Current and inverter.

DaveKerry

DaveKerry

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Hi all, thanks for reading.
We collect our new to us motorhome early in December, as our pervious one didn't have an inverter we didn't use things like cook pots, kettles and microwaves, on our new one we are having a 2Kw inverter, 350w of solar and a 360ah lithium battery all fitted by the dealer. Instead of guessing which appliance I can get and waiting for a fuse to pop or not, I would like to know how I find out what will work with our set up. I have a slow cooker that is 200-240w at 230v that is 2amps but at 12v its 20amps, the fuse in the domestic plug is 16amp so would the fuse from the battery be larger or smaller than 20amp. I don't know the fuse that will be protecting the inverter, although I guess there is a formula for calculating this.

More of a basic question is what wattage of slow cooker, coffer machine and maybe a microwave should I be looking to buy to be used with the new system.

Thanks for any help.
Dave
 
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Hi Dave

If your slow cooker is 240w at 230v it will use near enough 1A but you are right on the 12v draw and 20 times as in 240/12, the fuse in the plug will be 13A at the most,

The fuse from the battery has to supply the inverter at its maximum which is most likely 4kw peak on a 2kw inverter so A=w/v 4000/12 = 333A so could be fused at 400A or more.

Whatever appliance you buy needs to be below 2kw so 1500w to be safe, then just keep an eye on the battery capacity as 360ah gives you maximum 4,300 watt/h but again on the side of safety and longevity for your batteries I would work on say 3000 watt/h, the appliances you mention tend not to run for long periods so you are safe but the likes of an oven or remoska that might be on for much longer are the ones to watch, Remoska at 400w for one hour is 400 watt/h so very doable, a kettle at 2400w is 200A at 12v, if that takes 6 minutes to boil takes 240watt/h or 20ah from your batteries.

Your solar at 350w is how you hope to put back some of the power and works basically the same so in anything other than perfect conditions you will be getting less that 350w and sometimes a lot less so for example to put back what that Remoska has just used you would need at least 4 hours at 30% solar yield.

Martin
 
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The inverter will be electronically protected and if you overload it, it will trip out and normally reset them by turning off then back on after removing the load.

The 12v current of your 240v devices will be 20 times the mains rated current plus 15% for inverter inefficiency.
So 2 amps at mains voltage will be 40 amps at 12v plus 15% = 46 amps.
 
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funflair said:
Hi Dave

If your slow cooker is 240w at 230v it will use near enough 1A but you are right on the 12v draw and 20 times as in 240/12, the fuse in the plug will be 13A at the most,

The fuse from the battery has to supply the inverter at its maximum which is most likely 4kw peak on a 2kw inverter so A=w/v 4000/12 = 333A so could be fused at 400A or more.

Whatever appliance you buy needs to be below 2kw so 1500w to be safe, then just keep an eye on the battery capacity as 360ah gives you maximum 4,300 watt/h but again on the side of safety and longevity for your batteries I would work on say 3000 watt/h, the appliances you mention tend not to run for long periods so you are safe but the likes of an oven or remoska that might be on for much longer are the ones to watch, Remoska at 400w for one hour is 400 watt/h so very doable, a kettle at 2400w is 200A at 12v, if that takes 6 minutes to boil takes 240watt/h or 20ah from your batteries.

Your solar at 350w is how you hope to put back some of the power and works basically the same so in anything other than perfect conditions you will be getting less that 350w and sometimes a lot less so for example to put back what that Remoska has just used you would need at least 4 hours at 30% solar yield.

Martin
Click to expand...
Thanks Martin I now know what to look for, I used to look at labels for carbs, now I'll be looking for watts lol.

Dave
 
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Lenny HB said:
The inverter will be electronically protected and if you overload it, it will trip out and normally reset them by turning off then back on after removing the load.

The 12v current of your 240v devices will be 20 times the mains rated current plus 15% for inverter inefficiency.
So 2 amps at mains voltage will be 40 amps at 12v plus 15% = 46 amps.
Click to expand...
Thank you Lenny

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funflair said:
If your slow cooker is 240w at 230v it will use near enough 1A but you are right on the 12v draw and 20 times as in 240/12, the fuse in the plug will be 13A at the most,
Click to expand...
So what do you do about the 13amp fuse fitted in the appliance plug ? Do you have to change it?
 
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Riverbankannie said:
So what do you do about the 13amp fuse fitted in the appliance plug ? Do you have to change it?
Click to expand...
If that's how it comes I would leave well alone, I was just making the point that the maximum standard fuse rating in a 3 pin plug would be 13A, it may well be less for that particular appliance.

EDIT as Lenny says as I possibly misunderstood the question ;)
 
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funflair said:
If that's how it comes I would leave well alone, I was just making the point that the maximum standard fuse rating in a 3 pin plug would be 13A, it may well be less for that particular appliance.

EDIT as Lenny says as I possibly misunderstood the question ;)
Click to expand...
Now I am more confused as usual ! I thought here below you said that the amps would be (in your example) 240 /12, that is 20 amps? Wouldn’t that blow the fuse?

funflair said:
If your slow cooker is 240w at 230v it will use near enough 1A but you are right on the 12v draw and 20 times as in 240/12, the fuse in the plug will be 13A at the most,
Click to expand...

By the way I don’t have an inverter just because all this electricery eludes me. You will laugh at this, i had a quiz question yesterday about the symbols in ohms law which I know to be amps = volts/resistance The question was which letter represents amps - I am afraid I assumed it would be A !

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Riverbankannie said:
Now I am more confused as usual ! I thought here below you said that the amps would be (in your example) 240 /12, that is 20 amps? Wouldn’t that blow the fuse?
Click to expand...
You are confusing 240v and 12v e.g if an appliance takes 2 amps at 240v and you run it from an inverter the inverter will draw 40 amps (+losses) from your 12v supply.
The output of the inverter is still only supplying 2 amps at 240v.
 
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Riverbankannie said:
Now I am more confused as usual ! I thought here below you said that the amps would be (in your example) 240 /12, that is 20 amps? Wouldn’t that blow the fuse?



By the way I don’t have an inverter just because all this electricery eludes me. You will laugh at this, i had a quiz question yesterday about the symbols in ohms law which I know to be amps = volts/resistance The question was which letter represents amps - I am afraid I assumed it would be A !
Click to expand...
Lenny beat me again ;)

In the quiz the answer is I V=IxR or I=V/R
 
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Lenny HB said:
You are confusing 240v and 12v e.g if an appliance takes 2 amps at 240v and you run it from an inverter the inverter will draw 40 amps (+losses) from your 12v supply.
The output of the inverter is still only supplying 2 amps at 240v.
Click to expand...
Got it ! 💡
 
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autorouter said:
Oh No... don't mention the R word:oops:. Things are confused enough as it is:giggle:
It's P = I x V
That's power (watts) = amps x volts
Click to expand...
To add to the confusion for AC that will only give you apparent power for true power P = VI cos Φ.
 
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Riverbankannie said:
I would go and get my slide rule from my desk - if I thought it would help !
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In your desk! I always thought Bristol was a bit behind. :LOL:

Just to make you feel good I've got one in the loft. When taking the kids round the Sicence museum over 20 years ago there was one on display. I said I've got one exactly the same as that, they looked at me as though I was from another planet.
 
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Lenny HB said:
In your desk! I always thought Bristol was a bit behind. :LOL:

Just to make you feel good I've got one in the loft. When taking the kids round the Sicence museum over 20 years ago there was one on display. I said I've got one exactly the same as that, they looked at me as though I was from another planet.
Click to expand...
I hope it is a British Thornton, still in its grey case. My dad said they had to define their own log tables and write them up in a little book to use in the classroom, but they could not use them in exams. I was allowed to use an electronic calculator in class, but only use log tables in school. At college we could use whatever we wanted in class but allowed calculators in exams...........

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68c said:
I hope it is a British Thornton, still in its grey case. My dad said they had to define their own log tables and write them up in a little book to use in the classroom, but they could not use them in exams. I was allowed to use an electronic calculator in class, but only use log tables in school. At college we could use whatever we wanted in class but allowed calculators in exams...........
Click to expand...
My husband’s is but mine in a sleeve not a case. I never really got the hang of it. I was working when I bought my first calculator - the sinclair. I still have it, it cost a week’s wages.
 
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68c said:
I hope it is a British Thornton, still in its grey case. My dad said they had to define their own log tables and write them up in a little book to use in the classroom, but they could not use them in exams. I was allowed to use an electronic calculator in class, but only use log tables in school. At college we could use whatever we wanted in class but allowed calculators in exams...........
Click to expand...
Yep still in its grey case well it Was last time I saw it over 20 years ago. :LOL:
 
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I have a very old slide rule made of wood, with ivory scales. Faber Castell, Made In Bavaria. No it's not got Roman numerals on it.:giggle:
 
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funflair said:
Lenny beat me again ;)

In the quiz the answer is I V=IxR or I=V/R
[/QUOTE
The conventional symbol for current is I, which originates from the French phrase intensité du courant, (current intensity). ... The I symbol was used by André-Marie Ampère, after whom the unit of electric current is named, in formulating Ampère's force law (1820).
Click to expand...
 
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I've got a plastic slide rule somewhere with the popular refrigerant evaporating temperatures and pressures on it, was very useful to be able to quickly see if the evaporating temp is low enough. It's all on the Bitzer app nowadays 😊👍

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You will use app 26 amps @ 12 volt d.c. to produce app 1 amp @ 240 volt a.c. This allows for inverter inefficiency and voltage drop. 1 100 amp standard lead acid battery new and fully charged will give you at best 50 amps usable current more like 40 before becoming dangerously discharged. Lithium a different story. Whatever mains appliance you are using only will require the same fuse rating as when on hook up. As their 240 volt requirements are the same.
 
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We have 400AH Lithium battery with 3kw inverter. The theory is all very well, but we have a Votronics 400A meter which shows me exactly what current is being consumed, and what state of charge of the battery is, so whether we're using coffee pod machine, microwave, electric drill (for adding more stuff) or the toaster, we can be completely confident that we're not about to kill the battery. By the way, we used 70mm cables and 300A fuse to the inverter.

Just fyi, I was horrified by how much 12V juice the (apparently) 800W microwave used. It typically uses 120A at 12V which equates to 1400W! So being able to measure what you're using is all-important.
 
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Unless the sun is shining I would avoid high current devices which heat for a prolonged time.
 
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