1000 watt inverter / 120v = >8.3a 12v load?

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Jared1048

Jared1048

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Apr 19, 2011
456
Hunter 31 Seattle
I'm new to inverters and I'm trying to figure out how big of one I'd need.


Does a 1000 watt inverter put a load on the battery of at least 8.3 amps? (thats not accounting for loss of power in conversion)

What kind of power loss % can be expected?

Thanks!
-Jared
 
J

JohnShannon

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Jan 4, 2010
1,037
Farr 30 San Francisco
In the first approximation the inverter converts DC power to AC power at 100% efficiency.

So the power load on the battery is equal to the load on the inverter.

If you have a 1000W inverter running and nothing is plugged in, then the load on the battery is low maybe a few watts.

Plug a 120W light bulb in and now the battery is also loaded with about 120W which would work out to 10A. The equation is power=VoltsXamps

As far as efficiency is concerned, you should see 90% but that should all be in the data sheets.

At full 1000W load you are going to have more like 80A load on the battery.
 
weinie

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
Im no expert, but I would think at least 83Amps... not 8.3.
 
Maine Sail

Maine Sail

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Feb 6, 1998
11,701
Canadian Sailcraft 36T Casco Bay, ME
1000W AC / 12V X 1.15 = 96A

This is the DC draw at an 85% inverter efficiency... Use 1.1 for 90% or 1.2 for 80%... Oh and at 96A you'll be lucky to maintain 12V at the batteries so you might want to figure on 11.5 or so instead which will bump the DC draw closer to 100A...

The simple safe guideline is; 50A for 500W or 100A for 1000W, 150A for 1500W & 200A for 2000W......

For AC the watts to current calculation looks like this;

1000W / 120V = 8.3A

For DC the watts to current calculation look like this;

1000W / 12V = 83.3A

Multiply DC amps by 1.1 (90%), 1.15 (85%) or 1.2 (80%) based on your inverters efficiency...
 
robertsapp

robertsapp

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May 28, 2009
764
Hunter 376 Pensacola, FL
So what would the impact be on your typical coastal cruiser's house bank? Let's say you're dealing with a pair of group 24 or 27 flooded cells, and the owner likes the idea of putting in a small inverter so that he can use the onboard 700 watt microwave at anchor. Is this going to kick the ever loving snot out of the batteries to intermittently discharge them at 70 or 80 amps for several minutes? Or can they shrug that off with no ill effects?
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
22,985
Catalina 34 224 Maple Bay, BC, Canada
Do the math: 80 amps for two minutes ((2/60))*80) = amp hours.
 
robertsapp

robertsapp

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May 28, 2009
764
Hunter 376 Pensacola, FL
Stu Jackson said:
Do the math: 80 amps for two minutes ((2/60))*80) = amp hours.
Click to expand
If that was in reply to my question, then thanks, but you apparently didn't understand what I'm asking. I'm not concerned about the total amp hours out, that's obviously easy to calculate. I'm concerned about how that type of discharge rate may potentially affect the lifespan, durability and/or function of a typical bank composed of some general off the shelf consumer grade deep cycle batteries. I thought maybe with all the work Maine Sail has done with batteries, he might have some idea of whether you'd be turning your plates into Swiss cheese or not.
 
Maine Sail

Maine Sail

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Feb 6, 1998
11,701
Canadian Sailcraft 36T Casco Bay, ME
robertsapp said:
If that was in reply to my question, then thanks, but you apparently didn't understand what I'm asking. I'm not concerned about the total amp hours out, that's obviously easy to calculate. I'm concerned about how that type of discharge rate may potentially affect the lifespan, durability and/or function of a typical bank composed of some general off the shelf consumer grade deep cycle batteries. I thought maybe with all the work Maine Sail has done with batteries, he might have some idea of whether you'd be turning your plates into Swiss cheese or not.
Click to expand
It won't hurt them unless you simply leave them discharged when your done and don't recharge. Deep cycle batteries get hit with huge loads in industry all day long. The place I buy my metal from supplies the BIW ship yard where they build destroyers. Their lift trucks are exclusively battery powered and they groan under the loads they are asked for every day.. They use simple deep cycle wet cells though much larger in capacity than what you'd have on a boat.

The Peukert effect kicks in though and you'll have less than the "rated" Ah's the higher the draw. Conversely, at loads less than your 20 hour Ah burn rate you'll get slightly more Ah's..
 
robertsapp

robertsapp

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May 28, 2009
764
Hunter 376 Pensacola, FL
Roger that, thanks. Good information in case I ever decide to become that guy.
 
BillyK

BillyK

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Jan 24, 2010
502
Catalina 310 Ocean City, NJ
Realize that a 800 watt microwave typically pulls 1200 watts to run. there are lights, the fan, the spinny thing.. i'm on the hunt for a microwave that will run on a 1000 Watt inverter..
 
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