Selecting the right inverter for an AC Fridge

wildebus

Full Member
I know ... another thread about AC Fridges in Campers, but I thought it may be worthwhile to just clarify what to look for in an Inverter if you want to run an Mains AC Fridge in your Motorhome or Campervan.
These are my tips on Inverter Selection for a Fridge.

1. What power rating is needed?
The kind of Mains AC fridge you are likely looking at using in a vehicle will be a Compressor Fridge as they are the most efficient in terms of Electric Consumption. But all devices with compressors (Fridges, AC units, etc) have a high 'in-rush' current each time they start up. The graph below shows a typical in-rush current that my AC Fridge pulls each time it starts up

In Rush Current)
by David, on Flickr
The Fridge started up around 07:50 and went off around 08:02 (a typical duration). While on, it pulls around 3A or so, but the key thing is that at each start-up, it demands 90A or more!
This high load is only present for a very short duration - maybe 1-2 seconds at the most - but it must be considered and the inverter has to be able to run with a 1000W load for at least 2 seconds and maybe 5 to be on the safe side.
So even if the fridge typically only uses around 36W when it is running, the Inverter needs to be able to cope with an approximate 1000W load for 5 seconds


2. What overload capacity is available and how is the overload handled?
Different Inverters deal with overloads in different ways. Typically advertised is a 2x rating - so a 1000W inverter will claim to have a 2000W maximum capability in overload terms.
However, the way an overload is managed varies tremendously between manufacturers. Very often the overload capability is nowhere close to the claim; often the overload state can only exist for a few tens of millseconds (to cope with a mains spike for example); Very very few Inverters will have an overload capability for the length of time a Compressor In-rush would be active.
Also, some inverters will reset on an overload so will restart themselves; others will go into a fail state and require manual intervention to start again (this is an important point which will be touched on later); Any inverter that deals with an overload situation by blowing fuses should be avoided completely.
For a setup that doesn't need regular manual resetting, the Inverter must do an automatic restart on overload


3. What type of Inverter - Pure Sine Wave or Modified Sine Wave?
Both types will run a typical compressor fridge, but the Pure Sine Wave version will be more efficient doing so, is less likely to generate 'humming' on the device, and is typically (this is a subjective opinion) a better quality product in terms of design and componentry.
Either Pure Sine Wave or Modifed Sine Wave will usually work, but Pure Sine Wave is always the better technical option


4. How much power is used by the inverter itself in running? How noisy & hot will it get running 24/7?
All inverters have overheads in terms of power consumption. The larger the inverter, the larger the overhead tends to be. Because the inverter required for a Fridge that is usually a very low power (~35W) device is actually a fairly large Inverter of approximately 1000W or more, the Inverter overhead can over the course of a day actually use more power than the fridge itself in that time!
Running non-stop, will the inverter have fans running constantly or will they run only when the unit has hit a set temp (bearing in mind the fridge is using a small fraction of the inverters capability 99.99% of the time)
Check the no-load Power Consumption of the Inverter (quoted in Watts) and multiply that by 24 (hours) to get the daily Wh use, then divide by 12 to get the Daily Ah consumption [quick check - take the quoted Power consumption in Watts and multiply by two to get the daily Ah]
Check if the Fans are constant or temperature controlled


5. Do you need the inverter for other uses?
If you have other uses for an inverter, this will of course be available for those as well. Remember that the Fridge will typically only be on and running for around 20% of the time. If you do have other Mains devices, then you will need to remember that when the fridge kicks on, it is an approx 1000W load, so with the Fridge running AND your other devices running, it may overload and cause it to reset. To avoid having to go bigger Inverter-wise for extra occasional AC uses, you can simply turn the fridge off for the duration if it is only for a couple of hours.
Size the Inverter for all potential mains AC uses for it (although not neccessarily simultaneously)


When I was looking at this originally for my own setup, I ended up looking at various options.
I wanted an inverter that was silent when running as it would be on 24/7 and once I could trust to be running all the time unattended. For this reason I chose to go with Victron for their reputation and 5 year warranty.

I wanted as small an inverter as possible capacity wise, but of course it still had to be able to cope with the in-rush current surge.
I tried the 12/250 and 12/375 Victron Pheonix Inverters - neither of those had an surge ability sufficient to start the Fridge.
I then tried the 12/500 Victron Inverter. This is actually a 400W Inverter, but will deal with a overcurrent surge up to 900W for a long enough duration to allow the Compressor to start. In fact, this is part of the design criteria:
"Full bursts - High Peak output
Some electronic devices - particularly those which have an electric motor - require short burst of energy on start-up which is much higher than their rated power consumption. No problem - the Phoenix inverter has industry-leading peak-power capability."
This 12/500 Inverter has performed as needed and expected since Christmas 2017

Recently I tried the Multiplus 12/800 (700W Inverter) and being just 700W, that also goes into an overload/surge situation, but as per the snippet above from Victron, this is not a problem - you get informed of the overload but the inverter carries on. The graph below shows what is happening each time the Fridge starts

Fridge Overload
by David, on Flickr
(note: the differences in the spike sizes are not because they really vary, just that they are not always captured at the peak point by the monitor app)

On point #4 - Inverter Overhead. Just to push the Victron again, its no-load power consumption is very low at around 8W (so about 16Ah/Day) but if you use the ECO mode where it will turn itself on every few seconds to look for a load and then off if it doesn't find one, then the average no-load consumption is round 3W (6Ah/Day).
This I thought didn't work that well with the Fridge but does in fact on further testing.

For point #5 - Other uses. I do have other low-power uses for this inverter, such as Laptop Charging, Internet Router, Electric Blanket, etc. Some, such as Internet Router, would not work well with an inverter that goes on and off to save power and may be something to bear in mind? For other high-power uses I elected to have a second Inverter so I could stick with a small cost-effective unit dedicated to the Fridge.


Because of the Power Surge ability of the Victron, the smaller 12/500 Victron Phoenix Inverter is actually close to being one of the cheapest Pure Sine Wave Inverters available when looking at options for use with my Fridge, so given that and the fact it is a top brand with a 5 year warranty, I don't think I would bother to look elsewhere.
Depending on the specific Fridge you may have or get, the in-rush could potentially exceed the 12/500 abilities and so the 12/800 might be required? (it would still hit an 'overload' but would carry on working, as the graph above shows)
 

Chris Evans001

Full Member
Very interesting... I have a durite modified wave 500w inverter that I plan to use for an AC fridge. It states a 3x peak but I can't for the life of me find any detail of how long it can sustain that for. Since it's a reasonable quality inverter, I can only hope it will sustain it for long enough.
 

wildebus

Full Member
Very interesting... I have a durite modified wave 500w inverter that I plan to use for an AC fridge. It states a 3x peak but I can't for the life of me find any detail of how long it can sustain that for. Since it's a reasonable quality inverter, I can only hope it will sustain it for long enough.
Be interesting if it works. You could easily test it by just connecting a load >500W on it and see how long it runs before reset (if you can count to 3 before it resets, should be ok).
 

Wissel

Free Member
Great info @wildebus

I've run the same setup as you for months now (same fridge and inverter) and it's performed brilliantly. The fridge works great (better than the £800+ motorhome fridges I've previously owned) and after the initial cost saving is forgotten, it's the silence I really appreciate,

Due to a string of bad luck I'm now planning a brand new van. The fridge is staying, but I've decided to go for a much more powerful inverter, the Victron 2k smart. This means I can run more via 240v than before as I'm already paying the zero load premium for the fridge. I'm aware I'll only get about 93% efficiency, but have the leeway on my battery capacity (400Ah LiFePO4) to not worry about just a few Ah per day. Also the 2k inverter can be set to an Exo mode that detects 1w (from memory). If this allows the fridge to work in Eco I'll save power.

The convenience of having 240v on tap has already made it easier to run a PC etc in my van, which I would never have considered had I not followed your fridge installation.
 

wildebus

Full Member
Pleased to be of help :) I will send you a link to a suggested new inverter which I think you will be happy with.
The ECO mode of the Victrons will work with the Fridge, but you will need to do some experimentation to get the right setting. With my setup I set it to go into the ECO (or AES Model as my unit has it) at 51W and come out of ECO at 64W.
It sounds a very small window to go on and off but seems to work for me :)
 

vanours

Free Member
I read with interest your post: 'AC Fridge In Campervan -.Power Consumption Test' and this one.
I have been running the same fridge as yours all be it the 43L version via a Victron MultiPlus 12/500/20 in my self-build for three years now.
I too get a loud buzz and overload warning when the compressor fires up due to the initial power demand surge.
I would like to no longer have this buzz noise so want to upgrade the inverter.
From what I've read in your posts the 12/800 would also make a loud buzz and go briefly into overload warning so would be pointless for me to upgrade to that model.
Would the 12/1200 no longer make the loud overload noise and cope silently with the fridge compressor start ups or would I have to go larger to say the 12/1600?
I look forward to reading your reply when you get the time.
 

wildebus

Full Member
Hi,
The 800VA inverter is still marginally too small to cope without the overload, so higher would be needed.
It can be hard to capture the full extent of the inrush, but I am pretty sure the 1200VA unit would be enough.

Something I looked at doing, but didn't persue as I ended up getting a Victron 12/3000 Multiplus, was getting a softstart kit. These essentially use a pre-charge capacitor to provide the in-rush power and could be a good alternative if you can work out how to install (domestic appliances are not my forté so I while I got the kit, I didn't follow though the fitting for the reason above).

I have no doubt you will be able to sell your current multiplus without any problem though if you do upgrade - they are super-robust units :)
 
Last edited:
Top