Need quick “electrical math” for an inverter purchase

[k9medic]

We stayed in the Bahamas at the same marina from July 6th through July 21st.

Our total electric bill was $723.91 plus $86.87 for the VAT. We used a total of 1113kw according to my math. That comes out to about 3.3kw per hour with everything on (3 A/C's hot water heater, etc...)

Skimping on the food in the freezer is not an option. We plan on packing the freezer with as much as possible and then bring over what we need when we fly over.

I did make an arrangement for one of the local guys that I know to check on the boat for me while I'm gone but I still need to solve for the electricity issue.

As dhays said, I can recoup a lot of electricity cost over the 5 months that the boat will be there that way.

I just looked into 6 volt deep cycle golf cart batteries. I could drop two of them in there and get 500aH and since they don't have to start anything, I could probably draw them down to around 30% which is 350 aH.

If I drop two 100 watt solar panels on the top of my hard top then I could possibly see 150 watts an hour. So let's say a total of 750 watts on a 5 hour day sunny day.

Is this math coming out better?

 

[High Wire]

Are your 66ah batteries 6v? It takes two 66ah 6v batteries in series to get 66ah at 12v. Double check your math.

 

[O C Diver]

I don't think this is going to work. Imo, there are too many failure points for unattended success. If it were me, I would want an inverter/ battery charger that would keep the batteries charged and switch to inverter if shore power went out. Then do a few months and see what you really consume in electric (shore power).

One of the calculations that's been missed is how much solar you will really need if you get the rare 2 cloudy days in a row.

Ted

 

[ghost]

I don't think this is going to work. Imo, there are too many failure points for unattended success. If it were me, I would want an inverter/ battery charger that would keep the batteries charged and switch to inverter if shore power went out. Then do a few months and see what you really consume in electric (shore power).

One of the calculations that's been missed is how much solar you will really need if you get the rare 2 cloudy days in a row.

Ted

This. Go for minimizing shore power use. If it never comes on, you won’t spend anything. Strongly suggest a dc fridge. Also might want to go with agm batteries so you can charge faster when available. Size your battery bank not just for capacity, but to maximize the fastest charge rate when power is available, typically that’s around 25% of the bank ah capacity. Going from memory, others feel free to correct me. A smaller battery bank has a lower acceptance rate than a large bank. You also want to maximize the percent of time in a “bulk” charging mode. The last 20% of the battery charge will take more time than the initial charge for little added charge. Downside here is most batteries don’t particularly like going long distances between full charges. Gone for weeks will likely shorten battery life.

You are asking a lot. Might be worth it considering the relative value of replacing the food n the freezer, before t this kind of hardware is not going to come cheap. A plane ticket for the cooler might well be a cheaper option.

 

[ranger58sb]

From a battery perspective I have 4 AGM batteries that are 66aH a piece.

Those are pretty small. What BCI group? 12V batteries in parallel? That'd be 264 Ah for that bank when new, 132 usable at 50% DoD.

I just looked into 6 volt deep cycle golf cart batteries. I could drop two of them in there and get 500aH and since they don't have to start anything, I could probably draw them down to around 30% which is 350 aH.

Most 6V GCs give approx 220 Ah each, so 220 Ah in series at 12V. Two pairs would be 440 Ah, etc.

The taller L16s in series would be about 300 Ah per pair...

Just because they're "deep cycle" doesn't mean manufacturers will recommend routinely drawing down below about 50% DoD.

Except for Firefly Carbon Foam AGMs, more $$$ each.

FWIW, our fridge/freezer units are AC/DC, so we don't lose anything to the inversion process when we run them on the batteries. Replacing perfectly good AC-only units would seem like getting into more serious money, though...

-Chris

 

[boathealer]

I just looked into 6 volt deep cycle golf cart batteries. I could drop two of them in there and get 500aH and since they don't have to start anything, I could probably draw them down to around 30% which is 350 aH.

Is this math coming out better?

Two 6-volt golf cart batteries would likely give you 220-250Ah, not 500. You need both of them in series to get 12V. Ah doesn't add together unless in parallel (you could have 500Ah at 6V).

 

[deblen]

 

[k9medic]

Well I dropped $20 on a kill a watt device today. The numbers will identify everything else I guess.

I got a hell of a deal on dockage for 5 months in the Abaco’s but an additional $300-400 a month for electricity would kill it.

I know I could turn the fridge and freezer on when I arrive but that would almost negate the “food savings” I plan on having.

In the end it’s only money I guess...

 

[john61ct]

Yes, in re-reading from the beginning, the whole system is designed for shore power, which is always cheaper than solar / DC.

If you want to go mostly-solar you need to put in much more efficient appliances and spend prolly at least $5K on supply and monitoring infrastructure.

Starting with consulting a pro who takes you walking through the boat, noting what can be salvaged and what needs replacing.

Big project, lots of owner time and energy.

In the meantime just pay for shore power.

 

[dhays]

If you want to go mostly-solar you need to put in much more efficient appliances and spend prolly at least $5K on supply and monitoring infrastructure.

Starting with consulting a pro who takes you walking through the boat, noting what can be salvaged and what needs replacing.

Big project, lots of owner time and energy.

In the meantime just pay for shore power.

Great advice.

 

[Wxx3]

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.

Yep. What he said.

That's why I changed by Subzero fridge and freezer which used 220 watts per hour to Vitrifrigo's which use 60 watts per hour. My battery bank is 900 amp hours.

Solar in the Bahamas will give you decent power, but you always can't count on it.

 

[Ski in NC]

I'd clean out the fridge and leave it off. NOTHING is worse than cleaning out a fridge full of rotten food (been there, done that), and the stink will permeate the whole boat.

Pay the freight to bring food over from the states. Or take the boat to Marsh Harbor and stock up there.

Did I mention the stink???

 

[High Wire]

Your luck with leaving the frig on for long periods without a power failure must be better than mine. I would bet money the first time I came back everything would be bad.
Give it away when you leave. Buy fresh when you come back. Get better at meal planning to minimize giveaways. Defrost and prop open doors when gone. Save all the expensive electricity fix money for fresh foods.

 

[k9medic]

Over the past 4 1/2 years of owning the boat I have had great luck with the power staying on. Of course this is in the United States.

Having spent a lot of time in the Bahamas I do understand that power fluctuations are common place and I am readily prepared for them when I am on the boat.

My kill a watt arrives tomorrow so this weekend I will see where I stand on a true number scale.

Ultimately I may wind up bringing my own food back and forth. We will be flying privately so that is an option for us.

If you have ever spent any time on the outer islands, you understand that the concept of fresh is relative.

I just dropped $2000 on some Sea Dek for the bridge so that has eaten into the potential solar budget.

 

[dhays]

That Sea Deck looks very nice.

 

[Bay Pelican]

Consider putting in a wind generator or two. while in the Bahamas our wind generator which was a small Air Breeze was producing more amps over a 24 hour period then our two 140 watt solar panels. Even in a slip we used the solar and wind power first and the shore power only to top up the batteries every night.

If you did this, depending on the size of your charger, I would suggest you wire in a timer so that the shore power would charge the batteries every night at 2 am or so.

Of course Bay Pelican had 12 volt refrigeration which was quite efficient.

 

[k9medic]

Dhays, thank you for the seadek compliment.


I had a chance to plug in my kill-a-watt on Friday afternoon.

Over the course of 38 hours and 42 minutes my fridge used 1.39 kw.

This comes out to 0.0359kw per hour, which at 65 cents per hour (Bahamas cost) means it cost me 56 cents per day to run the fridge.

I haven’t plugged it into my freezer test but I’m figuring about the same cost for it.

That puts me at about $1.15 average per day for both unless I’m doing something wrong with the math.

Over a 30 day period that’s only $34-35 dollars.

I’m not sure where I came up with the $375 per month electricity charge.

 

[O C Diver]

Dhays, thank you for the seadek compliment.


I had a chance to plug in my kill-a-watt on Friday afternoon.

Over the course of 38 hours and 42 minutes my fridge used 1.39 kw.

This comes out to 0.0359kw per hour, which at 65 cents per hour (Bahamas cost) means it cost me 56 cents per day to run the fridge.

I haven’t plugged it into my freezer test but I’m figuring about the same cost for it.

That puts me at about $1.15 average per day for both unless I’m doing something wrong with the math.

Over a 30 day period that’s only $34-35 dollars.

I’m not sure where I came up with the $375 per month electricity charge.

Glad you made the effort to determine actual consumption. Did you simulate comparable conditions with the Bahamas (air conditioning off and windows open or closed as in the Bahamas)? Refrigerators use a lot more energy trying to cool inside a 90+ degree boat.

This then brings you to the second point from my original post. There is a significant possibility that the power will go out at some point. Do you have enough battery capacity and inverter capacity (if necessary) to sustain your refrigeration for a reasonable period of time?

I don't think this is going to work. Imo, there are too many failure points for unattended success. If it were me, I would want an inverter/ battery charger that would keep the batteries charged and switch to inverter if shore power went out. Then do a few months and see what you really consume in electric (shore power).

One of the calculations that's been missed is how much solar you will really need if you get the rare 2 cloudy days in a row.

Ted

Ted

 

[k9medic]

Right now I don’t have the depth that I really want for battery capacity.

I plan on paying one of the dock masters to act as a caretaker while I’m out but that might not be enough.

Ugh. All the thoughts...

 

[sdowney717]

Your luck with leaving the frig on for long periods without a power failure must be better than mine. I would bet money the first time I came back everything would be bad.
Give it away when you leave. Buy fresh when you come back. Get better at meal planning to minimize giveaways. Defrost and prop open doors when gone. Save all the expensive electricity fix money for fresh foods.

One mode of power failure, is the marina owner coming by and unplugging the boat. A marina I used to be at years ago, he did not charge us for power. When his power bill got too high, he would unplug boats. I would get to boat and see the plug laying on the ground, WTH?? I started tieing it on. Later he told me he has a power meter and he plugged into my boat and claimed I was using too much power. All I had running was the fridge and the charger. I told him when he unplugs my fridge and plugs it back in immediately of course the power demand spikes from a stalled compressor. The guy really was eccentric. Claimed I was running electric heaters and costing him a fortune. A short while afterwards, I went to a much better marina and half the drive from the house and no boat tax. I have saved almost $1500 now versus the other place. Cut my fuel bill for the car way down too, that is not figured into that savings. And no 40 minute drive on the interstate to get to boat anymore.

And I still have free power and water for $30 less per month. They don't mind me using power like the AC as long as I am at the boat.

Last I heard he has gotten alzheimer's. He fights with many people, always been scrappy. I am somewhat leery of going back there for a haul out, but his son who is cool level headed guy is running the place now..

 

[k9medic]

I would probably wind up hurting somebody if they did that!

At the end of the day we may just wind up turning everything off.

 

[john61ct]

Over the course of 38 hours and 42 minutes my fridge used 1.39 kw.

Nit to pick.

Watts, therefore kW, also amps, are an instantaneous measure, often varying every minute, or on/high vs off/low for a certain ratio of time.

Average energy used per unit of time is AH or watt-hours.

Note AH on 12VDC are about 10x the quantity at 120AC, not including inverter inefficiencies.

WH are the same no matter the voltage.

 

[k9medic]

Can you help me understand what you said?

I calculated that amperage over the course of that same time is 12 amps AC which comes out to about 127 amps DC. That would be about 3.3 amp hours right?

 

[john61ct]

You just can't say "something used X amps" over a time period, only AH.

"It is **now** using X amps, and **now** using Y" makes sense,

amps is a flow rate, not a quantity of energy.

Say you have a 12V fridge that burns 4A while the compressor is running.

At a certain temp differential (A) it runs the compressor 30min per hour.

When ambient is cooler B, it runs only 10min per hour.

Set as a freezer C it runs 45min per hour.

So AH per 24 hours Y is

4A times X Duty Cycle times 24

A. X = 50%, Y = 48AH
B. X = 16.7%, Y = 16AH
C. X = 75%, Y = 72AH

Same for watts vs watt-hours, just multiply amps times the voltage.

 

[O C Diver]

If you used the kill o watt meter to measure power consumed over that time period, then your ok. It won't give you an accurate representation of consumption if you are reading the power consumption in snapshots of time. Ideally I would have liked to see you run the test for a week to get a more average consumption. Your test was fine and the results are the same type as watching the utility power meter on the side of your house as far as kilowatts consumed is concerned.

Ted

 

[k9medic]

Ted, I did use the kill a watt meter for this.

I was disappointed to find that it didn’t calculate an average amp load for that time. I could see the amp draw when the compressor was running but after that it didn’t really tell me much.

Perhaps I was asking too much out of a simple meter. I was hoping for something that showed the average and peak amp draw over the time that it was plugged in.

 

[TomandJeri]

Ted, I did use the kill a watt meter for this.

I was disappointed to find that it didn’t calculate an average amp load for that time. I could see the amp draw when the compressor was running but after that it didn’t really tell me much.

Perhaps I was asking too much out of a simple meter. I was hoping for something that showed the average and peak amp draw over the time that it was plugged in.

You can get the average current value by dividing the kWh figure by the elapsed time, both from the meter. That's the average energy rate in watts, but it says nothing about the peak value, nor the duty cycle. Divide by 120V to get average current.

Multiply by 10 and again by 0.8-0.9 for inverter efficiency (depends on the inverter) to get an approximate average current draw on DC.

Mine is showing 1.81 kWh and a hair shy of 48 hours since I reset it. 1810 Wh/48 h is 37.5 Wh/h (or 37.5 W average energy usage rate). Dividing by 120V gives an average current draw of 0.31A. Multiply by 10 (120V/12V) for an approximate DC average current, and divide by 0.8 for inverter losses (may be a bit better with a good inverter) resulting in 3.9 amps average over a 48 hour period. So every hour it's connected it draws 3.9 Ah from the batteries (on average), if it's running on an inverter off batteries, of course.

Another interesting thing you can get from this is that, using mine again as an example, I get about 1A when it's running. The math shows an average current of 0.31A. So it's running about 30% of the time it's plugged in.

http://www.trawlerforum.com/forums/s4/norcold-vs-regular-refrigerator-39946-2.html


There's a bit more to it involving power factor (the PF value on the meter), but I'm reluctant to get into the weeds on that.

 

[O C Diver]

Ted, I did use the kill a watt meter for this.

I was disappointed to find that it didn’t calculate an average amp load for that time. I could see the amp draw when the compressor was running but after that it didn’t really tell me much.

Perhaps I was asking too much out of a simple meter. I was hoping for something that showed the average and peak amp draw over the time that it was plugged in.

I think we're saying the same thing. The test showed killowatts consumed over a time period. You were able to extrapolate that into how many killowatts / $ it would be in the Bahamas per month. It seems batteries and inverter are now a less important consideration.

Ted

 

[TDunn]

OK if you used 1.39 kilowatts over 38.7 hours that is 1390 watts/38.7 hr = 35.9 watts/hr so lets round up to 36 watts/hr. Assuming 80% efficiency for your inverter (worst case assumption) gives 36 watts/hr/0.8 = 45 watts/hr from the batteries.



Your batteries need to be able to power the system for about 14 hours a day (night plus early morning/late day when solar is minimal). So the batteries need to be able to supply 14 hr times 45 watts/hr = 630 watts per day assuming they get fully recharged every day. 630 watts per day is about 630/12 = 52 amp-hr per day. So you need a minimum of 104 amp-hr of battery capacity if you are going to get a decent life from your batteries.


As far as solar is concerned in the Bahamas you will get about 6 full sun hours each sunny day. Your system will draw 24 hr x 45 watts from the batteries to run the system via the inverter = 1080 watts per day from the batteries. Solar has to provide at least that much charging. AGM batteries have between 80% and 90% charging efficiency (varies with battery quality). So assuming 80% charging efficiency you need 1080/.8 = 1350 watts per day from your solar. If you get 6 full sun hours of charging a day that works out to a minimum of 1350/6 = 225 watts of solar. Since not every day is sunny, I would go with at least 400 watts of solar to be on the safe side. In addition I would definitely buy an MPPT controller since a PWM solar controller will raise your solar panel requirement by about 25%.


Note this assumes that your 1.39 kw over 38.7 hours is accurate and that you batteries are in good condition.

 

[Wood]

Well I dropped $20 on a kill a watt device today. The numbers will identify everything else I guess.

I got a hell of a deal on dockage for 5 months in the Abaco’s but an additional $300-400 a month for electricity would kill it.

I know I could turn the fridge and freezer on when I arrive but that would almost negate the “food savings” I plan on having.

In the end it’s only money I guess...

Rent some freezer space from a local for a couple hundred a month?