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Old 05-31-2014, 09:33 AM   #15
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The amp-hour size of your battery bank is a limit on how much power the inverter can draw, but need not be a limit on the size of inverter you purchase. On the other hand, there is no point in buying extra inverter capacity you can't use, unless perhaps you plan to upgrade the battery bank in the future, or move the inverter to another RV.

You really need to come at the problem from the other direction. Figure out the wattage you feel you need, then spec the inverter that can deliver it PLUS a battery bank that can deliver the necessary amps for the desired number of hours. If you are unable to meet the battery amp-hour demands, then you need to reset your power usage expectations and that will in turn reduce the size of the inverter and the battery demand. It's a matter of matching wants and needs to a budget, something most all of us grapple with every day.

A common mistake in assessing battery capacity is to overlook voltage. An inverter requires a minimum voltage, often 10.0-10.5dcv, in order to produce usable 120v power. A high amp rate reduces the battery voltage, which generally means you cannot expect to get a full 235 AH from your 2 x 6v batteries and still have adequate voltage for inverting. Even a brief moment of high amp draw might force the battery voltage below the inverter's minimum, resulting in an inverter shut-down. The 5-hour rate is typically much more representative of actual battery capacity when using an inverter, but it depends on how much amps you expect to draw.

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Old 05-31-2014, 12:05 PM   #16
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Originally Posted by ardbark View Post
I'm finally getting around to adding an inverter and want to make sure I match the Inverter size to the house battery pack.

I've got my eye on the Xantrex PROWatt SW series, it seems to be the favorite of lots of veteran RV'ers. It's a pure sine wave model which my computer will enjoy, and comes in several sizes which I have to pick from. Hence my question.

I have two 6v house batteries each rated at 225 amp hours @ 20 hours and 185 AH @ 5 hours. The most load the inverter will see at one time will likely be no more than house lights (all 12v) and a laptop and monitor, or a flat screen TV and DVD player if we're watching TV, maybe a small 110v sound system, awning lights (110v) when we're on the patio, those sort of things. No hair dryers, no residential fridge (although I wouldn't mind keeping that in mind if/when the Norcold finally dies). So no really big loads. If I need AC or the microwave I'll fire up the genny.

I get adding up watts from all the toys and picking an inverter size to match, but is there a formula for matching Inverter watts to rated Amp Hours for the batteries?

With just the two batteries, you'd want no more than a 1KW, even though you don't even need that much with a 400 watt, capable of doing the job. Just so you know, the lights have nothing to do with the inverter, except to rob some battery power from it. If your puter is a laptop or otherwise running off of it's own battery, it won't care about the wave form and most other things you mentioned won't either, unless maybe you install a residential fridge with an ice maker and it's funny electronics. If I ever replace the fridge, I expect to pay about $400 for a small one that will hold more than the OEM and an ice maker will be the furthest thing from my mind. I don't even like them, with all the different water supplies, entering the coach. There's convenience stores all over that sell the stuff, when needed. I don't want to be chopping on the woodwork either with enlarging the opening and removing well needed storage drawers and such. I've found that it's hard to go very far without passing a grocery store and could probably get by with less than that, even.
Just so you know, we have a 1KW mod sine wave inverter and run our OEM fridge on it, when going down the road and for short periods in between. I installed a relay as well, so that it switches over when a 120vac external source is present and whether it be from the shore or genset. You can shut the electric off at the fridge and when on propane for now.

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