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Old 01-21-2025, 01:38 PM   #1
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Inverter Question

I apologize in advance for starting another thread when there is one just below asking a similar question to mine,

A little background,earlier this year the wife and i sold our coach and downsized to an 18 ft travel trailer.In our enthusiasm with the new trailer I missed something.
The refrigerator is 120 volt only,no gas or DC.Im working on an inveter setup to power the refrigerator while traveling.Our travels will be from shore power to shore power approx 7-9 hours.
here is my setup
The refrigerator (4 cubic ft) pulls Approx 390 watts on start up
running useage is around 90 watts
i have a 1000 watt pure sinewave inverter,My plan is to wire the refrigerator to a solo circuit and connected to a transfer switch,The power from the breaker box will go to the inlet side of the transfer switch,while the other inlet will be connected to the inverter.
I have two lead acid batteries installed with approx 100 Ah each.(parelled 200Ah)
does this setup sound feasible.
I cannot find a formula to give me my DC amperage draw to the Inverter.

Should I consider a bigger inverter or would the larger size be more inefficient for my needs


Also in the other post it was mentioned using the seven pin plug to provided battery power while driving.
I own a starter /alternator rebuild shop and can list several reasons why this is not a great idea so im wanting to set my system up so it doesnt pull from the tow vehicle
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Old 01-21-2025, 01:45 PM   #2
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If you go pole to pole, you really don't need to keep power on the refer. We have gone almost 24 hours without power and all temps were still in the safe range. Just don't keep opening the refer to "surf" the contents
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Old 01-21-2025, 02:00 PM   #3
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Quote:
Originally Posted by dons2346 View Post
If you go pole to pole, you really don't need to keep power on the refer. We have gone almost 24 hours without power and all temps were still in the safe range. Just don't keep opening the refer to "surf" the contents
I agree with you,,BUT my wife ,,,,She once suffered from food poisoning from a faulty refrigerator and is in upper management in the food service industry so she is OCD about checking food temps and all the hazards.its easier for me to do this than to worry her.
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Old 01-21-2025, 03:52 PM   #4
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The conversion is 10 times the 120 volt draw.

120 volts ÷ 90 watts = 1.33 amps.
12 volts ÷ 90 watts = 13.3 amps.

If the fridge runs 50% of the time, it uses 6.6 AH + conversion losses. 7 AH .

PS: I'm with you on steady cooling in my fridge.
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Old 01-21-2025, 04:17 PM   #5
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I concur with twinboat's math.

I agree with you, there should never be a need to charge from the alternator if the overall RV power system was designed properly.

But I would suggest just using a simple 30A DPDT switch instead of a more complicated and expensive transfer switch. Have both the Shore and Inverter power hot legs be the switch Inputs and have your refrigerator circuit be the switch Output such that the switch can be toggled from Shore Power, Off and Inverter Power as desired. Here's link to switch:

https://leviton.com/products/1288

We have a 17 feet fiberglass Castia trailer that has 50A 120/240VAC 12,000 Watt shore power capability and we use a 3,000 Watt Inverter powered by a 100AH LiFiPO4 battery with a 270A maximum discharge current capability that creates a fully independent 120VAC 20A 2,400 Watt circuit for our microwave, refrigerator and several outlets that can be used with or without shore power. We use this switch approach for this fully independent circuit which enables us to have 14,400 Watts when we have shore power which we can then use for our propane/electric 6,500 Watt Tankless Water Heater, our electric 3,500 Watt Induction Cooktop and our other electric appliances and thereby not use any propane at all. Details can be found in our Casita trailer modification writeup if interested:

https://www.borstengineeringconstruc..._2023_Trip.pdf

Gayle & Bob
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Old 01-21-2025, 04:27 PM   #6
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Quote:
Originally Posted by 2Bargos View Post
I own a starter /alternator rebuild shop and can list several reasons why this is not a great idea so im wanting to set my system up so it doesnt pull from the tow vehicle
So, you don't want me to tell you to put the inverter in the truck and run 120v to the trailer?


Everything you have will work great. The pure sinewave inverter should be more than 90% efficient at the 400w start up, so that current is fine. The efficiency will be less at 90w but not enough to worry about in this situation.
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Old 01-21-2025, 05:17 PM   #7
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Quote:
Originally Posted by twinboat View Post
The conversion is 10 times the 120 volt draw.

120 volts ÷ 90 watts = 1.33 amps.
12 volts ÷ 90 watts = 13.3 amps.

If the fridge runs 50% of the time, it uses 6.6 AH + conversion losses. 7 AH .

PS: I'm with you on steady cooling in my fridge.
Thanks thats the info I was needed


SafariBen,,, Your right and the way you described would work best.
Let me explain,,If used correctly an inverter tied to the tow vehicle is a good thing ,,,but you can easily over work you alternator and not even realize it
Alternators are NOT battery chargers they are designed to maintain a system and anything added to that adds additional strain.
Pull in and stop some where ,you refrigerator or what ever else is plugged in is pulling on battery while vehicle is parked

You start tow vehicle up and drive down road then alternator is trying to maintain it's load plus charge a now weakend battery
People will comment "My alternator is a kbillion amps"it may be but it's duty cycle isn't. I have see more than one alternator on tow vehicles that were literally melted inside from trying to charge a dead Trailer battery

On today's vehicles with all the accessories most alternators are already running on the high side of there duty cycle
adding anything just adds more load to an already strained system

Let me add this example,take any vehicle you own run the engine about half throttle producing about half it horsepower the engine will last for years
Now take that same engine ,push it to its max every running second and see how long it will last

That's just my take ,I may be wrong but just passing on what I've experienced in my business
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Old 01-21-2025, 05:25 PM   #8
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That is a good idea sunny ,,hadn't thought about going that route.I already have my transfer switch so I probably go with it
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Old 01-21-2025, 08:19 PM   #9
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Quote:
Originally Posted by 2Bargos View Post
I agree with you,,BUT my wife ,,,,She once suffered from food poisoning from a faulty refrigerator and is in upper management in the food service industry so she is OCD about checking food temps and all the hazards.its easier for me to do this than to worry her.
Just to let you know, my wife was in food service management for 30 years, both institutional and college residential service. I think she knows what she is talking about.
If she feels ore comfortable having power to a refer 24/7 so be it.
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Old 01-22-2025, 07:08 AM   #10
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Quote:
Originally Posted by 2Bargos View Post
The refrigerator (4 cubic ft) pulls Approx 390 watts on start up
running useage is around 90 watts
i have a 1000 watt pure sinewave inverter,My plan is to wire the refrigerator to a solo circuit and connected to a transfer switch,The power from the breaker box will go to the inlet side of the transfer switch,while the other inlet will be connected to the inverter.
I have two lead acid batteries installed with approx 100 Ah each.(parelled 200Ah)
does this setup sound feasible.
I cannot find a formula to give me my DC amperage draw to the Inverter.

Should I consider a bigger inverter or would the larger size be more inefficient for my needs


Also in the other post it was mentioned using the seven pin plug to provided battery power while driving.
I own a starter /alternator rebuild shop and can list several reasons why this is not a great idea so im wanting to set my system up so it doesnt pull from the tow vehicle
When we had the inverter for the residential fridge it was setup like this:
Transfer Switch:
https://www.solarflexion.com/product...nt=Catch%20All

Inverter but a 1000-watt version:
https://www.solarflexion.com/product...nt=Catch%20All

Our setup was the one inlet on the transfer switch was for 120VAC from the breaker panel. The outlet went to the outlet for the fridge. The 12VDC came from the outlet on the bottom of the inverter thru the plug on the transfer switch.
When traveling, turn the inverter on, and you have 120VAC to the fridge. I always left the inverter on; in case you lost power while away from the rig. While I was doing it this way we had a seasonal site in Ohio and was away from the rig during the week.
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Old 01-22-2025, 09:05 AM   #11
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That's a good point and valid reason for using an automatic transfer switch if you only have a 120VAC refrigerator. Our RV was purchased and modified primarily for dry camping. Our refrigerator automatically uses propane if it doesn't have electric from our Inverter or if shore power is lost. We also have an electronic monitoring system that in conjunction with Starlink Internet provides the status of our RV and alerts us if power is lost, etc.

We also use propane for our air conditioning, kitchen & outdoor cooktops, furnace and tankless water heater when dry camping. One of the first steps in designing and optimizing a RV for dry camping is eliminating appliances that have long duration and high electric loads. Another critical step is minimizing water usage and having a way to easily replenish it and dispose of the black and gray water. Then you can dry camp indefinitely as long as you can source food, water and propane.

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