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Why do we use avg power when sizing inverter?

ramiws

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The internet is full of this statement and example (also in Will videos):
"Watt is instantaneous power. 1000W load running for one hour is 1kWh. "
So that means if all your loads sum up to 1000W then you need at least 1000W inverter.
All of this does not make sense to me.
If a device has rating of 1000W it means this is the average power consumed over a whole hour, right? it does not mean it will suck 1000W at any given second. because if it really a load that sucks 1000W as instantaneous power and it was running for an hour that means if we are measuring and recoding it every second it will be 1000Watt/Second which means 1000watt.sec * 3600 = 3600kWh and not 1kWh.

So why the whole internet takes the average power of appliances as base of sizing the system? a device that is rated 1000W is actually on average is just consuming 1000W/3600= 0.27W So why do I need 1000W inverter to drive such small load of 1W

Please assume the load is pure resistive load for the sake of simplicity.
 
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which means 1000watt.sec * 3600 = 3600kWh and not 1kWh.
You need to divide the above answer by 3600 to get kWh. I am not familiar with Watts per second as a unit of measure. It is 1000 Watts for an instance, a millisecond, a second, a minute or an hour if you do not turn it off. If you want an average you have to divide the sum of the samples by the number of samples. That is the definition of a mathematical mean (average).
 
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1000w load is a 1000w load. That is power.
Power times time is energy......power consumed over a time period(s)
1000 watts for 1 second, 1000 watt * 1 seconds
1000 watts for 1 minute, 1000 watt * 1 minutes
1000 watts for 1 hour, 1000 watt * 1 hours
100 watts for 10 hours, 100 watt * 10 hours
 
You need to divide the above answer by 3600 to get kWh. I am not familiar with Watts per second as a unit of measure.

He's likely confused with the definition of a Watt as a Joule per second. A Wh is 3600J.

The internet is full of this statement and example (also in Will videos):
"Watt is instantaneous power. 1000W load running for one hour is 1kWh. "
So that means if all your loads sum up to 1000W then you need at least 1000W inverter.
All of this does not make sense to me.
If a device has rating of 1000W it means this is the average power consumed over a whole hour, right? it does not mean it will suck 1000W at any given second. because if it really a load that sucks 1000W as instantaneous power and it was running for an hour that means if we are measuring and recoding it every second it will be 1000Watt/Second which means 1000watt.sec * 3600 = 3600kWh and not 1kWh.

So why the whole internet takes the average power of appliances as base of sizing the system? a device that is rated 1000W is actually on average is just consuming 1000W/3600= 0.27W So why do I need 1000W inverter to drive such small load of 1W

Please assume the load is pure resistive load for the sake of simplicity.

What you describe is not at all the approach of this forum, and has been pointed out, you are missing a couple concepts.

A rating is not an average at all. It's typically maximum continuous power or a peak. My fridge has a 325W power rating. It uses about 310W whenever it's on with very little variation. It runs about 16 hours per day using 16*310 = 4960Wh/day.

If you look at the energy audit spreadsheet, it doesn't do it the way you think it does. It looks at Wh and is based on ACTUAL energy usage of a given device and its run time. I have many devices that use a fixed power whenever they're on. I have many that vary. I have many that surge to a very high power and then taper to a continuous power.

For devices that may have erratic power consumption, it is routinely recommended to run a kill-a-watt meter on it to understand its actual Wh consumption.
 
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it uses about 310W whenever it's on with very little variation. It runs about 16 hours per day using 16*310 = 4960Wh/day.

ok so 310W draw when running. how do you calculate the consumption if it running for 5 seconds? 310W*5 = 1550w.Sec which is 0.43wh. ok thank you guys I think I got that part

But it still means that if my loads rating is 1000W, I need at least 1000W inverter to run them
 
ok so 310W draw when running. how do you calculate the consumption if it running for 5 seconds? 310W*5 = 1550w.Sec which is 0.43wh. ok thank you guys I think I got that part

But it still means that if my loads rating is 1000W, I need at least 1000W inverter to run them

Possibly even more, depending on what it is. Examples:

My RV fridge, since it's just a heater, could run on a 400W inverter very easily. It would not run on a 250W inverter.

My RV A/C unit uses about 1500W. However, I need either a 4000W+ high-frequency inverter to power it due to its very high surge current or a 2000W low-frequency inverter with a genuine 4000-6000W surge rating for several seconds. While many cheap, lightweight high-frequency inverters have a surge rating, the time period for those surges is too low.

The inverter continuous power rating needs to be greater than the loads.

The title of this thread is inaccurate. We do not use average loads. We use peak loads. Devices are rated primarily for peak or maximum continuous power.
 
Yes I thought that the power wattage on devices is the average for an hour given that some are constant consumption and some are erratic with surge. I thought that manufacturers put the average power on the label to make it easy to calculate the hourly consumption.

Now I got it. It is the peak power.
 
Now I got it. It is the peak power.
Not exactly. If you define peak as surge, then it is not peak. It is the "normal" consumption after the starting surge. There is no measurement that I know of that takes into account the starting surge and consumption since it may last only a second and the device might run for days. Sometimes surge is stated as LRA, which is the Amps it might draw upon startup but that gives you no clue about how long that surge lasts. Note, this only usually matter with motors and other devices that have surge loads.
 
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