Minimum inverter for small refrigerator

[Steve961]

I have started working on a solar power system for a remote off-grid cabin. While I just joined this forum, I do have to say I have learned a lot from reading the posts over the last few weeks - what a great resource this is.

Something that I have not been able to find though is guidance on the sizing of an inverter for a small refrigerator. My overall power needs will be fairly small - mostly a few CFL lights and a laptop computer now and then. The one "large" item I would like to have is a small 5 cu ft refrigerator. I would probably do one of those chest freezer to refrigerator conversions that I have read about.

I would also really like to go with a sine wave inverter, but cannot justify the cost of the larger ones. I do see that Morningstar has a 300 watt (600 peak) sine wave inverter that is available for a reasonable price. Could this inverter run a small/efficient chest refrigerator along with a few CFL lights?

Thanks for your help.

Steve

[Cariboocoot]

First off, you are right in considering a sine wave inverter if you want to run a 'frige. AC induction motors don't run well off modified square wave. Some will, some won't ... but it shortens their life in either case.

Will a 300 Watt inverter run a 'small' 'frige? That all depends on what that 'frige actually draws. The running current is one thing; the start-up surge something else. I have a full-size refrigerator that draws around 240 Watts peak when running, but the start-up is a sudden burst of about 3 times that - and it varies with the 'state' the 'frige is in. Mine also has auto defrost which draws 500 Watts - and then requires the compressor to work at maximum in order to re-cool the freezer.

Small 'friges aren't necessarily more efficient than big ones. And surge ratings on equipment of any sort are sometimes optimistic. Ideally you run a device through a Kill-A-Watt meter first and see what its power requirements really are. Failing that, getting the specs (Amps @ Volts) for the unit is a good starting place. Those tags are often a work of science fiction, however: sometimes the draw is higher, sometimes lower.

Consider some of these Samlex inverters too:
http://store.solar-electric.com/sasiwain1.html

[john p]

In simple terms no,,I do see that Morningstar has a 300 watt (600 peak) sine wave inverter that is available for a reasonable price. Could this inverter run a small/efficient chest refrigerator along with a few CFL lights
on a standard refrigerator you are going to need a sinewave inverter of about 1000 watts continuous.. As a refrigerator draws about 8 times is running consumption on startup.. so if it draws about 200w on 120v??? it will draw about 1600w on startup....
Just about everyone is going to tell you the high start up draw is because of the type of motor but that is rubbish, And told many times by people that should know better..

[icarus]

Most small fridges are terribly inefficient. On a per cuft basis they are terrible relative to a bigger energy star fridge. I suggest you get a Kill-a-watt and test any fridge you are interested in INCLUDING starting amps.

A couple of suggestions include finding the best fridge that suits your needs in conventional fridge, or alternatively find a good Propane fridge. It is my opinion that while propane fridges are ultimately less cost effective when used full time, the equation fall on the side of the Propane when used occasionally. (week ends, holidays, seasonally)

Full time I wish I had gone with a conventional, but by adding insulation to the case, and fans to the evaporator and condenser they burn only a tiny bit of Propane per month. Just for the record, we run as many as a dozen Propane fridges in remote bush cabins, and when installed and used properly they work very well.

Good Propane fridges can be had quite reasonably from wrecked or used RV's. There are some things to look for in a used fridge, but if you buy carefully you can get a bargain. I am currently using one I bought used for $200 and it has run perfectly for 3 years. I have some that have come free, and a couple I found at the dump!

Good luck.

T

[mike95490]

Maybe. Maybe the suresine can start your specific fridge. It's rated at 300W cont, 600 for 10 min, which implies the surge could be factored off the 600W figure, since heat sink capacity seems to be the wattage driver.
Find a vendor with a 60-90 day return policy. The suresine is a grand power miser.

[john p]

kil-a-watt meters and similar are not that good at accurate detecting of startup current, as they only take a reading every second and many times miss the peak start up current.
A300w inverter is extremely unlikely to be adequate to start any refrigerator

[Steve961]

Thanks for all of your help. It looks like I will be reevaluating my refrigerator and/or inverter choices. I will probably start scouring craigslist to see if I can find a used Propane refrigerator. Without an electric fridge, my solar panel and battery needs would definitely be reduced.

[crewzer]

An AC/DC fridge might be worth considering. Running from DC improves the unit's efficiency and eliminates the need for a DC-AC inverter. The fridge probably won't be inexpensive, but you'll be able to get by on a smaller/cheaper PV system and avoid the inverter cost.

See: http://www.thetford.com/HOME/Product...7/Default.aspx

12V DC CFL's are also available, as are DC-DC converters for laptops.

HTH,
Jim / crewzer

[mike95490]

For weekend, or occasional use, propane is affordable. For long term use, a energy star fridge, and a solar array for it, seems to be the better choice, unless you like endless propane bills.

[nigtomdaw]

I did a cost comparision for people living full time in Sunny Spain and taking into account the full time off gridder already has a suitably sized system with an inverter charger set up then the payback time for a new A+ Fridge Freezer and 350 watts of PV extra to run it, compared to a New LPG Fridge Freezer and LPG was just less than 3 years in favor of the solar outfit !

Not a bad payback time IMHO