KWH TO KW- generator sizing

J

Jacob Gerstner

Member
Location
Michigan
Occupation
Journeyman
Hello All,

I have a customers whose utility company reads the monthly usage in Kwh. I’m trying to use that monthly read out (I have 12 months of usage in kwh) to determine the size of generator I need. My question is, how do I convert kwh to kw? If I divided the kwh to to the number of hours in a month I get half a kw. This doesn’t seem right, Any help on this would be great! thanks so much.
 
For the most part it can't be done, with any accuracy.

You could use nearly all the kWhrs in first few days of the month at a high rate and then sit nearly idle the rest of the month. Your average you figure out may not be enough for the generator to be able to handle what is the true peak demand.

With smart metering you possibly can ask the POCO if they can give you demand data though, this will tell you what their demand history is like. I'd be looking for peak demand over say a 12 month period and from that you possibly could even discover there may be items that you could shed to keep the demand within what a smaller generator can handle.
 
As Kwired said, that will just give you the average Watts consumed, not a very useful figure. If they don't have demand history, best thing to do with just meet with the client and turn everything on in the house, minus anything that will be load shedded, and clamp it and see what you get.
 
Besoeker3 said:
They are simply different measures - kWn is energy, kW is power or rate.
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Yes, but if you have two different places that had 1000 kWhr of usage over a month's time, how do you know one didn't use most of it within a short time and the other one somewhat averaged it out over the whole month? That first one needed "more power" while it was actually using that energy.
 
kwired said:
Yes, but if you have two different places that had 1000 kWhr of usage over a month's time, how do you know one didn't use most of it within a short time and the other one somewhat averaged it out over the whole month? That first one needed "more power" while it was actually using that energy.
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They are simply different units - don't conflatelate them..
 
Besoeker3 said:
They are simply different units - don't conflatelate them..
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I guess that's what we are trying to tell the OP.

One is a component of the other, but time is also the other component Only way you can determine power from an energy measurement is if the power level is constant throughout the time period.
 
An analogy for the OP: What you are trying to do is figure out how fast a car can go, based on its annual mileage.

You can only convert kWh to peak kW if you have knowledge of the time pattern of energy use.

The utility might be able to supply 15 minute demand data, but even this is approximate, since it comes from energy used in a 15 minute period.
 
Utility bills are not a good way of sizing a generator.

Generac has some sizing data on their site which is fine if you’ve never done one before.

My preferred method is to walk through the property with the customer and count up all the loads. If it’s very high, then we talk about what we can load shed.

Don’t forget about well pumps and pressure pumps for off grid sites, ask about how many freezers they have, (one customer had I believe 5, that all needed to stay on), don’t forget about motor inrush, and remember that many systems will start as soon as power is applied, so you could potentially have multiple motors starting together.

The big 4 in residential is dryer, heat system, water heater, and range/stove. If those are electric, you’ll be installing a much larger unit, or load shedding.

Check for a hot tub.

And then there is EV chargers. Some people are not interested in charging them with a generator, some it’s a necessity.
 
kwired said:
I guess that's what we are trying to tell the OP.

One is a component of the other, but time is also the other component Only way you can determine power from an energy measurement is if the power level is constant throughout the time period.
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kW is power. kVA is energy.
 
Welcome to the forum.

Let me try to simplify it.

Kwh is a measure of electrical power consumed over a period of time.

Kw is a measure of power being consumed at any one moment in time.

A generator must be sized to meet the peak consumption of Kw at any one moment.

Thus, knowing the Kwh is of no use. You need to determine the peak power demand.

Now, with this understanding, read through the previous responses again.
 
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LarryFine said:
Welcome to the forum.

Let me try to simplify it.

Kwh is a measure of electrical energy consumed over a period of time.

Kw is a measure of power being consumed at any one moment in time.

A generator must be sized to meet the peak consumption of Kw at any one moment.

Thus, knowing the Kwh is of no use. You need to determine the peak power demand.

Now, with this understanding, read through the previous responses again.
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So if my utility company can’t provide peak KW then I need to size off either a load calc or a more general size based on loads ect. Thanks so much for this response and help.
 
Jacob Gerstner said:
So if my utility company can’t provide peak KW then I need to size off either a load calc or a more general size based on loads ect. Thanks so much for this response and help.
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Bingo. Just like the power company, you have to size your source to meet the largest peak of usage, regardless of the duration. If you need 100Kw for one minute per month, you need a 100Kw source.
 
winnie said:
An analogy for the OP: What you are trying to do is figure out how fast a car can go, based on its annual mileage.
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Tell that to insurance companies when they factor mileage on your vehicle into your premium costs. They don't seem to care where you may have put those miles on at just that you put them on.
 
Evans a load calculation can be problematic. If all you need to run is a heat pump that has an MCA of 25 amps, a 25 amp generator most likely won't start that hvac unit. Similar issue with any big motors like a well pump.

In most applications, a load calculation will provide a number much larger than needed. My house which has a load calculation of 150+ amps rarely draws more than 30A on a given phase. Most of my appliances are gas though, so that helps a lot. But the air conditioner has a locked rotor amp draw of about 110 amps.
 
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