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Home Backup Generator - Electrical Questions
Posted on 1/10/24 at 12:24 pm
Posted on 1/10/24 at 12:24 pm
I'm considering a backup generator for my house and I've received conflicting information regarding what my electrical requirements are so I come to the brilliant minds of TD to tell me what I need. My house is mostly gas with the main consideration for the generator being AC and Fridge/Freezers. I have a 4-ton AC unit. I have gas for heat/stove/oven/water heater.
My power meter gives me a reading of the max draw my house has ever had and that comes out to 8.35 kVA. I don't know what that means in kW so I've run a few different calculators online and don't get anywhere near the 20kW power level that everyone wants to sell me. It will run on natural gas so I know the rating drops to 18kw, but just seems like overkill. What am I missing? Is the goal overkill since this is like an insurance policy?
My power meter gives me a reading of the max draw my house has ever had and that comes out to 8.35 kVA. I don't know what that means in kW so I've run a few different calculators online and don't get anywhere near the 20kW power level that everyone wants to sell me. It will run on natural gas so I know the rating drops to 18kw, but just seems like overkill. What am I missing? Is the goal overkill since this is like an insurance policy?
8
Posted on 1/10/24 at 12:50 pm to NewIberiaHaircut
A kVA, volt-amp, is equal to kW, watt, as volts x amp = watt; so kVA=kW.
You need to take into consideration, surge watts, energy required to start an electric motor, which maybe several times higher than the running watts, energy required to keep the motor running after it starts. Example, it takes much more electrical power to start a HVAC compressor than to keep it running after it starts.
More than likely the 20kW generator is sized properly with some cushion built in. Maybe you can go smaller but is there much savings in installing a a 16 kW stand by generator vs 20 kW - I don’t know but I wouldn’t think so.
Surge Watts vs Running Watts
You need to take into consideration, surge watts, energy required to start an electric motor, which maybe several times higher than the running watts, energy required to keep the motor running after it starts. Example, it takes much more electrical power to start a HVAC compressor than to keep it running after it starts.
More than likely the 20kW generator is sized properly with some cushion built in. Maybe you can go smaller but is there much savings in installing a a 16 kW stand by generator vs 20 kW - I don’t know but I wouldn’t think so.
Surge Watts vs Running Watts
This post was edited on 1/10/24 at 1:37 pm
Posted on 1/10/24 at 12:59 pm to CrawDude
Get an electrician to put an amp meter on your service and turn on everything you want the generator to run. Turn major appliances on 30 to 60 seconds apart. Add a safety factor and use this to size your genny.
Most gen. are sized using propane such as 15kw or 20kw. Using NG you will loose about 10-15% so size accordingly.
A few years back Cummings specs where based on using LG.
Most gen. are sized using propane such as 15kw or 20kw. Using NG you will loose about 10-15% so size accordingly.
A few years back Cummings specs where based on using LG.
Posted on 1/10/24 at 1:04 pm to NewIberiaHaircut
Do you want an automatic standby generator or just a setup to get you by in an emergency?
Posted on 1/10/24 at 1:15 pm to NewIberiaHaircut
If you have a 4 ton AC unit and all other big loads are gas, you should be fine with a unit under 14-16kw. The biggest hurdle you have to jump over is the fraction of a second surge for your HVAC, which on a 4-ton unit can be around 115 amps. Anything else won't draw much, save for 1,000 watts for a toaster, 1,000 watts for a microwave, etc.. Even my 4-ton AC after it starts is only drawing about 3,200 watts.
That said, the questions of what you want to power, what you want to plan to power in the future, your expectations on what life inside your home needs to look like during an outage. etc.. also come into play. If you want to live like the power isn't out, then you will need to size it bigger.
A big reason why everyone gets the 22kw generac is because price wise it's not much more than the 20, and in some cases costs less. So if you need a 20, and a 22 is around the same price, upgrading makes sense whether you need it or not.
I built my own generator. My mathematical max running load is 10,282 watts on natural gas. I've load tested it to 10,200 watts. I have a microair soft starter on my 4-ton ac unit, which dropped my AC surge amps from 117 to 48. My generator starts and runs that plus all the lights, fans, small appliances, etc.. in the house.
That said, the questions of what you want to power, what you want to plan to power in the future, your expectations on what life inside your home needs to look like during an outage. etc.. also come into play. If you want to live like the power isn't out, then you will need to size it bigger.
A big reason why everyone gets the 22kw generac is because price wise it's not much more than the 20, and in some cases costs less. So if you need a 20, and a 22 is around the same price, upgrading makes sense whether you need it or not.
I built my own generator. My mathematical max running load is 10,282 watts on natural gas. I've load tested it to 10,200 watts. I have a microair soft starter on my 4-ton ac unit, which dropped my AC surge amps from 117 to 48. My generator starts and runs that plus all the lights, fans, small appliances, etc.. in the house.
This post was edited on 1/10/24 at 1:23 pm
Posted on 1/10/24 at 1:24 pm to NewIberiaHaircut
My parents just put in a 24Kw and their house is mostly electric (except furnace)and it will run everything.
The generator will run all of the following appliances simultaneously:
4 ton AC
Double oven (both on)
Cooktop
Clothes dryer
If a 24 Kw can run all that at the same time, a 20 for your needs might be overkill. Also, my parents do not have a soft start on their AC. The auto switch for the generator just waits 5 min to start the AC so everything doesn’t start at the same time during the auto cutover.
The generator will run all of the following appliances simultaneously:
4 ton AC
Double oven (both on)
Cooktop
Clothes dryer
If a 24 Kw can run all that at the same time, a 20 for your needs might be overkill. Also, my parents do not have a soft start on their AC. The auto switch for the generator just waits 5 min to start the AC so everything doesn’t start at the same time during the auto cutover.
Posted on 1/10/24 at 1:49 pm to TheBoo
quote:
f you have a 4 ton AC unit and all other big loads are gas, you should be fine with a unit under 14-16kw. The biggest hurdle you have to jump over is the fraction of a second surge for your HVAC, which on a 4-ton unit can be around 115 amps. Anything else won't draw much, save for 1,000 watts for a toaster, 1,000 watts for a microwave, etc.. Even my 4-ton AC after it starts is only drawing about 3,200 watts.
My AC pulls 124 amps according to the sticker.
quote:
That said, the questions of what you want to power, what you want to plan to power in the future, your expectations on what life inside your home needs to look like during an outage. etc.. also come into play. If you want to live like the power isn't out, then you will need to size it bigger.
We don't use as much power as it is and would conserve during an outage.
quote:
I have a microair soft starter on my 4-ton ac unit, which dropped my AC surge amps from 117 to 48.
The generator salespeople start talking in circles when I mention this. I don't care about the incremental cost differences between larger units I just want straight answers. It appears the sales folks have less understanding about electricity than I do at this point.
Posted on 1/10/24 at 2:20 pm to NewIberiaHaircut
quote:
It appears the sales folks have less understanding about electricity than I do at this point.
This is not hard to accomplish.
You are like me then. I don't need a big standby so we can live business as usual during an outage. 12-14-16kw is plenty for us, so long as it can start the AC. I recommend getting a soft starter to drop the surge on your AC unit then you can go with a smaller unit that is within your comfort zone.
Briggs makes a vertical 12kw (10.8kw ng) unit that's rated to start a 4 ton AC.
This post was edited on 1/10/24 at 2:28 pm
Posted on 1/10/24 at 2:35 pm to NewIberiaHaircut
what about your clothes dryer? Or would you worry about connecting?
Posted on 1/10/24 at 5:19 pm to NewIberiaHaircut
just for reference, I have a 2700 sqft home with 4 amd 3 ton ac compressors. Gas water heater, heater, and stove. I can pretty much run the whole house except for the 4 ton unit with an 11 kW generator converted to Natural gas. The 3 ton unit cools all bedrooms and an upstairs living room, so I'm good with that. Sure I can't run my microwave, dryer, blemd up some margaritas, and start the AC up at the same time, but otherwise it can handle it.
Posted on 1/11/24 at 7:49 am to TheBoo
I definitely will be purchasing a micro air soft start. Looks like a must have.
Posted on 1/11/24 at 7:30 pm to NewIberiaHaircut
I had an outage in June and got to test my standby out:
My house is 4300sqft with 3 AC units (2.5, 3, 4T).
Electric dryer (unfortunately)
1 fridge/freezer, 1 upright fridge, 1 upright freezer
Microwave
2 wall ovens
Gas stove, 2 gas water heaters
Then usual stuff.
My measured load was 11.5kW. This was a hot day. All AC running.
Turned on the microwave, both ovens, dryer, and that bumped it to 23.2kW where it ran pretty steadily.
Forgot about the toaster. Would’ve bumped it a bit more.
My house is 4300sqft with 3 AC units (2.5, 3, 4T).
Electric dryer (unfortunately)
1 fridge/freezer, 1 upright fridge, 1 upright freezer
Microwave
2 wall ovens
Gas stove, 2 gas water heaters
Then usual stuff.
My measured load was 11.5kW. This was a hot day. All AC running.
Turned on the microwave, both ovens, dryer, and that bumped it to 23.2kW where it ran pretty steadily.
Forgot about the toaster. Would’ve bumped it a bit more.
Posted on 1/11/24 at 9:03 pm to NewIberiaHaircut
CrawDude nailed it in exactly why a generator appears to be oversized for your electrical demand in your home. Of all the 240V devices fed from your home panel, the starting current (or locked-rotor amps, LRA) of your AC compressor is the biggest driver in generator size. LRA is typically 4-8 times the full-load current (or rated load amps, RLA) so the rush of energy needed to put that compressor in motion is high. But it doesn't last long at all so if it can be reduced, you're golden.
100% smart move. I have two 3 ton units and both are on Micro Airs. They don't lie when they say the LRA is reduced by ~70%. This has been true for both of mine - LRA reduced from 79 and 84 to 25 and 28 respectively. So if your nameplate reads 115 LRA, expect around 35A.
After considering the size of your HVAC compressor(s), the next determining factor in generator size are the other 240V loads in your home - dryer, cooktop, oven, water heater, etc. These won't have near the inrush of your HVAC and are mostly resistive loads but once running they play a part. For example, a typical electric water heater is 4500-5000W. That can jump you from one size to the next for your generator.
As others have said, you could probably get by on a 14kW or 16kW by reducing the inrush on your AC with a soft starter. But the price difference isn't high and you leave yourself spare capacity in the generator for later.
EDIT: Forgot to mention that kVA is a measure of kW while taking into account something called "power factor." It is mostly irrelevant for homes since it applies mostly to rotating machinery. So for your case, as CrawDude said, kW = kVA.
quote:
I definitely will be purchasing a micro air soft start. Looks like a must have.
100% smart move. I have two 3 ton units and both are on Micro Airs. They don't lie when they say the LRA is reduced by ~70%. This has been true for both of mine - LRA reduced from 79 and 84 to 25 and 28 respectively. So if your nameplate reads 115 LRA, expect around 35A.
After considering the size of your HVAC compressor(s), the next determining factor in generator size are the other 240V loads in your home - dryer, cooktop, oven, water heater, etc. These won't have near the inrush of your HVAC and are mostly resistive loads but once running they play a part. For example, a typical electric water heater is 4500-5000W. That can jump you from one size to the next for your generator.
As others have said, you could probably get by on a 14kW or 16kW by reducing the inrush on your AC with a soft starter. But the price difference isn't high and you leave yourself spare capacity in the generator for later.
EDIT: Forgot to mention that kVA is a measure of kW while taking into account something called "power factor." It is mostly irrelevant for homes since it applies mostly to rotating machinery. So for your case, as CrawDude said, kW = kVA.
This post was edited on 1/11/24 at 9:07 pm
Posted on 1/12/24 at 12:09 am to bapple
All of that is exactly what I was thinking but couldn’t get any generator guy to articulate it. Sounds like my only issue is the inrush for the 4 ton AC and the soft start is already on the way. So if I can reduce that inrush current, I’ll be golden. According to my electric meter the max my house has pulled is 8350 watts so I just can’t see why the last guy I spoke with wanted me to get a 24kW generator. 
Posted on 1/12/24 at 8:09 am to NewIberiaHaircut
You also dont want to size your generator to run at 100% load either. Generally, want them to run at about 80%.
Its probably what he has in stock. Generator availability has gotten better, but its still 6+ months if they dont have one on the shelf(maybe not that 12kw briggs though).
quote:
I just can’t see why the last guy I spoke with wanted me to get a 24kW generator
Its probably what he has in stock. Generator availability has gotten better, but its still 6+ months if they dont have one on the shelf(maybe not that 12kw briggs though).
Posted on 1/12/24 at 9:05 am to NewIberiaHaircut
quote:
According to my electric meter the max my house has pulled is 8350 watts so I just can’t see why
Ideally if you could get your average running load to about 50-60% of the generator rated load it'll lend to the longevity of the generator. If you calculate an average usage of 6-7000 watts including the AC, then easy, get at least a 14kw generator.
Posted on 1/13/24 at 2:09 pm to NewIberiaHaircut
One thing I learned is that “20kw” or “22kw” is achieved with propane. A 20kw on natural gas is really 19kw. Something to consider if you are against the upper limit.
Posted on 1/14/24 at 11:05 am to CrawDude
KVA is not equal to KW . It is a function of what kind of load you have -Real vs. Apparent power. If you have a large inductive or capacitive load, there could be a large difference !
This post was edited on 1/14/24 at 11:08 am
Posted on 1/14/24 at 9:52 pm to kew48
quote:
One thing I learned is that “20kw” or “22kw” is achieved with propane. A 20kw on natural gas is really 19kw. Something to consider if you are against the upper limit.
Correct. My 22kW Honeywell has a nameplate rating of 19.5kW on nat gas. It's not wise to be on the knife edge of capacity anyway so if he's getting that close, he should bump up a size.
quote:
KVA is not equal to KW . It is a function of what kind of load you have -Real vs. Apparent power. If you have a large inductive or capacitive load, there could be a large difference !
The power factor from a utility is generally 0.9 and above for residences. I'll pull this good quote I found:
quote:
The bulk of electricity in houses is used to either heat things up (space heaters, ovens, cooktops, water heaters) or cool things down (air conditioners, refrigerators.) These either have intrinsically good power factor (heating elements are resistive, i.e. p.f. 1.00) or they come with power factor correction built-in (air conditioners).
The things you measured are mostly electronic devices, so they have poor power factor, but they also don't draw much power compared to the heating/cooling devices listed above.
--
Contrast this with industrial sites, where a large part of the load is AC induction motors with rated power factors between 0.80 to 0.90 (and less than that if they're less than fully loaded). There can be 10 MW worth of induction motors in a decent size plant - I know of ore crushing and grinding mills which are driven by a 10 MW induction motor each.
In summary, you can use kW = kVA for home generator calculations.
Posted on 1/15/24 at 2:35 am to bapple
2200 ft2. We use a EM5000SX Honda (gasoline) generator. I had an electrician (neighbor) install a transfer switch to disconnect from Entergy before switching to generator power. We shutoff the water heater and A/C breakers before switching to generator power. For A/C we use a window unit and for heat we use an electric radiator style heater that is the most energy efficient. Entergy power is never off for long.
This post was edited on 1/15/24 at 1:31 pm
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