I have to find a generator to power a chest freezer for a work project.
The freezer is plated ".69kWh/24hr" and I would like to rate it in some units I understand i.e. watts or KVA in order to select the power source.
Can anyone help please? I suspect it's a ridiculously simple calculation but I'm lost.
TIA
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I'd say it's not going to be simple as dividing the total kWh figure for the 24 hours by 24. When the freezer is at temperature the compressor switches off and uses nothing. So it average out at 0.69kWh over the period but at times will peak a lot higher. Especially when freezing for the first time.
Having said that the smallest petrol generator you can find should be enough I would have thought. We had an issue with our fridge freezer in the cellar (door left ajar so it frosted up and then kept running).... it didn't use much power when on all the time.
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>> I'd say it's not going to be simple as dividing the total kWh figure for
>> the 24 hours by 24. When the freezer is at temperature the compressor switches off
>> and uses nothing. So it average out at 0.69kWh over the period but at times
>> will peak a lot higher. Especially when freezing for the first time.
>>
Plus, being a motor, the startup current will be higher still - typically three times higher than the running current.
It wouldn;t surprise me if the start up current were around 3-5 amps, for less than a second.
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That is just an average figure.
0.69 kWh/24h divided by 24 h equals an average of only 29 Watt.
The peak will be much higher.
Edit - just as rtj70 says!
Last edited by: Crocks on Mon 26 Mar 12 at 22:41
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Can you find the compressor motor, it may have a plate giving more useful information?
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if it helps I was always told anything less than 2.2kva is useless for most 240v essentials, anything bigger is better! - but make your Tea/Coffee with Gas!!
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I don't think any division needs to be done
The figure given is the average over a day so you don't get a high figure over the afternoon or a low figure overnight
Get a generator built for 24 hour running rated above 1 KW/h
IMHO.
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>> Can you find the compressor motor, it may have a plate giving more useful information?
>>
I agree; that will give the running current. Then factor in an allowance for the starting current, especially if you're planning to use an inverter generator.
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The medium sized chest freezer I bought recently has a compressor rated at 90w. So even with start current, it won't take much to run it.
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I’m making some basic assumptions here.
The compressor will probably be running for about 50% of the time. This gives it a power input of 58 Watts, assuming there are no cooling fans fitted.
This gives a typical cooling capacity of about 100 watts, which is about right for a chest freezer.
A typical compressor to do that would be somewhere about 5cc displacement.
The figures I have for one of those show a locked rotor current of 7 ½ amps. (The instantaneous current needed for a fraction of a second to start the motor turning.)
Normal running current is shown as about 0,75 amps. Running with a full “warm” load I would expect this current to stay below 1 amp
Figures taken at typical operating temperatures.
HTH
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"William Owen Smith" has measured the power consumption of all sorts of domestic appliances. tinyurl.com/lan785 The chest freezer he monitored used 155W in the cooling mode and a total power of 2.58kWh in 24 hours. That's a 24 hour consumption of 3.7 times what yours is. It's probable/possible therefore that the power consumption of yours in the cooling phase is something like 155/3.7W, i.e 42W.
Last edited by: L'escargot on Tue 27 Mar 12 at 10:15
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Thanks for your help, all.
I'm going to use my own 0.8KVA generator to dry run the fridge from warm to see if the starting load will trip it and go from there.
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>> I'm going to use my own 0.8KVA generator
I'm a bit late to this one, but from my plant/tool hire days I'd have said a 1KVA (the usual minimum retail model) would be fine, even accounting for the fridge's start-up current.
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>> I have to find a generator to power a chest freezer for a work project.
>>
Lateral thinking hat on:
are you planning for rolling power cuts, à la 1972?
A summer of discounted melted ice cream? ;-)
Last edited by: AnotherJohnH on Tue 27 Mar 12 at 13:07
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I'm going to use my own 0.8KVA generator to dry run the fridge from warm>>
It will more than likely be fine. When it does start, let it run. Most motor stalls in refrigeration compressors are caused by them being turned off and then back on again too quickly before the pressure in the system has equalized. This usually takes about 5 minutes.
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Convert kWh/24hr for me - rtj70
