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Sabreman 05-19-2008 11:26 AM

Installing 700 Watt Inverter
Intended Use: Recharge laptop, maybe run small coffee maker. That's it.

Background: I have a 150 Watt West Marine inverter that kept cutting off last season when the laptop was charging. Since the laptop power brick gets hot in proportion to the laptop battery level (real hot the lower the charge), I thought that it may be drawing quite a bit of juice and exceeding 150 watts. Since power = voltage * current, that would be about 12.5 Amps (150 watts/12 VDC = 12.5 amps). This is an enormous amount of current (I don't recall what my ammeter read) for a laptop to draw, so I now suspect that I'd drained my batteries below the inverter threshold, EXCEPT that I was on shore power at the time with the charger running, so it shouldn't have been an issue.

Plan: Since they're so inexpensive and to avoid inverter limitations, I just eBay'd a 700 Watt inverter that I plan to hard wire into the electrical system. All installation instructions that I've read call for a direct hookup to the batteries. Obviously, I don't want to do that and want to wire it to a spare breaker on my power panel.

Question to the Sailnet Wizards: Can anyone see a problem with wiring to an UNUSED breaker on the boat's electrical panel? Will the inverter work ok not being direct connected to the batteries? I suspect that the manufacturers don't want someone to use a cigarette lighter and its skinny wires to draw 700 watts so they recommend a direct connect.

Thanks, all.

sailingdog 05-19-2008 11:30 AM


The problem is that a 700 Watt inverter will be drawing about 59 amps at maximum draw. I don't think you happen to have a spare 75 AMP breaker on your panel, or the wiring to support that kind of load there.

700 Watts / 12 Volts = 58.333 Amps

You'd be much better off cabling it directly to the batteries and putting a fuse in the line someplace.

This is one reason inverter/charger units, like the Freedom 10, are so nice. They have to be heavily wired to the batteries to support their charging and inverter function anyways... :)

Sabreman 05-19-2008 11:43 AM

Sailingdog - "will be drawing about 59 amps" Agreed. I'd thought of that but figured that running just a laptop or coffee pot wouldn't get anywhere near 59A. Your response however, made me realize that I'd be wasting the inverter capacity wiring to a 30A or 15A fuse. Good catch, I'll just have to make sure that I properly size the fuse and cable to the batteries. The cable run will be about 7' taking into account proper routing.


chucklesR 05-19-2008 11:53 AM

I have a 700w inverter that I just alligator clip to my batteries when I want to use it, otherwise I use either a 100w or 300w that plugs into normal 12v outlets. The 300w runs laptops just fine.
Simple and easier.

Valiente 05-19-2008 12:11 PM

SD's comments are spot on. Basically, you can wire the thing properly with a 50 amp in-line fuse, or just never draw that much power with it by running something smaller or running big wires to a smaller circuit breaker.

Another consideration, and one perhaps more flexible, is what I did: I got a 700 watt "power pack" with a light and an air pump and two AC outlets that is charged while at dock and then kept in a locker.

Canadian Tire

Basically it's a 28 Ah AGM in a box. It carries enough juice to charge VHFs and to charge/run laptops for several hours without the cost or hassle of wiring, and with the convenience of portability. I've used it to run sanders and Dremels for up to an hour when I didn't feel like running 150 feet of extension cord across the yard, and it's been nice to have in the Zodiac to run truly bright spotlights to see debris in the water ahead when running at night.

Oh, yeah, and you can start your engine a couple of times with it.

Another alternative is this, of course. I tested it with a 1500 W hair dryer on "melt head" and it didn't flinch.

chucklesR 05-19-2008 12:21 PM

My dad gifted me with a 110v dc single cup coffee maker that draws 500w, I' use it to make that first cup of coffee without waking the Admiral up.
I reckon firing up the Honda would short circut my plan :)

If what I'm reading is right I could wire my 700w into my 50 amp windlass breaker, it's installed right there in my Nav station/battery locker about 3 inches from where I keep the inverter anyway.
I was just afraid of running the windlass and forgetting it turn off my laptop but really that should not be a problem - or if it would it would just trip the breaker anyway, not melt the boat.

sailingdog 05-19-2008 02:15 PM


I don't see why you couldn't do that... you wouldn't get the full 700 Watts out of it...but you'd get 500 or so from it. :)

Sabreman 05-19-2008 02:41 PM

chucklesR - That's exactly what I'm thinking. I'm not sure how big my largest breaker is (been simplifying the boat for 3 years and pulling obsolete gear OFF), but a worst case is to add a 50A breaker in lieu of a lower rated one.

chucklesR 05-19-2008 03:02 PM

It's on my list of things to do (wiring it in) while I sail about at anchor this (memorial day) weekend. I'm seriously looking forward to some quality time of tinkering about in the morning.
First, I'll make a cup of coffee with the old alligator clip, then I'll wire it in.

sailingdog 05-19-2008 03:05 PM


Originally Posted by chucklesR (Post 316648)
It's on my list of things to do (wiring it in) while I sail about at anchor this (memorial day) weekend. I'm seriously looking forward to some quality time of tinkering about in the morning.
First, I'll make a cup of coffee with the old alligator clip, then I'll wire it in.

Don't get zapped. :)

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