Converting KW/hr to amps

[Coppersmith]

I have a prospective client with a big house for whom I need a ballpark guess on generator sizing to keep everything powered (i.e. no load shedding). The only information I have is (1) she has a 225 amp service; and (2) her peak average daily usage is 132 KW/Hr.

In the thread http://forums.mikeholt.com/showthread.php?t=125282 the OP asks about this conversion. In the last post, ggunn states: "you can convert kilowatt hours to amperes if you know the voltage at which your loads operate and assume a steady state load (a big assumption). If, for example you consume 100 kwh in a day at 240VAC and assume a steady state load, that's 100 kwh divided by 24 hours, or a 4.17 kw load. 4.17 kw divided by 240V is 17.36 A."

This is basically the information I'm looking for and yes I know there is not a direct conversion. Since nobody responded after ggunn's post can somebody first of all verify the above thought process is correct and then tell me if I am barking up the wrong tree with the following ballpark estimate:

132 KW/hr divided by 24 hours = 5.5 KW/hr divided by 240V = 22.92 amps ave per hour

and therefore if I assume most of the usage is during an eight hour period, I could guess that:

132 KW/hr divided by 8 hours = 16.5 KW/hr divided by 240V = 68.75 amps ave per hour

and so a generator of say 18,000 watts would probably be more than enough?

What say you? (I know there is a lot of guessing here.)

 

[augie47]

From my experience there is a major flaw in that the average load (KWh/hrs) has little to do with the actual load at any given specific time and your generator will need to supply the max load.
I have found KWh to be of little value in these matters.

 

[charlie b]

I agree with Gus. Unless you have a good reason to believe the load is constant (or nearly enough to being constant), then the total KWH in a day means nothing. Consider a 200 mile trip down a road with a 55 MPH speed limit. A man drives this trip in 4 hours, so his average speed was 50 MPH. But he received a speeding ticket! How could that be? Because he was going 30 MPH for part of the trip and 90 MPH for part of the trip.

The generator will need to be sized for the fastest part of the trip (the highest measured value of KW). The only way to guarantee that the generator will never be overloaded is to size it for the entire 225 amps of the service. That equates to 54 KW. If you want to save the client money by justifying a smaller generator, then get the client to pay for a load measurement. Set up a meter just like you would for a 30 day load study for which the intent is to add load to an existing facility (NEC 220.87). You are not adding load, so you don't have to go 30 days. But you need to factor in things that might not be running during your test but will be running on the generator. Air conditioning comes to mind.

 

[tw1156]

I have a prospective client with a big house for whom I need a ballpark guess on generator sizing to keep everything powered (i.e. no load shedding). The only information I have is (1) she has a 225 amp service; and (2) her peak average daily usage is 132 KW/Hr.

In the thread http://forums.mikeholt.com/showthread.php?t=125282 the OP asks about this conversion. In the last post, ggunn states: "you can convert kilowatt hours to amperes if you know the voltage at which your loads operate and assume a steady state load (a big assumption). If, for example you consume 100 kwh in a day at 240VAC and assume a steady state load, that's 100 kwh divided by 24 hours, or a 4.17 kw load. 4.17 kw divided by 240V is 17.36 A."

This is basically the information I'm looking for and yes I know there is not a direct conversion. Since nobody responded after ggunn's post can somebody first of all verify the above thought process is correct and then tell me if I am barking up the wrong tree with the following ballpark estimate:

132 KW/hr divided by 24 hours = 5.5 KW/hr divided by 240V = 22.92 amps ave per hour

and therefore if I assume most of the usage is during an eight hour period, I could guess that:

132 KW/hr divided by 8 hours = 16.5 KW/hr divided by 240V = 68.75 amps ave per hour

and so a generator of say 18,000 watts would probably be more than enough?

What say you? (I know there is a lot of guessing here.)

The POCO Meter's I've come across in Texas cycle to show the peak kW demand of all time; with the newer smart meters, one can also request the previous year's data which includes peak demand as well. Have you gone to check the meter?

 

[jaggedben]

Or see if the utility provides 15 minute interval data online. Some make this available through the customer's online account. May be called 'Greenbutton' data. Get it in a spreadsheet and search for the highest 15 minute value.

 

[Ingenieur]

use a peaking factor of 4
132000/24 x 4 = 22 kw / 27 kva range

better yet
just add up the big elec loads
cooling
wh
cooking
dryer
etc
and add a w or two per sqft for misc

the smart meter reading is a good idea

btw units are kwh not kw/h
so kwh/h = kw
no need to factor current into sizing
some loads will be 120 and some 240
power is all that matters

 

[ggunn]

I have a prospective client with a big house for whom I need a ballpark guess on generator sizing to keep everything powered (i.e. no load shedding). The only information I have is (1) she has a 225 amp service; and (2) her peak average daily usage is 132 KW/Hr.

In the thread http://forums.mikeholt.com/showthread.php?t=125282 the OP asks about this conversion. In the last post, ggunn states: "you can convert kilowatt hours to amperes if you know the voltage at which your loads operate and assume a steady state load (a big assumption). If, for example you consume 100 kwh in a day at 240VAC and assume a steady state load, that's 100 kwh divided by 24 hours, or a 4.17 kw load. 4.17 kw divided by 240V is 17.36 A."

Thanks for quoting me correctly and completely. I qualified what I said by saying that assuming that the load is constant is a big assumption. When you are sizing a generator, the most important consideration is the peak load it will need to service, not the number of kWh it can deliver. The limitation on the peak load it can handle is a function of the size of the generator; the limitation on the number of kWh it can supply is more a function of the size of the fuel tank.

 

[Coppersmith]

Thanks for quoting me correctly and completely.

Bet you were surprised to see a quote of yours from eight years ago.

 

[Jraef]

...her peak average daily usage is 132 KW/Hr...

That could have been 3.168kW per hour for 24 hours, or 100kW for 1 hour and 1.4kW for the other 23 hours, or 33kW for 3 hours, 10kW for 3 hours, and 167W for the remaining 18 hours. You have no way of knowing because it all comes out to the same 132kWH for a day. But if you give her an 18kW generator, it's too big for the first scenario and too small for the other two. You just don't have enough information to make a reasonable choice.

 

[robertd]

What happens when a motor load, like central A/C starts on a home generator? Do you need to use the running load or starting load when sizing the generator? Can home generators take a small starting overload?

 

[electrofelon]

I would just turn a bunch of stuff/everything on and take an amp reading. That will result in the most appropriately sized generator. YMMV, but I have never been asked by an inspector for calcs or data on an optional generator install, and if they did I would just make them up.

 

[Coppersmith]

That could have been 3.168kW per hour for 24 hours, or 100kW for 1 hour and 1.4kW for the other 23 hours, or 33kW for 3 hours, 10kW for 3 hours, and 167W for the remaining 18 hours. You have no way of knowing because it all comes out to the same 132kWH for a day. But if you give her an 18kW generator, it's too big for the first scenario and too small for the other two. You just don't have enough information to make a reasonable choice.

I suppose that's possible, but it's not likely. Using a little common sense, odds are the air conditioning and water heater loads were on 24 hours. Most everything else was off while people were sleeping (except a few security and night lights) and peak usage was around dinner time when large heating appliances in the kitchen were being used. So it's not crazy random.

 

[JFletcher]

A generator can handle the inrush of a AC compressor starting, unless it is very nearly loaded to capacity, then it might trip... Unlikely though.

Need to find out if the customer wants to be able to run everything in the house on the generator, or maybe just specific loads like heating and cooling, water heater, dryer, etcetera. If they have an electric range and oven, they made very well be willing to buy a smaller generator and leave that out of the equation... ditto for things like a hot tub, second HVAC unit, and other typical large loads.

A 225 amp service will probably not see more than a hundred amps 98% of the time. I believe the largest air-cooled generator that Generac makes is a 24 KW unit, which is good for 100 amps. Unless this customer has electric everything, like baseboard heat, water heater, range, oven, dryer, Spa, Etc, is very unlikely that you will run over 100 amps for any sustained amount of time.

Typically, on houses that we have installed generators, we go with a 17 to 22 KW Guardian series. When we load test these, it is often difficult to get over 80 amps sustain, last time we do it we had to turn on both HVAC units, the dryer, the oven, all four stove burners, and run the hot water to kick the water heater on. With a 22 to 24 KW unit you almost have to try to intentionally overload it.

If any of the major appliances or HVAC is gas heating versus Electric, you can dip down into the 14 to 18 KW range quite easily.

 

[Coppersmith]

I would just turn a bunch of stuff/everything on and take an amp reading. That will result in the most appropriately sized generator. YMMV, but I have never been asked by an inspector for calcs or data on an optional generator install, and if they did I would just make them up.

My only worry is that I give the HO what they want which is the ability to turn on all they normally turn on during a power outage. Money is no object for these people and if I said to get a 225 amp generator they probably would, but I wouldn't feel like I did my job if I let them waste that much money. I'm sure my recommendation will have a healthy reserve capacity. Just not crazy.

 

[JFletcher]

My only worry is that I give the HO what they want which is the ability to turn on all they normally turn on during a power outage. Money is no object for these people and if I said to get a 225 amp generator they probably would, but I wouldn't feel like I did my job if I let them waste that much money. I'm sure my recommendation will have a healthy reserve capacity. Just not crazy.

In that case I would turn on everything they normally run, worst case scenario with electric heating or air conditioning running full blast, whichever load is higher, water heater, dryer, oven, say two stove burners, and maybe some heavier 120-volt loads that have more than a few minutes run time, like a washing machine and add 1500 watts each for a hair dryer, vacuum cleaner, and a kitchen appliance like a Fry Daddy or coffee pot... microwaves and garbage disposals don't run long enough to even be considered. Take or calculate an amp reading based on that and go from there.

In the rare case that they will be pushing the generator to its limits, making sure your 120-volt loads are as balanced as they can be on each leg is somewhat important... You would not want a 105 amp load on a leg and an 85 amp load on b-leg.

I've only ever seen two house generators that were larger than 24 KW, one was a 36 and the other a 54, the latter of which supplied power to a 7000 square foot house with numerous outbuildings.

Just because they have a 225 amp service panel does not mean that they are anywhere near that capacity.... the load calculations could have showed a hundred and five amps, and the original electrician could have put in a two hundred amp service panel

 

[Jraef]

AC and water heater are CONNECTED all day, but not “on” the entire time, they cycle on and off, as does the refrigerator.

I like the idea of running everything they might want at the same time and measuring. Open all the doors and windows on a hot day so the AC runs, open the fridge door, open the hot water tap, turn on all the lights and appliances you can, then look at the watts being consumed.

 

[JFletcher]

AC and water heater are CONNECTED all day, but not “on” the entire time, they cycle on and off, as does the refrigerator.

I like the idea of running everything they might want at the same time and measuring. Open all the doors and windows on a hot day so the AC runs, open the fridge door, open the hot water tap, turn on all the lights and appliances you can, then look at the watts being consumed.

:thumbsup:

Now is also an excellent time to upgrade all of their lighting to LED. If they have any old tungsten filament flood lights, halogen floods, incandescent flood lights, or any other old inefficient lighting, time to get rid of that stuff. One generator upgrade we did, by changing out all of the interior 65 watt flood bulbs for 10 watt LEDs, we cut out over 2 kilowatts of lighting load, which in turn allowed us put in a slightly smaller generator. Or with the lighting upgrade, you can put in the same size you would have and have more headroom to run other things.

Wiping out over 2 KW of lighting load also lets the air conditioning run a fair bit less. And if your customers truly have a blank check budget for this generator upgrade, , they may also be served well by upgrading insulation and windows. You won't make any money off those upgrades however older windows and shallow / low R-value insulation lose an incredible amount Heating and Cooling.

A panel and crawl space inspection for electrical are good things to upsell the customer since you're going to be in both anyway. And you can always pitch the idea of additional features, like low voltage Landscaping lighting, USB receptacles, centralized router for 5- bar reception high speed internet everywhere in the house, camera system, Etc. Anything you think they might need or want, pitch it to them while you're there.

 

[Coppersmith]

:thumbsup:

Now is also an excellent time to upgrade all of their lighting to LED. If they have any old tungsten filament flood lights, halogen floods, incandescent flood lights, or any other old inefficient lighting, time to get rid of that stuff. One generator upgrade we did, by changing out all of the interior 65 watt flood bulbs for 10 watt LEDs, we cut out over 2 kilowatts of lighting load, which in turn allowed us put in a slightly smaller generator. Or with the lighting upgrade, you can put in the same size you would have and have more headroom to run other things.

Wiping out over 2 KW of lighting load also lets the air conditioning run a fair bit less. And if your customers truly have a blank check budget for this generator upgrade, , they may also be served well by upgrading insulation and windows. You won't make any money off those upgrades however older windows and shallow / low R-value insulation lose an incredible amount Heating and Cooling.

A panel and crawl space inspection for electrical are good things to upsell the customer since you're going to be in both anyway. And you can always pitch the idea of additional features, like low voltage Landscaping lighting, USB receptacles, centralized router for 5- bar reception high speed internet everywhere in the house, camera system, Etc. Anything you think they might need or want, pitch it to them while you're there.

All good ideas.

 

[kwired]

her peak average daily usage is 132 KW/Hr.

What exactly does that mean? Having the word peak in there makes it a little confusing. When peak is used it is usually only describing peak KW or KVA in recorded during a certain time period like 15 minutes or an hour. That sort of data would be useful to you. But if you just know the average daily usage is 132 kW/Hr you don't know if it is at a stead rate or if a majority is all used at a certain time of day.

Load shed devices can help you here. If you shut down an AC compressor or a water heater (tank type heater) for 5-15 minutes while some other load gets priority, owner may never know it shut down. And it is only happening when the utility power is out, so not like it will happen on a daily basis either.

 

[ggunn]

Bet you were surprised to see a quote of yours from eight years ago.

Time flies. It doesn't care if you are having fun or not.

BTW, from context I understand what you mean, but when you write "kW/hr" it means kilowatts per hour, which doesn't make sense. Kilowatt hours is power and time multiplication, not division.