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Old 03-26-2013, 06:50 PM   #1
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Inverter sizeing

This may have been covered before but I'm looking for info or a good link to figuring out what size inverter I should install. I'm guessing there is some simple equation for this?
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Old 03-26-2013, 06:54 PM   #2
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Gotta give us some more info.
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Old 03-26-2013, 07:07 PM   #3
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Originally Posted by jukesy View Post
This may have been covered before but I'm looking for info or a good link to figuring out what size inverter I should install. I'm guessing there is some simple equation for this?
add up the total watts your items will consume. Make certain that the combined start up draws from all motors plus other draws do not draw over the surge rating of the inverter. Then add some wattage for expansion. This would be the minimum size that I would use.
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Old 03-26-2013, 07:58 PM   #4
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add up the total watts your items will consume. Make certain that the combined start up draws from all motors plus other draws do not draw over the surge rating of the inverter. Then add some wattage for expansion. This would be the minimum size that I would use.
Yep, that's about it. Now if you and your crew can exercise some sense, you can reduce this rating by just knowing not to try to run everything at once. For example, don't run the microwave while the coffee pot is brewing.

Of course, nothing is quite that simple. You need enough battery power to supply the current needed by the inverter. Usually, that will be multiple batteries connected together as a "bank".

Doing such an install is a stretch for the average DIYer unless you have some electrical background and the ability to follow the installation instructions and the ABYC electrical code.
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Old 03-26-2013, 08:22 PM   #5
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Doing such an install is a stretch for the average DIYer unless you have some electrical background and the ability to follow the installation instructions and the ABYC electrical code.
This is one job that if there is any question of your ability to install to hire a professional. Inverters usually ground in a different way than the AC panel. In fact, on most if you connect it to the AC neutral bus bar you will blow your inverter. They are not to be trifled with. Remember there is a lot of current going in and out of an inverter. Do it right.
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Old 03-26-2013, 09:06 PM   #6
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Close your browser (or keep it open a minute to go to Amazon) and go buy a copy of the invaluable "Boat Owner's Mechanical and Electrical Manual" by Nigel Calder. He will walk you through all this in easy to understand, but detailed terms in the first few chapters of the book. But don't skip to the inverter chapter, read from Chapter 1 onward.
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Old 03-26-2013, 10:31 PM   #7
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I did it the other way around. I opened my checkbook and bought the biggest inverter I could afford then sized my load accordingly.
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Old 03-27-2013, 01:02 AM   #8
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Thanks for all the info. Much appreciated.
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Old 03-27-2013, 03:06 PM   #9
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Inverter Sizing A big one or multiple smaller ones?

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I did it the other way around. I opened my checkbook and bought the biggest inverter I could afford then sized my load accordingly.
I used this approach on my old boat and suspect it may apply to most. I bought my first inverter back in 1991. Back then the bigest and best seemed to be a 2000W Trace. Unfortunately it didn't fit my budget. I bought a 1500W Vigel. It served my needs (Microwave, toaster, coffee maker, fans, TV etc) well until early in the 2000's when it caught fire. It was only running a fan at the time. I woke up thinking I was hearing an electrical arcing sound. I was. Fortunately I was on the boat and no harm was done except the inverter. But it did get my attention. I replaced it with a much cheaper 1750W. That still works, but I am a bit hesitant to install it in the trawler due it's age The burning inverter is still a clear memory. I need a bit more capacity and I want an inverter/charger to take better advantage of Genset time and shore power than I get with my 40 amp charger. I am wondering about the use of a single large inverter/charger vs. using two or three smaller units.
Here is my logic for multiple units, I welcome comments and opinions, my mind is not made up yet.
My AC needs are too light to run the 7.7KW genset unless the Air Conditioners, electric stove and/or hot water heater are being used. And even then I've got excess power that could be used to charge the batteries faster than the 40A. can. My 840 AH battery capacity could utilyze a 100A to 150A charger. But I need the ability to turn down the charge rate if I am stuck with only a single 30Amp shorepower connection.
My AC inverter loads would be refrigerator, currently running nicely off a cheap 1000W inveter. Occasional use of the microwave, coffee maker, vacumn cleaner, fans, a 1HP 120V AC washdown pump. and the Admiral's CPAP machine.
Inverters seem to be more efficient when running near their capacity as compared to running a very light load. So my reasoning is to continue to run the fridge off a dedicated inverter with no charger just slightly bigger than the minimum. The admiral's CPAP can run off a portable unit plugged into a 12Vdc outlet. It pulls very little juice. The other stuff could run (not at the same time) off a 1500W to 2000W inverter/charger. To run with a single inverter, I think I would need a 2500W to 3000W. I think I can go the multiple inveter route cheaper. This is because the fridge doesn't seem to care if it's fed a modified sine wave. The 12V supply wires can be significantly lighter. And I like the added redundancy if one of the beasts misbehaves. The idea of a 3000W inverter running all th time seems wastefull of battery energy.

Your thoughts??

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Old 03-27-2013, 04:12 PM   #10
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Do you want a stand alone inverter or an inverter/charger combination? We have a combination. We wanted the largest charger that came with a reasonable sized inverter. We ended up with an Outback that has a 125 amp charger and 2800 watt inverter with automatic switching. We spend about 75% of the year at anchor and since we run the generator to charge our batteries the charger size was important. The inverter size is also important though. We can run any of the house AC loads including the water-maker, vacuum cleaner, power tools, microwave, etc. When we bought Hobo, she had an old undersized, modfided sine wave inverter/charger. We were able to upgrade to the Outback by changing some wire sizes, few breakers and replacing the unit.
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Old 03-27-2013, 05:52 PM   #11
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Old 03-27-2013, 07:06 PM   #12
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I bought the inverter, bought the books, and then opened my check book. At least I have those books. I went with a 1000 watt True Sine Wave. I can run everything (including microwave) but the shop vac trips it at start up. Oh well.
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Old 03-27-2013, 09:32 PM   #13
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LARRY,
I am thinking a stand alone for the fridge and a combo for the main inverter. The minimum main inverter will start the 1 HP washdowm pump I need to find out the starting current for it is. 1500 Watts might do it depending on surge capability. The charger I would like to be in the range of 100 amps. I know I won't find that in a 1500 W inveter. But I do see a Magnum 2000W/100A. I am afraid any charger bigger than 100A will be too much for a single 30 amp shorepower plug. Perhaps if it could powershare or could be derated it would work. Bigger would be better if it powered by the gen set. I prefer the combo unit for price and ease of installation. A standalone 100 amp charger seems to be nearly as expensive as a combo inverter charger.
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Old 03-28-2013, 06:05 AM   #14
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I am afraid any charger bigger than 100A will be too much for a single 30 amp shorepower plug.

A bigger problem is if your noisemaker is under about 10KW the full output of a large charger can not be obtained.

There is nor enough power under the sine wave of a small unit to make big DC amps.

A 150A truck alternator dual belted on the noisemaker , with a temp compensating smart charger does the best charge in the least time.

Leaves the output of the set useful for AC loads.
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Old 03-28-2013, 07:37 AM   #15
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Craig: Not sure what Magnum you are looking at but those which operate from their better remote controls are easily programmable to gear down the charge amps as other loads come on line, according to what your shore power is. You can down load the manuals for their inverters and remotes (the programming options are in the remote manuals). Also a 3000w inverter only draws on the batteries according to what is being demanded of it at any given point in time. You can look at the specs and see that it's "at rest" draw is minimal and no different from that of a smaller inverter. Once again, the Calder book is really outstanding on explaining all this in detail... you have to read though and not just skip ahead to the inverter chapter.

100amps 12 volt is 10 amps 120, plus whatever the charger's efficiency rating adds so can be managed.
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Old 03-29-2013, 06:20 AM   #16
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The low cost of a cheapo 1500W unit at under $200 makes it a good choice for many.

However IF I were spending those big bucks for a "better" unit the one feature that would make it worthwhile is the amp limiting/following and amp infill ability.

The ability to start an air cond on a crap 15A circuit or a second AC on a 30A power hose , or use a small noisemaker and be able to start big induction loads would be worth the extra price.

Otherwise why bother with a big dumb inverter?
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Old 03-30-2013, 02:48 AM   #17
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Quote:
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This may have been covered before but I'm looking for info or a good link to figuring out what size inverter I should install. I'm guessing there is some simple equation for this?
A resource I found very helpful are the white papers of Victron energy; of course it is also about their products, but anyway contains a lot of useful info about everything around energy on board and with different sample solutions for different needs

http://www.victronenergy.com/upload/...9%20-%20EN.pdf

Succes
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Old 03-30-2013, 04:38 AM   #18
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It simply came with the boat.

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