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Watts to Amps please!!


mrsmelly

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Ok so I am just playing with stuff to see how much juice it uses. I am going to buy an inverter to have in stock as a spare but dont need it big. My fridge shows 628 watts on start up for less than half a second then drops  down to forty four very quickly whilst running. My question is what capacity inverter needs to cope with the initial surge to start the fridge? Taa

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Just now, ianali said:

Inverters are generally measured/sold by watts aren’t they? Therefore do you just need a wattage above the 600 odd measurement you have? 1,000 watt inverter? Somebody will be along to tell me why I’m wrong I bet.

:o OOh blimey I hadnt thought of that lol. Well I am fick innitt. You may well be right but it doesnt seem much? I was hoping somett like a 1000 watt verter would do but will wait for more answers ?

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2 minutes ago, mrsmelly said:

:o OOh blimey I hadnt thought of that lol. Well I am fick innitt. You may well be right but it doesnt seem much? I was hoping somett like a 1000 watt verter would do but will wait for more answers ?

Yes I would wait. I know very little about anything and I’m in the pub,which is always a dangerous place to give advice from.

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6 minutes ago, ianali said:

Inverters are generally measured/sold by watts aren’t they? Therefore do you just need a wattage above the 600 odd measurement you have? 1,000 watt inverter? Somebody will be along to tell me why I’m wrong I bet.

You are wrong?

 

The fridge’s rated wattage is its steady state consumption. However when the compressor motor starts up, there will be a much higher short term “surge” demand, which could overload and trip an inverter rated at the fridges rating. How big the surge, how tolerant the inverter is of short term overloads, are variables that are type specific and therefore hard to anticipate. So either add a healthy chunk of extra inverter power, or risk a trial and error process.

Edited by nicknorman
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23 minutes ago, mrsmelly said:

Ok so I am just playing with stuff to see how much juice it uses. I am going to buy an inverter to have in stock as a spare but dont need it big. My fridge shows 628 watts on start up for less than half a second then drops  down to forty four very quickly whilst running. My question is what capacity inverter needs to cope with the initial surge to start the fridge? Taa

How are you measuring that? If you are using a storage scope then fair enough, if any sort of meter I doubt the meter would have registered the initial part of the start up surge and that may well be much higher than the 628 Watts. Not 100% sure about AC stuff and very modern electronically controlled motors but motors tend to have a very low resistance until they start spinning and then it drops as they speed up.

Edited by Tony Brooks
Chaged DC to AC which is what I thought I had written
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1 minute ago, nicknorman said:

You are wrong.

 

The fridge’s rated wattage is its steady state consumption. However when the compressor motor starts up, there will be a much higher short term “surge” demand, which could overload and trip an inverter rated at the fridges rating. How big the surge, how tolerant the inverter is of short term overloads, are variables that are type specific and therefore hard to anticipate. So either add a healthy chunk of extra inverter power, or risk a trial and error process.

Yes, but isn’t the 628 Watts the surge demand? Anyway at least I won my bet. 

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1 hour ago, mrsmelly said:

Ok so I am just playing with stuff to see how much juice it uses. I am going to buy an inverter to have in stock as a spare but dont need it big. My fridge shows 628 watts on start up for less than half a second then drops  down to forty four very quickly whilst running. My question is what capacity inverter needs to cope with the initial surge to start the fridge? Taa

 

Although you no longer appear to need to have an answer to your topic heading, the calculation is simple - divide the watts by the volts to get the amps.

So using your figures, 628 watts divided by 12 volts  is 52 amps. Depending upon the distance between your inverter and fridge, you could need a very fat cable for that.

 

 

Edited by David Schweizer
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12 minutes ago, David Schweizer said:

 

Although you no longer appear to need to have an answer to your topic heading, the calculation is simple - divide the watts by the volts to get the amps.

So using your figures, 628 watts divided by 12 volts  is 52 amps. Depending upon the distance between your inverter and fridge, you could need a very fat cable for that.

 

 

Not quite.  The distance between the batteries and the inverter needs thick cables - there ought to be 240V AC between the inverter and the fridge, so your calculation becomes 628/240 => 2.6A.

 

 

Edited by TheBiscuits
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31 minutes ago, David Schweizer said:

Although you no longer appear to need to have an answer to your topic heading, the calculation is simple - divide the watts by the volts to get the amps.

So using your figures, 628 watts divided by 12 volts  is 52 amps.

Divide by 10 to take into account inverter losses. So 63A in your example. 

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Not sure what the rated wattage of our fridge freezer is, but can say that it starts no problem on an inverter genny of 1.5kW rating, and used to start OK on a (now deceased) non inverter genny of 1kW max rating. It gives the genny a workout when it kicks in for a couple of seconds, so don't underestimate the start load of the compressor. 
 

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Just some practical experience. I have a cheapo modified sine wave 600W inverter as a backup for the Victron when it went wrong. This struggled to start the fridge compressor sometimes. 1000W or more should be safe though. Will depend on fridge, inverter, phase of the moon and colour of socks of course.

 

Jen

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1 hour ago, TheBiscuits said:

Not quite.  The distance between the batteries and the inverter needs thick cables - there ought to be 240V AC between the inverter and the fridge, so your calculation becomes 628/240 => 2.6A.

 

 

 

Yes, thanks for that,  I meant to say "between the batteries and the inverter" - Senior moment

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20 hours ago, WotEver said:

As the fridge compressor is a motor you really should ensure it’s sine wave although others seem to cope just fine with msw.

 

8 hours ago, Detling said:

Pure sign is best, you can run motors off msw inverters but the motor will run hotter and this will probably shorten it's life.

As it’s important I thought it worth repeating. 

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One other thought I have come across several 1000w inverters that would not start my fridge (LEC) the only common thing is that they were all high frequency inverters, these are usually smaller and lighter than low frequency ones. One I even emailed  the manufacturer and was shown how to turn off the inrush protection but it still shut down.

 

Edited by Loddon
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49 minutes ago, Loddon said:

One other thought I have come across several 1000w inverters that would not start my fridge (LEC) the only common thing is that they were all high frequency inverters, these are usually smaller and lighter than low frequency ones. One I even emailed  the manufacturer and was shown how to turn off the inrush protection but it still shut down.

 

As a generalisation low frequency inverters have more surge power than high frequency inverters, which is what you just demonstrated.  Good to hear reality matches theory.

 

added - actually I should have said theory matches reality.

Edited by Chewbacka
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3 hours ago, Loddon said:

One other thought I have come across several 1000w inverters that would not start my fridge (LEC) the only common thing is that they were all high frequency inverters, these are usually smaller and lighter than low frequency ones. One I even emailed  the manufacturer and was shown how to turn off the inrush protection but it still shut down.

 

Its a bit of a minefield. My set up is first class and does everything I ask of it ,however its an all singing and dancing combi and I do not like combis as for me they are too complicated if I have a problem therefore I intend having seperates ready onboard for when the inevitable happens. I will then have to suss out how to rewire some bits I suppose.I have a travel power anyways so I need an inverter to run the proper fridge 24/7 with a bit for telly and charging in reserve so thinking of a quality 1000 or 1200 watt.

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As part of my work I have measured the peak inrush current and duration of large commercial chiller compressors, but the principles for domestic are the same.

 

Inrush was typically around 7-8 times steady state current and lasted for around 500 milli-seconds.

 

So say 15 amps inrush for a fridge that draws a continuous 2 amps.

 

My Victon 3000VA combi has an overload capacity of 5500VA for an unspecified amount of time.

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17 minutes ago, cuthound said:

As part of my work I have measured the peak inrush current and duration of large commercial chiller compressors, but the principles for domestic are the same.

 

Inrush was typically around 7-8 times steady state current and lasted for around 500 milli-seconds.

 

So say 15 amps inrush for a fridge that draws a continuous 2 amps.

 

My Victon 3000VA combi has an overload capacity of 5500VA for an unspecified amount of time.

My gadget says the fridge runs at 44 watts so say eight times that would be 352 watt thingies then so that means a ???? inverter capability for about half a second?

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41 minutes ago, mrsmelly said:

My gadget says the fridge runs at 44 watts so say eight times that would be 352 watt thingies then so that means a ???? inverter capability for about half a second?

Like I said aaaaages ago... a 1kW PSW inverter should be fine :)

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