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Old 08-06-2018, 08:36 AM   #1
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Need quick “electrical math” for an inverter purchase

I’ve discussed this many times before and had my butt handed to me when it came to inverter plans but I need some down and dirty answers.

I need to run two Marvel 6.1 cubic feet appliances - one fridge and one freezer.

They show to draw 2.8 amps AC so I rounded it to three amps.

I want to use a cheaper 3000 watt modified sine wave inverter to power these when I’m away from the boat.

From a battery perspective I have 4 AGM batteries that are 66aH a piece. I plan on supplementing this with a 100 watt solar panel for charging. The boat is in the Bahamas.

Will this work for my needs?
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Old 08-06-2018, 08:47 AM   #2
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3 amps at 110 AC would equal only 330 Watts or max 400 Watts with an inefficient inverter. A 3000 watt inverter would be serious overkill, and would also be more power than your batteries can comfortably source - even if you include your solar panel.

If the fridge is the only thing you're running on AC plus a couple of other laptop chargers and stuff, I would go with a 1000-watt inverter. I use a pure-sine inverter, so I can't comment on the modified sine ones unfortunately.
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Old 08-06-2018, 08:57 AM   #3
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Yep. This is strictly for the fridge and the freezer running so by my math I would be looking at about 700 Watts.

These two items would be the only thing left on while I am not on the boat. When I am on the boat I would be plugged into shore power.

I’m just wondering if those batteries with the small amp hour rating would work.
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Old 08-06-2018, 09:07 AM   #4
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I see. So you have 264Ah total, We'll assume you want to discharge them to 50%, that's 132 Ah. At 700 Watts your fridge will be pulling (700W/12V = ) 60 Amps from your batteries. That's only two hours of run time.

700 watts for a fridge/freezer combo is incredibly high. How did you come to that figure? Is it pulling that much energy all the time? A medium-size window air conditioning unit would pull about that much energy - a fridge should be far less.
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Old 08-06-2018, 09:21 AM   #5
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My fridge/freezer pull about 100ah /day natively on dc. Not the best fridge (tundra), about 8.5 cuf. Good efficient freezer around 6cf.

These are my biggest loads, and I have a 1200ah bank of agm.
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Old 08-06-2018, 09:44 AM   #6
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3A @ 120V apiece so about 34A @ 12V assuming 90% efficiency. 68A for both. Assume they run 50% duty cycle (might be more in the Bahamas), that's 816 AH in 24 hours out of your 264 AH battery bank. You'd need to fire up your charger every 4 hours, and since it would take a minimum of 4 hours to charge the batteries, you might just as well let the genset run 24/7. The inverter isn't the problem, the energy source is.
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Old 08-06-2018, 09:47 AM   #7
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Refer you to Calder's . "Boat Owners Mechanical and electrical manual" since you needed to ask in the first place. Need to calculate all loads
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Old 08-06-2018, 09:55 AM   #8
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power usage.

It's important to consider the "start up" or surge current of motor driven loads such as fridge & freezer devices.
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Old 08-06-2018, 10:00 AM   #9
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Originally Posted by caltexflanc View Post
Refer you to Calder's . "Boat Owners Mechanical and electrical manual" since you needed to ask in the first place. Need to calculate all loads


I’ve calculated the loads from this sales ad -https://m.grainger.com/mobile/product/MARVEL-Refrigerator-1LBF5 and thrnnised this site to conver AC to DC - https://www.batterystuff.com/kb/tool...-inverter.html

This comes out to 64amps. I’m assuming thst is per hour while running?
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Old 08-06-2018, 10:15 AM   #10
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K9 your situation confuses me a bit but...

You have two appliances and each draws 2.8 amps when running? So when running that is 5.6 amps or 5.6 x 110 = 620W AC. If you figure that the inverter will be about 80% efficient, that means you need about 620W / .8 = 770W DC to run them.

770W DC is about 64 amps. As was pointed out, if your battery bank is 264Ah (that is if the batteries give you 100% of their rated Ahs) you will only get 2 hours of run time before your batteries hit 50%.

Appliances don’t run 100% of the time however. The 2.8 amps is only when the appliance is running. Since you aren’t on the boat presumably you wouldn’t be opening the fridge or freezer so the run-time will be less. We can guess as to what the run-time would be, but if the guess is a little off you could have flat batteries and spoiled food.

Say the run-time is 25%. Instead of 64amps/hr you would draw 16 amps/hr. This means you could run the appliances for 8 hours off the inverter before hitting that 50% point on the batteries (8 x 16 = 128)

As for the solar. A 100W panel will only give you 8 amps/hour to the batteries on a perfect day with full sun. That might be enough to run just one of your appliances during that perfect day and leaves you nothing for actually recharging the batteries.

So based on faulty math and very questionable assumptions, I’d guess you could run the appliances for 12 hours during the OR 8 hours during the night off that battery bank. I wouldn’t trust my math so you shouldn’t either but I think it is clear that a battery bank of 264 AH is not anywhere near large enough to keep your food safe if you are away from the boat for more than a few hours.
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Old 08-06-2018, 10:28 AM   #11
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Thanks Dave. That’s the math I was following too.

We will be on the boat once a month so the rest of the time I need reliable inverter power.

Perhaps time to add some
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Old 08-06-2018, 10:34 AM   #12
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Not answering your question but I wold pay the few extra dollars and get a full sine wave inverter. From a former modified sine wave user.
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Old 08-06-2018, 10:36 AM   #13
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Almost all that has been said about above your situation is true. You can get a better handle on the power requirements of your fridge/freezer by plugging them into a Kill A Watt. That fairly cheap device is the AC equivalent of a battery monitor. It gives instantaneous watts and cummulative watt hours of power used.


Hook up one or both of your fridge/freezer units for a few days and see what the average watt hour consumption is for a day. In my experience the nameplate current is always high and your running cycle time may only be 1/4 or so. It all depends on ambient temps, how much you open it each day and if you put warm food inside.


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Old 08-06-2018, 11:04 AM   #14
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Are you sure that's the model fridge you have/are getting?
That's an explosion-proof fridge for hazardous locations:
https://www.agamarvel.com/scientific...tion/haz-6ear/

Also, the amperage quoted from your Grainger link will not be the amps it pulls most of the time, but the ultimate maximum it would ever pull in the worst case scenario, probably with some safety factor. In other words, this number is not what you're looking for when sizing your battery bank.
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Old 08-06-2018, 11:07 AM   #15
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Quote:
Originally Posted by djmarchand View Post
Almost all that has been said about above your situation is true. You can get a better handle on the power requirements of your fridge/freezer by plugging them into a Kill A Watt. That fairly cheap device is the AC equivalent of a battery monitor. It gives instantaneous watts and cummulative watt hours of power used.


Hook up one or both of your fridge/freezer units for a few days and see what the average watt hour consumption is for a day. In my experience the nameplate current is always high and your running cycle time may only be 1/4 or so. It all depends on ambient temps, how much you open it each day and if you put warm food inside.


David




The kill-o-watt device is invaluable for things like fridges that have surge start up current, running current, and duty cycles. The name plate rating is usually none of the above. It’s otherwise a total guess.

That said, it does sound like you have a total energy deficit between power consumption and power available. Long term, as in more than a day, battery capacity really doesn’t matter. It instead all about power production vs consumption. Production comes from your panels, and 100w of panels will produce about 500wh of energy per day. My refrigeration consumes maybe 1500wh per day. I think you are pretty far off from something that will work.
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Old 08-06-2018, 11:32 AM   #16
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Agree fully with using a Kill-a-Watt meter. One problem with refrigeration is the defrost cycle. When my old Sub-Zeros went into their defrost cycles the wattage went over 400 watts (measured on a Kill-a-Watt.)

I used modified sine wave for 10 years and then true sine wave for the second ten years. My advice is buy the true sine wave. It is more efficient on refrigeration even when the modified will work. Modified will not work on some new units so be careful. Also down the road you may buy something else which won't work on modified sine wave.
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Old 08-06-2018, 12:44 PM   #17
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Originally Posted by Westiculo View Post
Are you sure that's the model fridge you have/are getting?
That's an explosion-proof fridge for hazardous locations:
https://www.agamarvel.com/scientific...tion/haz-6ear/


Well... maybe he has a gasser with a propane stove. ;-)
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Old 08-06-2018, 12:50 PM   #18
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It looks like I will invest in a kill a watt.

As for the fridge model, Sea Ray has it listed as a marvel 6.1 ft.³ fridge. When I looked up that model all that came up was what was listed in Grainger.
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Old 08-06-2018, 01:39 PM   #19
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If you are running off batteries + solar for more than a day or two at a time,

You really should get as hyper-efficient a fridge & freezer as possible, ideally 12V native rather than going through an inverter.

You will still likely need more panelage and a bigger bank.

Unless you have a genny or powerful well-adjusted alternator setup and don't mind burning dino juice an hour or two each morning before the solar charging starts.
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Old 08-06-2018, 02:00 PM   #20
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Unfortunately I’ll be gone from my boat for up to three weeks at a time so no chance to charge daily unless solar.

I’d hate to turn the fridge and freezer off the whole time though.

Currently I “burn” about 0.7kw per hour A/C running my freezer,
Fridge, ice maker and one air conditioner set at 80 degrees for humidity control.
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