How to size a 12VDC inverter

On paper, you can look up an appliance’s load, or look at it’s nameplate. My fridge says 11.6A & it’s in the sense detected device topic; model PSS28KSHSS

Traditionally 12A x 120VAC = 1440W is how you’d calc backup generator or inverter size. Address peak load later. We’ve had power outages & would like to avoid throwing groceries away. Also, be able to use the fridge normally during an outage. Generators do not match output to load well & come with many disadvantages. An inverter matches perfectly & has other advantages.

An electric car with a 12VDC battery can power an inverter. Find the rating on 12VDC charger, ie 1000W. Can I power my fridge? Traditional calc says NO, 1440 > 1000.

Use Sense to show YES. Add a smart plug like Kasa HS110 to measure total load of fridge. Sense alone may not find all the fridge components so it would show partial load. You could move the C clamps to your fridge circuit. Let the smart plug feed data to Sense for graphing for as long as you want to simulate a power outage; 4 days for me.

From 9:45AM Tuesday to Saturday, smart plug recorded many 64W cycles that I assume is the compressor. Sense also found this, I could hear it & named it Fridge. For the 1st time I also saw 2 600W loads I assume is the defrost. Sense had found a 364W load I named Fridge 2 but I didn’t confirm. 3W is standby.

Sense + smart plug shows I can power & use the fridge normally during a power outage. A pure sine 800W inverter + extension cord + maybe a towel under the window is all that’s needed. 1600W should be the peak rating.

A 1000W inverter would let me power other loads & is the max the car supports. Repeat Sense + smart plug for any appliances you need during a power outage. 90W circulating pump keeps house warm, etc.

So why are experts (1 EE, 1 Mech E) saying “that can’t be right”? Sense alone or + Kasa are missing the startup peak. I expect >1000W then 64W tail, which does seem low. Do I have an induction compressor? I don’t think so. I trust Sense & Kasa are accurate enough to size the inverter. Doing so provides a big advantage over starting the gas emergency generator. If there’s an error; please let me know.

Your refrigerator is marketed as having a “new high efficiency compressor” and “you may hear a high pitched hum” while it is operating (from p43 of the owner’s manual). Refrigerator compressors are now often driven by inverters and use brushless motors. There is no high inrush of current from that arrangement.

Those two tall peaks do look like defrost heaters.

Your calculations for power requirement are ok for DC power and AC devices that have primarily resistive loads (e.g. heaters, irons, ovens, …). With a fridge (and washers/dryers), and because of the compressor’s motor, there will be a reactive/inductive component (coils of the motor) to the power and you need to accommodate those as well. Read up on ‘Power Factor’, also called ‘Cosine Phi’. This is one of the reasons variable AC power is often tricky. Related story: Wanted to replace a speed controller for my ceiling fan recently, went to HomeDepot and took the old switch/controller with me … the gentleman there mentioned the old controller was in fact a dimmer (variable resistance), and not suitable for motor/inductive loads …

If I’m right that @chrisspy has a compressor driven by an inverter, then that’s a DC motor without high inrush and the power factor is not applicable to them. The inverter circuit is likely driven from a switching supply. If you can put your refrigerator on a Kill A Watt you could see the power factor (although Kill A Watts don’t have lab accuracy, it will be close on an AC motor). The owner’s manual talks about a high pitched hum when running; that’s not typical for a 60Hz motor.

chrisspy… Don’t forget when you have your final number (watts required) to multiply it by 1.10 to 1.20 for invertor efficiency. Inverters do not run at 100 percent. There are the losses to consider… My 2 cents… Gerry

Cleaned the fridge coils yesterday. From Jan 19 to Feb 23, the pattern I posted previously simply repeats (<700W peak briefly) as monitored by Kasa HS110. demiller9 correctly clued me in to the efficient compressor. There’s a lot of capacity remaining on the 1200W inverter I can use during the next outage.
Plug monitoring of loads is best, Sense is 2nd best, paper calculation is 3rd best for sizing.

There is also this to consider if the electric car is a Tesla.:

From your article: “We see a lot of automakers building bi-directional chargers into electric vehicles these days to directly access the main battery pack.
Just today, we reported on the unveiling of the new Hyundai Ioniq 5, which is equipped with that technology.
Tesla has been reticent in the past about integrating that capacity in its vehicles, but Tesla CTO Drew Baglino recently said the automaker plans to implement it in future vehicles.”

3.6kW 220V included with Ioniq 5. Many will want V2L so Tesla should reduce it’s warranty, not void or people will buy cars that can. They could also offer a BIG Powerwall discount to Tesla car owners & steer people to buy 2 products.
OTOH, thread is about sizing the inverter to measured (not calculated) loads.

My point is that more accurate sizing is done by monitoring or looking at someone else’s monitoring of your specific make & model. If those graphs are for your appliance, great. Oversizing happens when you don’t use specifics & rely on a generic or typical refrigerator, for example. Oversizing happens when you use the 11A (1400W) on the appliance nameplate.
Sense does show details needed; I disagree with your statement & ask you to look my graph posted Jan 23.
Sense alone may not identify all 'fridge components so a smart plug is needed for 120V. A 2nd pair of clamps are needed for 240V.
From Sense + smart plug monitoring I know this 1200W inverter can power the 'fridge & more. Without monitoring, ‘professionals’ would say I cannot & advise buying a big gas generator or Tesla Powerwall.

Don’t be rude about educating me.
The long time period was chosen because this fridge does not have a 1000+W startup spike like you say it should. If it did it would have shown.
The compressor is on this high resolution graph with x axis ticks marking 60 seconds. There is no startup spike.
This fridge has a special compressor that allows it to be high efficiency & inverter friendly during a power outage. Now, when I replace appliances I look for “inverter friendly during a power outage” but all that is advertised is Energy Star or other non-specific info when I really want to see a high res load graph during steady state use.

These folks claim “Biggest Benefit of inverter refrigerator is definitely the lack of surge.” LG INVERTER REFRIGERATORS — northernarizona-windandsun
I do believe Sense has the resolution to detect a surge while the graph may show a lower resolution. The graph would show a surge if it was there.
Maybe someone wants to post these detection & graphing resolutions.

Sense has revealed some of these monitoring intervals.

  • Underlying voltage and current sampling to compute power is done in the million samples per second range.
  • Because we are dealing with AC power, you don’t want to aggregate or produce an RMS value for anything less than a full cycle (US = 1/60th of a second). There are instantaneous surges higher, but also instantaneous drops to zero (or even lower in some cases) immediately thereafter.


  • Sense produces half second RMS values for the power meter, so you are seeing the accumulation of 30 AC cycles.

Thanks Kevin1
Million/sec is plenty of resolution to detect a startup spike & so is displaying 30 AC cycles; for the purpose of sizing an inverter IMHO