Need quick “electrical math” for an inverter purchase

[k9medic]

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?

 

[Westiculo]

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.

 

[k9medic]

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.

 

[Westiculo]

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.

 

[ghost]

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.

 

[DDW]

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.

 

[caltexflanc]

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

 

[skipper2]

power usage.

It's important to consider the "start up" or surge current of motor driven loads such as fridge & freezer devices.

 

[k9medic]

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/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html

This comes out to 64amps. I’m assuming thst is per hour while running?

 

[dhays]

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.

 

[k9medic]

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
Batteries!

 

[ktdtx]

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.

 

[DavidM]

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

 

[Westiculo]

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/products/hazardous-location/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.

 

[twistedtree]

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

[emoji106]

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.

 

[Bay Pelican]

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.

 

[dhays]

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/products/hazardous-location/haz-6ear/

Well... maybe he has a gasser with a propane stove. ;-)

 

[k9medic]

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.

 

[john61ct]

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.

 

[k9medic]

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.

 

[DavidM]

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.

How do you know that? IF it is from a monthly power bill then it is accurate, but who knows how much of that is the air conditioner.


But if you are looking to run the fridge and freezer for weeks whiie you are away, you need a different set. A well insulated and designed 12V 6 cu ft fridge and separate freezer compartment should only take 75 amp hours daily with no one there to open and close it. That can be covered with 300-400 watts of solar panels. The cost of the panels and solar controller will be about the same as your 3,000 watt inverter. Unfortunately efficient 12V fridge/freezers are expensive, about $1,500.



David

 

[janice142]

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/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html

This comes out to 64amps. I’m assuming thst is per hour while running?

I have a 3.1 cubic foot two door Haier that in ambient temps of 80 degrees uses 60Ah per day as measured by my Kill-a-Watt meter. Recently I posted a series on my website which may offer insight.

Article here:
Powering the Refrigerator article on janice142

In a nutshell you need more solar AND more batteries.
Also please don't forget as your boat heats up more power will be required to keep the refrigerator cool. I have two fans blowing at the back of my reefer. One is pointed towards the compressor and the second pulls hot air out of the compartment.

Adding: The fans are only turned on when the starboard side of Seaweed is facing the sun.

The Kill-a-Watt meter is a great tool. Don't take it as gospel however when it is giving you figures inside an air conditioned boat.

Good luck.

 

[k9medic]

My power readings are from my meter and comparison photos between two times over an unoccupied period of 96 hours.



Ill check out the blog. It sounds as if the easiest way would be to just replace my appliances with 12v and boost my batteries.

 

[john61ct]

Sorry if I missed it, but are you leaving your boat on the hook for days or where shore power is available?

If the latter then should no issue with a good marine charger.

 

[k9medic]

Sorry if I missed it, but are you leaving your boat on the hook for days or where shore power is available?

If the latter then should no issue with a good marine charger.

At a marina in the Bahamas with metered electric. .65 cents per kw though. If I use .8/hr that’s about $375 in electricity per month just to sit there maintains a cold fridge and freezer.

Dockage is already expensive and I’m planning on flying back and forth monthly so saving at least $300 a month is important.

 

[dhays]

At a marina in the Bahamas with metered electric. .65 cents per kw though. If I use .8/hr that’s about $375 in electricity per month just to sit there maintains a cold fridge and freezer.

Dockage is already expensive and I’m planning on flying back and forth monthly so saving at least $300 a month is important.

Hmm... at $375 per month, you could recoup the cost of additional solar and battery capacity pretty quickly.


The other option would be to simply not keep food frozen while you are gone. Use the freezer when you are on the boat and the rest of the time just have the fridge running. That would cut your power load by over half. I'd look at swapping out that 100W panel for a really good (and expensive) high capacity panel or two. I use a single panel rated at 365W. In the summer here it is able to keep up with a 12v fridge/freezer as well as 12v freezer/cooler in the cockpit. The days are long but it gets shaded quite a bit from the arch and radar etc...

 

[Sealife]

Another consideration is marinas in the Bahamas have electrical outages more regularly than what you may be accustomed to. Inverter could drain batteries to LBCO point typically set way to low rather quickly with your assumed usage and battery capacity.....

 

[kchace]

Lots of good advice on here. You need a lot more solar AND a lot more battery bank to confidently do what you want to do.

Ken

 

[sdowney717]

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/products/hazardous-location/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.

'Auto cycle defrost eliminates the need to manually defrost the refrigerator'

It is the defrost timer that turns on a small heater, that is where they get the advertised 3 amps number. Normal running this is likely around 1 amp. so the figuring out this battery requirement ought to consider it.

I also went with a 3000 watt MSW inverter, been happy enough with it. I am not using the MSW all that much, it is only for if the gen wont run and we are out away from shore power, or want peace and quiet.

One day I forgot to turn on shore power, or the grandkids had turned off the switch after coming back to slip. 3 days later, the inverter had flattened the house batteries, even with the battery charger on. My shore power, I put in a separate relay, so when I plug in to the shore dock plug, the battery charger will come on regardless if the relay for the AC system of the whole boat is set to off. But my house batteries were also 6 yr old starter batteries and they had self destructed with shorted cells. I replaced them with Duracell 'deep cycle technology' 12v batteries from Sam's club.


My auto defrost 10 cu foot fridge draws 0.9 to 1.1 amps with the compressor, and maybe 5 to 6 amps when defrosting cycle is on. I have an AC digital amp meter on both ac lines. Easy to see volts and amps real time.

 

[john61ct]

I would rely on mains, bank as a fallback for outages, shut down the heavy consumers as much as possible while away, go more "voluntary simplicity" provision for each trip out.

Next choice capital investment, really go as efficient as possible on appliances, beef up the solar and put in good monitor logging consider it an experiment, constant quality improvement until everything is reliable, ROI payback from the $300 / mo savings.