Minimum inverter for small refrigerator

Steve961 · September 2009

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 · September 2009

Re: Minimum inverter for small refrigerator

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 · September 2009

Re: Minimum inverter for small refrigerator

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 · September 2009

Re: Minimum inverter for small refrigerator

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 · September 2009

Re: Minimum inverter for small refrigerator

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 · September 2009

Re: Minimum inverter for small refrigerator

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 · September 2009

Re: Minimum inverter for small refrigerator

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 · September 2009

Re: Minimum inverter for small refrigerator

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/Products/NorcoldRefrigeratorsHome/DE0061EV0061/tabid/197/Default.aspx

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

HTH,
Jim / crewzer

mike95490 · September 2009

Re: Minimum inverter for small refrigerator

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 · September 2009

Re: Minimum inverter for small refrigerator

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

solorone · September 2009

Re: Minimum inverter for small refrigerator

Just doing a quick read here, and it looks like the one thing that is being left out of the equation, is reliability. Unless you have a dual inverter system, which I do not, then you are putting your self at risk should there be an inverter failure, not to mention dealing with a protracted time without sunlight. Right now it cost only .66 a day to operate a 18 CF LP RF, granted the LP units original cost is 2 to 3 X a AC unit. It looks to me if you factor it all in, then the reliability would give LP the lead.

The unknown factor here is LP cost, for me it is $2.20 per gallon, but I also buy in bulk.

Cariboocoot · September 2009

Re: Minimum inverter for small refrigerator

solorone wrote: »

Just doing a quick read here, and it looks like the one thing that is being left out of the equation, is reliability. Unless you have a dual inverter system, which I do not, then you are putting your self at risk should there be an inverter failure, not to mention dealing with a protracted time without sunlight. Right now it cost only .66 a day to operate a 18 CF LP RF, granted the LP units original cost is 2 to 3 X a AC unit. It looks to me if you factor it all in, then the reliability would give LP the lead.

The unknown factor here is LP cost, for me it is $2.20 per gallon, but I also buy in bulk.

Things can go wrong with an LP 'frige too. Like when the tank runs out in the middle of the night. The price of that gas goes up and down too. Mostly up. I lucked out in buying the last 40 lbs. for $26.04 (propane is still better for heat - as in stove and hot water).

I've had both: ran propane for years. It was expensive and cumbersome to put up with. The capacity was smaller too. It did go out from time to time. Once the burner baffle failed and it stopped cooling even though the flame was lit.

I've switched to electric, and don't regret it at all. The 'frige is bigger, cheaper to replace than a propane unit. The electric is far more dependable, and can be backed up with a generator if necessary. We're here most of the year now, and the electric is definitely better than propane under those circumstances.

I've never had an inverter fail yet, which means I've got two good back-up units from previous systems (as the load requirements have increased so has the inverter capacity). Plus four generators. You tend to acquire stuff over the years. Never yet had to use any back-up system - in twelve years.

nigtomdaw · September 2009

Re: Minimum inverter for small refrigerator

Also one other important plus for electic ff is truthfully Ive yet to have an ice cold beer from any gas fridge in Spain, My fridge is set at 1 degree C , 3 years without trouble !

icarus · September 2009

Re: Minimum inverter for small refrigerator

solorone wrote: »

Just doing a quick read here, and it looks like the one thing that is being left out of the equation, is reliability. Unless you have a dual inverter system, which I do not, then you are putting your self at risk should there be an inverter failure, not to mention dealing with a protracted time without sunlight. Right now it cost only .66 a day to operate a 18 CF LP RF, granted the LP units original cost is 2 to 3 X a AC unit. It looks to me if you factor it all in, then the reliability would give LP the lead.

The unknown factor here is LP cost, for me it is $2.20 per gallon, but I also buy in bulk.

Said perfectly. In fact you can reduce the LP consumption a bit by adding evaporator/condenser fans to an lp fridge. I run a tiny 24vdc fan inside and a larger 24vdc on the outside (I run them off 12vdc so they run almost silently) This has reduced my burn time by ~30% Building the fridge in, with an additional 2" of foam insulation on the outside of the cabinet, sides top and door drops that run time another 50%. Adding a cold air intake in the summer behind the fridge and a hot air exhaust helps as well. For the fridge, water heater, and cookstove we burn ~60# of propane every 5 weeks (12 gallons?) The other thing I toy with is running mine on 120 vac when the batteries are fully charged. I would need a few more panels, and for the 1-2 hours of extra sun would only save me maybe 20 minutes of propane at 1500 btu/hour. Pretty long payback for an extra 100watts of PV.

As for reliability. My neighbour has a non electronic Dometic that has been burning CONTINUOUSLY for ~22 years with our a failure. (An automatic change over regulator with dual tanks keep the fridge going when one tank empties and is cheap to buy) I have a Dometic americana series one that has run for 10 years only blowing the 12vdc fuse once for some reason.

As I have suggested before, they can be expensive to buy, but if you shop around, there are used ones all over, and they can be had quite reasonably if you buy carefully.

I agree with Mike however, that if you are going to live full time, bite the bullet, add the PV and the inverter, and buy the energystar compressor!

Tony

solorone · September 2009

Re: Minimum inverter for small refrigerator

nigtomdaw wrote: »

Also one other important plus for electic ff is truthfully Ive yet to have an ice cold beer from any gas fridge in Spain, My fridge is set at 1 degree C , 3 years without trouble !

I know what you mean, but that was in the old days, modern gas 18 CF units will freeze everything in it if you are not careful. I am very happy with new units, and amazed at how well they work.

Cariboocoot wrote: »

Things can go wrong with an LP 'frige too. Like when the tank runs out in the middle of the night. The price of that gas goes up and down too. Mostly up. I lucked out in buying the last 40 lbs. for $26.04 (propane is still better for heat - as in stove and hot water).

I've had both: ran propane for years. It was expensive and cumbersome to put up with. The capacity was smaller too. It did go out from time to time. Once the burner baffle failed and it stopped cooling even though the flame was lit.

I've switched to electric, and don't regret it at all. The 'frige is bigger, cheaper to replace than a propane unit. The electric is far more dependable, and can be backed up with a generator if necessary. We're here most of the year now, and the electric is definitely better than propane under those circumstances.

I've never had an inverter fail yet, which means I've got two good back-up units from previous systems (as the load requirements have increased so has the inverter capacity). Plus four generators. You tend to acquire stuff over the years. Never yet had to use any back-up system - in twelve years.

Yes I have had a gas unit fail, had to have the back replaced, and cooling in the summer in Georgia was a tad rough, but it worked fairly well.

I have had an Inverter fail, burned up after being damaged by letting the genset run out of gas, a serious no no for a Trace, likewise I had a small backup unit. My fear is being gone and lightening taking the system down, we deal with a lot sky fire here.

I guess I need to have a look at the current AC units on the market, need to stay current, still get a lot of calls about being off the grid. There are a lot of dreamers out there.

icarus wrote: »

Said perfectly. In fact you can reduce the LP consumption a bit by adding evaporator/condenser fans to an lp fridge. I run a tiny 24vdc fan inside and a larger 24vdc on the outside (I run them off 12vdc so they run almost silently) This has reduced my burn time by ~30% Building the fridge in, with an additional 2" of foam insulation on the outside of the cabinet, sides top and door drops that run time another 50%. Adding a cold air intake in the summer behind the fridge and a hot air exhaust helps as well. For the fridge, water heater, and cookstove we burn ~60# of propane every 5 weeks (12 gallons?) The other thing I toy with is running mine on 120 vac when the batteries are fully charged. I would need a few more panels, and for the 1-2 hours of extra sun would only save me maybe 20 minutes of propane at 1500 btu/hour. Pretty long payback for an extra 100watts of PV.

As for reliability. My neighbour has a non electronic Dometic that has been burning CONTINUOUSLY for ~22 years with our a failure. (An automatic change over regulator with dual tanks keep the fridge going when one tank empties and is cheap to buy) I have a Dometic americana series one that has run for 10 years only blowing the 12vdc fuse once for some reason.

As I have suggested before, they can be expensive to buy, but if you shop around, there are used ones all over, and they can be had quite reasonably if you buy carefully.

I agree with Mike however, that if you are going to live full time, bite the bullet, add the PV and the inverter, and buy the energystar compressor!

Tony

I used a fan on the back last year, and I vent the heat, but do not have a fresh air inlet. I have not thought about a fan inside, PM me with some details if you will.

Likewise I had a Dometic, had to replace the back once, just junked it this year, it was 25 years old and had been failing for a few years.

I guess I just assumed everyone had bulk LP tanks, unless they lived on an island:D

icarus · September 2009

Re: Minimum inverter for small refrigerator

Both the evaporator and condenser fans are 24vdc running off 12vdc. They pick up their power from the fridges 12vdc connection at the control board (or anywhere else you have 12vdc handy. I have a snap disc thermostat on the hottest part of the condenser, so when the burner is lit and the ammonia solution starts to circulate, once the condenser temp gets to be ~100f it clicks on, turning both fans on. It stays on until the temp drops to ~90f. (adjustable for hotter climates) (About $10 from Grainger). Of course once the burner goes off, the condenser gets cool, making the t-stat open, turning off the fans. The two fans together draw ~.5 amps@ 12vdc. Running 24 vdc fans them run almost silently.

The ouside (condenser fan DRAWS air up over the coils, reenforcing the natural draft. There is a fresh air vent in the back wall drawing in cool outside air. The inside (evaporator) fan hangs just below the evaporator cooling fins and draws air down, forcing it towards the bottom of the fridge. It serves to keep the air circulating in the fridge evening out temps, and helping especially when the fridge is full.

On a side note. The biggest reason for absorption fridge (LP) failure is running out of level, ( a particular issue with RVs(. The second is rusting out of the coolant loop where it goes into the box. This is more of a problem with fridges that are turned on and off frequently allowing condensation to form on the tubing. Fridges that run full time don't suffer from this problem as much.

It is important to understand that absorption fridges circulate as a result of the thermosiphon action of the coolant. It needs the PROPER heat to boil the solution and get it to rise. It then needs to cool the condenser quickly to allow the solution to settle again, causing circulation. Too little heat in the burner, or not enough cooling of the condenser will cause the system to work poorly if at all. A system that circulates too slowly ( mostly due to poor condenser cooling) will develop a weak spot in the tubing and allow the burner to prematurely burn a hole in the tube, or more likely will allow the solution to solidify slowly resulting in poorer and poorer interior cooling. Sometimes these solids can be "broken up" by shaking the fridge, but usually it is too late.

Good luck,

Tony

Photowhit · September 2009

Re: Minimum inverter for small refrigerator

I like the suggestion of checking them out.

I've had 2 small fridges both draw @ 100 watts running. Many larger electrical motors have a soft start, basically loading capacitors for a short time before starting the motor.

Not sure if this would apply to smaller motors like this.

System2 · July 2010

Re: Minimum inverter for small refrigerator

I have a small 400watt inverter running off my van's battery. I found a small fridge for $50 at home depot that draws 0.7 amps continuous. Startup is right around the max the inverter can handle.

I installed a 2 watt solar battery maintainer in the window to keep the battery topped off when the fridge isn't running. Works like a charm. 8)

On particularly hot days the battery dips below desirable level. Most likely due to the fridge running more often.

icarus · July 2010

Re: Minimum inverter for small refrigerator

.7 amps on 120 vac is 7 amps on 12 vdc, or ~84 watts. If it is a compressor fridge as opposed to a thermal electric cooler, the start up current might be 5-10 times that for an instant.

On another note, a 2 watt panel will probably not even keep a car battery in float. It might put out ~.1 amp,, maybe on a perfect day!

Tony

System2 · July 2010

Re: Minimum inverter for small refrigerator

The thing about running a small fridge is that the t-stat will shut it off for periods of time. The drain on the battery is not constant. The idea is that the batteries will refresh during these times. My inverter also goes to rest when battery voltage drops too low. This allows the solar to refresh the battery also.

This arrangement works well for me. Soon I will be upgrading to larger panels and mounting them to the roof of my vehicle. Thinking I will put two 50 watt panels up when the funds are available.

icarus · July 2010

Re: Minimum inverter for small refrigerator

Please note: A 50 watt solar panel will be lucky to put out ~ 200 watt/hours of power on a perfect day, enough to run your 84 watt fridge for ~2.5 hours all things being equal. Depending on temperature, I would guess the duty cycle of the fridge is likely to be ~50%, or 12 hours, or ~1kwh/day. (Small fridges are notoriously inefficient. So your 50 watt panel is likely to only give you ~ 10-20% of your daily uses under ideal circumstances. If you have it on an RV, you are very unlikely to have ideal circumstances.

I strongly suggest that you read and understand the following links:

http://www.batteryfaq.org/
http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

If your inverter is going to low voltage disconnect you are not likely to be getting fully charged at all, and you are risking prematurely killing your battery.

Minimum inverter for small refrigerator

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