In this article:

Why those standby generator calculators and worksheets leave out VERY important details on choosing what size generator to buy.

Why heat pumps, AC, and even auxiliary heat strips are a game-changer for selecting the right generator.

Why “LRA” or “locked rotor amps” should be the first number you consider.

Other points on when you should seek help, and how.

Point #1: Lazy marketing out there passing for generator sizing advice.

This one really grinds my gears.

Take a walk through the various “generator sizing tools” and worksheets standby generator manufacturers have out there.  They’re sloppy and aren’t really designed to help you choose what size generator.  They’re more about selling you a generator as quickly as possible.

Many of them rely on the square footage of your home as the primary factor and use very, VERY broad averages.  Most of them will list 50+ appliances — most of which are fairly light loads — and completely ignore the heavy hitters, like HVAC.

Oh yeah, minor detail, the heating system that keeps you from dying during that winter outage is the biggest influence on what size generator is appropriate for your home.

And the generator size worksheets gloss right over this.

32 appliances listed from Christmas lights to phone chargers, but virtually no help on what size generator will actually heat or cool your home.
32 appliances, no heat pump.

Here’s the generator sizing worksheet from Home Depot, one of the nation’s highest volume sellers of generators.  It’s sure to list that 10w cell changer and 20w string of Christmas lights… but nothing more than a furnace fan blower for HVAC.  Read the fine print.

The “Standby Generator Sizing Calculator” from GE takes a different approach to generator sizing than Home Depot.  It actually allows you to select “4 ton Heat Pump.”  It recommends one option: their 17kw standby generator.  Convenient enough from the company that Edison built, right?

Not all heat pumps, even of similar size, are created equal.  This brings me to my next point to help you understand what size generator could work for you.

Point #2: Heat Pumps and LRA

That 17kw unit is rated up to 64 amps.  That sounds like “a lot” to many homeowners.  With GE or the generator’s manufacturer telling you that this will handle a 4 ton heat pump, you might just pull the trigger based on that.

You shouldn’t.

I don’t run a hypothetical heat pump on hypothetical electricity or leprechaun tears or unicorn farts.  It’s a real thing that has data, measurements, and operational standards, so let’s rely on those.

Knowing that, go down to your basement to check the amperage listed on the data plate.  Here’s what you’ll see (this is mine):

A Carrier heat pump data (or name) plate noting LRA, LRA, FLA, and minimum volts and helping to explain why this is essential info to find what size generator you will need for your home
YOUR heat pump data, namely LRA, will likely be the largest influence on what size generator will be right for your home.

So, by seeing that data plate, you’re already doing more than many homeowners trying to select the right size of standby generator.  Nicely done.  Gold star.

Let’s discuss what we see:

RLA

RLA stands for “Rated Load Amps,” and is not a number you should rely on to guide you in determining what size generator will work for your needs.  This number is a UL (Underwriter’s Laboratories) specification where they take the Maximum Continuous Amps and multiply by .641.  Because Maximum Continuous Amps is based on when the compressor overload will trip, multiplying it by .641 basically gives it some headroom.  RLA has more to do with system design than the amps your heat pump pulls in operation.

RLA DOES NOT stand for “running load amps,” and has little to do with the amperage draw of your compressor.

LRA

Next, ladies and gentlemen, is the most important factor for determining what size generator you will need.  LRA stands for Locked Rotor Amps, and is not a theoretical number: it is the number of amps your compressor will draw to get started.  It is also called “inrush current” for the same reason, because for that first half second, it takes a LOT of power to get that compressor running.

Wonder why your lights may dim for half a second when the heat pump kicks on?  It’s because of LRA.

Well, my heat pump is a 4 ton unit, the same as we used in the GE example.  My heat pump’s requirement to start the compressor is 96.0 amps as you can see above.  96.0 LRA.

Despite the manufacturer’s generalized recommendation, unless you can account for that 32 amp shortfall, you’re either going to:

A) overload the generator and have it trip the overload and shut down

B) heavily strain the generator, not trip the overload, and just destroy your generator over time

C) not supply enough power to the heat pump and damage it (potentially along with any other appliance running at the time)

FLA

FLA stands for “Full Load Amps” which is the maximum amperage the blower motor will draw.  So this is up to ANOTHER additional 7 amps of continuous load onto the generator.  Right there is more than 10% of that GE unit’s rated maximum amperage, so it too is a significant factor in what size generator you should consider.

Other heat pump influences on what size generator you’ll need

Generators sometimes have surge amp ratings to be able to handle heavier amps for a limited period.  Many times you will see this published for commercial generators.  Also, there are reports of home standby generator manufacturer’s citing specific surge amps on the phone.  Get it in writing, because, otherwise, good luck on a warranty claim.

Another thing that some talk about is “permissible voltage dip” to satisfy LRA… basically starve the compressor with just enough power to get it moving.  Same say you can allow 30% voltage dip, which, if you look at the data plate above, is a no-no.  Minimum is 197V, which is only 15% less than from 230v.  More than 15% voltage dip in my case, and either the heat pump will shut down or I’ll damage it.

Remember also that you’re going to similarly starve ALL of the other appliances/motors running at the time with that “dip.”

My stance is this: if I am going to pay thousands for a home standby generator and thousands more for installation, why would I skimp and risk damaging that investment or the thousands of dollars I have in appliances, heat pump, etc?

You know what the definition of a “brownout” is? A drop in voltage.  Why would I want to pay to create my OWN damaging brownouts?

For example, I recently had to have my well pump pulled and rewired because of damage it suffered during a brownout.  This brownout was pre-generator, but the damage was done.  It was $900 to pull and rewire my pump, and would have been around $2500 if I needed a new well pump.

The fact that the worksheets talk around the heat pump/LRA issue isn’t confidence inspiring, so buyer beware.  You may want to talk to a licensed electrician or electrical engineer to take some quick measurements and make a recommendation about what size generator they would recommend for YOUR specific needs.

Point #3: Use YOUR appliance ratings.

The next consideration in determining what generator size you need is that you aren’t using guesses or averages.  Catalog the essential and non-essential appliances you would want to run in your home during an outage.  Use a standby generator’s worksheet (Kohler’s “Expert Level” option does a good job of capturing the basics). Populate it with the data from YOUR appliances.

Maybe the appliances will need less power, maybe more, but at least you’ll be making decisions based on real data.

An added bonus?  Create that record of your specific power needs and send it to the generator manufacturer’s tech line.  They’re there to help you determine what generator size you need, and you’re potentially spending a lot of money, so why not rely on their expertise?

An additional bonus to this approach is that they couldn’t as easily duck out of responsibility.  They can’t duck a generator failure by saying ‘your loads were more than the averages.’  Maybe this is being overly cautious, but again, you’re spending a lot of money and they offer the service… use it.

Point #4: Heat Strips, Auxiliary Heat, and Emergency Heat

Heat strip used to help add capacity to heat pumps... it can also significantly increase loads or overload your generator. It's a significant consideration for what size generator will be needed.

If your heat pump has an electric backup, it has an internal auxiliary electric heater. That “heat strip” that is basically a big resistance heating coil.  Based on my earlier point on heat pump compressors and LRA, you might think, “I’ll just run auxiliary heat and avoid the compressor altogether.”

Two points here:

Auxiliary heat adds an even higher electrical load on top of your heat pump compressor alone.

In many systems, auxiliary heat likely runs BOTH the compressor and the heat strips.  This would be a huge amperage load.  In my case, it could be 96 amps of LRA + 7 amps for blower + 40 amps for a 20kw heat strip.  Yes, you read that right, those heat strips consume massive amounts of electricity themselves.  These ain’t a hair dryer.

Emergency heat isn’t optimal for you or your home.

Emergency heat typically runs ONLY the heat strips, but again, that’s a very significant load.

If you’ve ever run heat strips, they pump out far hotter air at the register than your heat pump.  In my home, the air temp from the heat strip is around 130 degrees when it’s usually closer to 90 degrees.  This can be really tough on wood floors, artwork, and is generally not very comfortable (ask me how I know).

Let’s say you go away for weekends or are using this standby generator for a vacation home.

If you have an outage on a very cold day and the aux heat kicks on and overloads an undersized generator, you’ve got a problem. The generator overload breaker has tripped, and the generator is disabled.  Many residential generators will not self-reset an overload condition.

You’re on your way to frozen pipes despite having a sizable investment in a standby generator to avoid that exact problem.

Support systems can play an important role in your system.

Your thermostat, generator, and HVAC system all need to be considered as a system.  I’ll write a full post on this, but it’s a big reason why I added Smartthings, Nest thermostats, and a battery backup to my broadband modem.  It’s neither expensive nor complicated.  I get a text if I lose power, I get a text when the generator kicks on, and I can still check temps in the house and the generator shed.

Zoning systems like an Arzel Boss add another layer of complexity if you want to be really redundant, but again, that’s another post.

Point #5: Give your generator a little headroom.

Another consideration in what generator size will suit your needs is giving it a little extra capacity.  If you’re really cutting it close on size, any factor reducing power could be enough to cause you problems.

From Generac specification sheet:

“Maximum kilovolt amps and current are subject to and limited by such factors at fuel Btu/megajoule content, ambient temperature, altitude, engine power and condition, etc.”

When you start stacking up all of the factors that can reduce standby generator output, it can definitely add up.  Don’t ignore it, prepare for it.  Remember that little engine in the real world is not necessarily going to match the output levels of ideal lab conditions.

To give you an alternate view on output headroom, read my article comparing the headroom of a used diesel vs a new 20kw natural gas standby generator.  Diesel standby generators generally are in a different class when it comes to power output, and especially amperage.  Something to consider.

Point #6: Undersized generator? Ultimately, it’s on you.

Maybe this one is already front of mind, but it bears stating:  This isn’t a hammer you’re buying.  You aren’t going to pop that 500lb Generac in the back of the ol’ Family Truckster and take it back to Home Depot.

When that generator is out of the box and placed, you’re going to have a hard time returning it.  After firing it up, you’re going to have a VERY hard time returning it.  This is not something you want to try to end up selling on Ebay or Craigslist after you find out it’s undersized.

Remember, doubling standby generator output from 11kw to 22kw doesn’t cost twice as much… it’s more like 25% more.  Factor in tax and installation, and the total cost of that 22kw is probably only to end up about 10% more than the 11kw.

I could also make this same argument for stepping up to a used diesel standby generator, as well.

All in all, the fact that you have ultimate responsibility makes selecting the generator for your needs and the decision on the generator installer/electrician/electrical engineer that much more important.  And you also don’t want to be that electrician’s “learning experience” first generator install!  I’m working on building out some guidance on this, so stay tuned.

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