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Batteries

Freezers & Fridges on Batteries: Inverter Sizing

In the previous part, we took a quick tour of the topic of freezers and refrigerators on battery power. I also gave some rough indications of the kilowatt-range inverter power that these refrigeration appliances take to operate properly, despite their modest running powers.

This article takes a closer look at inverter sizing for freezers and fridges. We will start with a more detailed table inverter power vs. freezer/fridge type and size, which is good for planning. Then, I will give you four formulas for a more accurate estimate of the inverter power requirement of a particular appliance model based on its specs.

Inverter sizing is a balance, and there are a few common pitfalls to be avoided. An undersized inverter is a total show-stopper, but oversizing is not recommended either. This information should help you pick an inverter that is powerful enough, but not any bigger, heavier and more expensive than necessary.

Inverter sizing chart

The table below gives the continuous output power required from a power inverter to run different freezers and fridges classified by volume capacity and type.

INVERTER POWER
REQUIREMENT

Appliance
Capacity (tot.)
[cu.ft]
Power,
continuous
[W]
Freezers:
Chest<10 cu.ft
10…30 cu.ft
1000 W
2000 W
Upright<10 cu.ft
10…20 cu.ft
1500 W
2500 W
Refrigerator-
freezers:
No-freezer <20 cu.ft1500 W
Top/Bottom
freezer
<10 cu.ft
10…20
2000 W
2500 W
Side-by-side/
French-door
<15 cu.ft
15…25
<30
2500 W
3500 W
4000 W

Note that the numbers on the table are:

  • minimum inverter power specs;
  • refer to inverter continuous power rating (not surge power); and
  • valid for a typical fridge/freezer in the class.

These recommendations are based on my own starting power measurements for a number of fridges and freezers. You can use the table for rough sizing, but if you have any reliable specs for the fridge or freezer you intend to power, I recommend using the rules below.

Inverter sizing rules

The inverter sizing chart above is OK for quick&rough work but is limited in that it uses average freezer/fridge power values.

For more accurate inverter sizing, I have formed four sizing rules based on individual freezer/fridge model specifications. These can account for model-to-model variation in the powering requirements.

1 Running watts or amps (Best)

The steady running power (W) or current (A) of a refrigerator/freezer are in my experience the best predictor of the surge power the appliance is going to draw on startup.

Rule 1: Inverter continuous power rating should be at least 10 times the freezer or fridge running power, both in watts:

Inverter Watts > 10 x Running Watts

Rule 2: Inverter continuous power rating should be at least 1000 times the running AC amps at 120V:

Inverter Watts > 1000 x AC Amps (@120V)

Rules 1 and 2 are difficult to apply because running watts/amps are rarely reliable or even available. Few freezer/fridge datasheets report them, and often the numbers given are unrealistically high. Use Rules 1 and 2 only with powers/amps you measured yourself.

2 Energy use (2nd best)

Most applicable inverter sizing rule is based on fridge or freezer yearly energy use – this is the kilowatt-hour (kWh) number of the Energy Guide label (or equivalent).

Rule 3: Inverter continuous power rating in watts should be at least 5 times the yearly energy use of the refrigerator or freezer in kilowatt-hours:

Inverter Watts > 5 x yearly kWh

Although not as accurate as Rules 1 and 2, the yearly kilowatt-hours are almost always available and reliable, since in most jurisdictions the refrigerating appliance manufacturers are legally obliged to test and report it.

3 Volume capacity (rough)

If you don’t know the running watts, amps or the energy use, you can fall back to using the volume capacity of the freezer or fridge to estimate the needed inverter size.

Rule 4: Inverter continuous power rating in watts should be at least 250 times the freezer or fridge volume capacity in cubic feet:

Inverter Watts > 250 x cu.ft

This rule is not accurate and does not give you an optimized inverter size. On the other hand, it is very simple and based on the one number you always should have on your freezer or fridge.

How I formed the rules

With fridges and freezers, a power inverter must be sized to handle the starting power. This momentary current is often 10 times the steady running current, and by far the toughest challenge for the inverter.

The challenge is that you rarely know the starting power of a certain fridge/freezer – it’s hard to measure and never given in the specs.

To solve the issue, I decided to find out how the starting powers relate to the fridge/freezer specs that are given. To do this, I

  1. measured the starting current for a number freezers/fridges
  2. saw how the starting currents scale with running power and amps, and
  3. how the running power/amps are connected to specs that are typically available (energy use, volume capacity, type)

This exercise gave me statistical connections between common specs and starting power, and allowed me to predict the starting power – and the inverter size – from the freezer/fridge specs that are typically available.

How high is the starting current really?

Most freezer and fridge powering instructions claim the surge power draw at startup to be 3 to 5 times the steady running power. But is this true?

Based on my own measurements, the fridge/freezer start surge power is in reality 10…12 times the running power – dramatically higher than the claimed values.

The backstory: I noticed that something is wrong with the 3-to-5-times rule when a 600W inverter (1000W peak) could not start a sub-100-watt, 11 cu-ft chest freezer. (A bigger 1000W inverter which I got next could start.)

I decided to investigate: I ran the battery cables through a current gauge, powered up the freezer and scoped the surge peaks at the compressor start.

The graph below shows the result: the starting current is hedgehog-shaped pulse with a mean at 100A, peaks touching 200A and a duration of just over 1s. Compared to the steady current of around 10A, the 100A mean is 10x higher. The peak is absolutely brutal and way over the “3 to 5x” running power – no wonder the 600W inverter tripped!

Freezer surge current at startup: the spiky peak current pulse has a duration of around 1 second and an average current of 100A, which is 10 times the steady running current! (The displayed currents were measured on the battery side of an inverter which powered the freezer.)

Continuing the investigation by scoping a few other freezers and fridges the same way, the starting powers I measured fell quite consistently into the bracket 10…12 times the running power. Both of the peak and the running power levels actually fluctuated a bit so the rule was not exact, but consistent enough to serve as a basis of inverter sizing.

Power cycling

Just as if the 10x surge current at startup would not be bad enough, it gets worse: starting power peaks pictured above happen not only when you power on the freezer or fridge, but actually many times an hour.

Why? Because freezers and fridges modulate their power by periodically switching the compressor on and off.

You can see an example of this power cycling in the graph below: the current draw of a freezer (through and inverter) is not constant, but a variable square wave with 9A current when ON and less than 1A when off.

The bad news for the inverter: there is the same horrible surge power peak we saw in the first graph at each compressor startup. You can see these surge peaks in the graph below as very narrow spikes in front of the ON cycles.

Freezers and fridges run their compressors in cycles, creating a few starting events every hour with current demand spikes. Measured from a 11 cu-ft chest freezer on a 1000W inverter.

Understanding fridge/freezer power specs

Inverter sizing starts from the freezer/fridge power specifications –these tell how heavy the appliance is to drive electrically. Specifically, the inverter must be sized on the freezer/fridge starting power (or surge power), which is around 10x the running power.

The problem is that starting power is never specified, so you need to use the available specs to estimate the starting power. The commonly reported power specifications for freezers and fridges are:

  • Energy use: Yearly electrical energy consumed in typical use, expressed in kilowatt-hours (kWh/y).
  • Current rating: Maximum continuous current draw (running amps) while compressor(s) are running. Expressed in amps (A) at AC line level (120V or 230V). Unreliable, often exaggerated or not reported at all. Not to be confused with fuse size.
  • Power rating: Continuous power draw (running power) while compressor(s) are running, expressed in watts (W). Very useful for inverter sizing, but rarely given.
  • Fuse: Required fuse size on the powering AC line circuit, important for taking the starting current. In practice, this is always “15A” or “15 to 20 A”.

The power specs and typical values are summarized in the table below.

Freezer/fridge
Specification
Typical
values
Notes
Energy use200…800 kWhmost reliable
Current rating0.7…3.5 Aoverrating
common
Power rating75…400 Wrarely given
Fuse15…20 A115VAC

Best freezer/fridge spec for inverter sizing?

Energy use (most cases)

Energy use (kWh) is the most useful freezer/fridge power spec for inverter sizing. It is indirect but reliable and always available for new machines. The reason is that the energy use is the main energy efficiency metric for freezers and refrigerators, and manufacturers are in most regions legally obliged to test and report it.

You can find this number on the Energy Guide (US), the Energuide (CA), Energy Label (EU and UK), or Energy rating label (AU). The energy use is the most reliable power spec that you will usually get.

Running Power or Current (if available)

Running power and current are the best freezer/fridge specs to base the inverter sizing on. These specs are directly linked to the compressor motor size, and should have the best correlation with the starting power too.

Sadly, these ratings are rarely given on freezer and fridge datasheets. And even when they are, they are often unrealistically high and therefore unreliable. It seems that manufacturers have a habit of exaggerating the running power and current specs, perhaps to discourage you from potentially undersizing fuses, breakers or wiring in the feeding circuit.

The only time I recommend using the running power/amps and Rules 1 or 2 is when you can measure these quantities yourself.

Understanding inverter power specs

Inverters have basically only two power-related specifications:

  1. Continuous power: An output power level that can be sustained indefinitely, measured in watts.
  2. Peak power: A higher power output that can be sustained for a short time. Also called surge power or intermittent power.

I have found only the continuous power useful in estimating its capability in driving freezers or fridges. All of the rules and the chart presented here therefore refer to the continuous power rating of the inverter.

Why not use peak power ratings?

The problem with the peak/surge/intermittent power ratings or power inverters is that the allowed peak duration is not specified.

This means that whatever peak wattage the inverter advertises, you never know for how long it can take it – 100 seconds, 10 seconds or 10 milliseconds (power line half cycle)?

To be useful in coping with the surge power of freezers and fridges, the inverter peak power capability should cover the starting current peak duration. I have measured the starting peak to be 1 to 2 seconds (see above).

Sadly, there is no way of knowing if the inverter can keep its spec peak power for this 1 to 2 second period.

Can the inverter be too powerful?

In principle, the inverter can never be too powerful for running a freezer or a refrigerator – larger will always work. If the sizing chart or formulas above give you a 2500W minimum power, for example, the inverter can be 2500W or larger.

There is one catch, though: bigger inverters have a larger no-load current. The inverter draws this current from your battery at all times, even if the freezer/fridge compressor is not running – you can see this in the power cycling graph above.

The no-load current is a waste, and the higher no-load current of bigger inverters means that they will drain your battery a bit faster.

Conclusion

In this article we have seen how you should size the inverter power output for running fridges and freezers on batteries. We learned that it’s critical that the inverter can handle the surge power required to start up the compressor of the cold appliances.

We also learned that the surge power is never specified must be estimated from the other freezer/fridge specs. For this, we got two practical tools: an inverter sizing chart and a set of sizing rules.

We saw that required inverters are pretty beefy: freezers and fridges take 1000W to 4000W continuous power ratings to start up properly.

The next and final part of the series will take up the other half of the dimensioning equation: the battery pack. Be sure to check it out!

Q&A

Will a 1000W inverter run a refrigerator?

A power inverter with a 1000W continuous power rating may be able to run energy-efficient small-to-medium freezerless refrigerators, but is underpowered for refrigerators in general.

Freezerless fridges typically require a 1500W inverter at a minimum for reliable powering; models that include a freezer may take anything from 2000W up to 4000W to start properly. For more information, see the inverter sizing table and sizing rules in this article.

What size inverter for a chest freezer?

Small chest freezers with volume capacity under 10 cu-ft can typically be driven with a 1000W inverter, while larger chest freezers (up to 30 cu-ft) typically require a 2000W to start properly.

Chest freezers are energy-efficient and take lower inverter power to run than other freezer types. The powering requirements still vary from model to model, and for best results the inverter sizing should be based on the freezer power specifications according to the rules above.

How many watts to run a freezer?

Common consumer freezers draw from 75W to 400W of steady power and take surge powers between 1kW and 4kW at startup. The power use depends on freezer size, type and energy-efficiency. Upright freezers use substantially more power than chest freezers on average.

When driving a freezer with a power inverter, you should size the inverter continuous current rating to be at least the freezer surge power. For best sizing practices, see the inverter sizing table and sizing rules in this article.