary. That was 401kwh annual consumption**

 

[funflair]

Thank you everybody. I was going to run a 230v fridge aa+ via an inverter. Only information I have is its consumption is 401kwh. I want an inverter just for this, smallest possible. Would be easier if you told me what to buy ?

Think you need to check those numbers,

Sorry hadn’t updated thread, I still make that just over 1kwh per day or just over 90ah which still seems a lot for a fridge, or are my sums wrong?

The inverter size would be determined by peak draw plus a safety margin but average consumption would be something like 50watts.

 

[HandyAndy]

I realised it was yearly consumption and that you were reading that off the aa+ sticker. ??
So you have great solar and a good sized battery bank. Great start. You are in a good position to have a inverter on all the time. In which case I would be looking across ranges of inverters that fit your power requirements in total output.
Once you have a list I would go back over it and look for the one with the lowest quiescent current draw because this "always on" number is the one that will effect things most. The fridge, once cold, shouldn't be that power hungry, yes a lower kwh pr year number will show you that it has better insulation but probably not a lot else.
If the compressor has to keep cutting in to keep it cold because you are opening the door often, it's going to use more power. But if that inverter is permanently on to cover it, then that will be more of a concern, not the loss through heat of the inverter while inverting but the loss while just sitting there doing nothing.

^{_{Subscribers  do not see these advertisements}}

 

[Reallyretired]

I have a 300w psw inverter, smallest I could find, that just runs the toothbrush charger (1w)
That must be pretty inefficient but probably better than using the big one.

 

[DBK]

Thank you everybody. I was going to run a 230v fridge aa+ via an inverter. Only information I have is its consumption is 401kwh. I want an inverter just for this, smallest possible. Would be easier if you told me what to buy ?

401Kwh over a year works out at about an average of a 50W load running 24 hours a day. Of course it will vary as the compressor will cut in and out but the average is 50W. Assuming of course the quoted figure is correct and not some fictional laboratory figure like car mpg claims.

So you could probably get away with a 500W inverter if you don't have a need for a hair dryer or coffee machine. I would go for pure sine wave to be on the safe side as the fridge might have clever electronics in it. There's a link in my signature below to a thread about fitting mine but there are others on here if you search.

Now for the difficult bit, is it going to work? In summer no question it will, but in the depths of winter I think it will be marginal assuming you will have other power drains like lights, TV, etc. Your 620W of solar will generate a fraction of that in winter, 200W peak perhaps but then only for 4 or at most 6 hours a day. You can save power by turning the fridge off at night but you are going to be counting amps.

I'm not saying don't try it but you may need to rethink in the light of experience of how it performs in winter, if you're going to use it in winter of course. You can always buy a genny.

 

[StitchesnFluff]

While you guys are chatting this through ......Therefore an inverter utilised only to charge 36v battery for my clip on handcycle to wheelchair battery would need to be ..........w? We have 2 x 90ah batteries and 150w of solar.

 

[HandyAndy]

While you guys are chatting this through ......Therefore an inverter utilised only to charge 36v battery for my clip on handcycle to wheelchair battery would need to be ..........w? We have 2 x 90ah batteries and 150w of solar.

Charger will use about 1-2 amps at 240v using ohms law 240w-480w
Direct conversion to 12v would be 20-40 amps @ 12v
A safely discharged 90 ah battery is about 45 ah usable if used gently (i.e not pulling 20-40 amps)
You effectively have 90ah usable of which you need to pull 20-40 amps pr hour.....
2.5 - 4 hrs and they are flat without solar input and that's not taking into account the massive drop in ah rating when you pull large currents. (i.e a 90ah (usable) battery won't give you 90 amps for 1 hr but it will give you 1 amp for 90 hrs.

Oh Please! ignore all that.... the charger uses 1-2 amps at 36v... I shall have to start again
1-2 amps at 36v is 36-72w
36w-72w @12v is 3-6 amps
That's more realistic
do I have to start over or can you work it from there

 

[HandyAndy]

delete, double post

^{_{Subscribers  do not see these advertisements}}