Inverter question

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retire09

Senior Member
If I need to run a 10 amp load for two hours from a 5000 watt inverter, how do I determine the minimum size and number of 12 volt batteries needed?
I have access to heavy equipment batteries but I don't have any specs for them at this time.
 

Smart $

Esteemed Member
Location
Ohio
Just the basics... but you didn't mention load voltage so I'm going to assume 120VAC

120VAC ? 10A ? 2hrs = 2400VAh load

2400VAh load ? 12VDCin ? inverter efficiency = minimum combined battery Ah rating

From there, you have to understand how battery Ah ratings can be misleading, enough to pad your minimum.

http://www.allaboutcircuits.com/vol_1/chpt_11/3.html

Also, 10A@120VAC will require over 100A@12VDC. Battery cable efficiency (i.e. size) will also affect the equation, but not written into the basic formula because everyone assumes the cables will be large enough.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
As a rule of thumb, with a load of 1200VA you should be using a 24 or 48V inverter, not 12V.
That way for your need of 2400VAH, requiring 12000VAH of battery capacity, you can put low voltage batteries in a single series string instead of putting ten 12V 100AH batteries in parallel.
Do not get trapped in a 12V box!
Look at 6V or even 4V or 2V deep cycle (RE) batteries to get the AH rating you need.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
As a rule of thumb, with a load of 1200VA you should be using a 24 or 48V inverter, not 12V.
That way for your need of 2400VAH, requiring 12000VAH of battery capacity, you can put low voltage batteries in a single series string instead of putting ten 12V 100AH batteries in parallel.
Do not get trapped in a 12V box!
Look at 6V or even 4V or 2V deep cycle (RE) batteries to get the AH rating you need.

And once you figure the battery amp-hours you need to power the load, double it so that your battery state of charge stays above 50%.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
And once you figure the battery amp-hours you need to power the load, double it so that your battery state of charge stays above 50%.
My example numbers threw in the more conservative factor of 5 for minimum 80% SOC. I just did not call attention to it, so thanks for clarifying.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
My example numbers threw in the more conservative factor of 5 for minimum 80% SOC. I just did not call attention to it, so thanks for clarifying.
Obviously I did not do the math. :D

When I was doing residential PV systems we frequently got calls from folks with large homes who told us that they wanted to "get off the grid". When we told them that to do that it would cost them a couple of hundred thousand dollars just for for batteries and those batteries would fill up their garage, they invariably changed their minds.
 
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mike_kilroy

Senior Member
Location
United States
And once you figure the battery amp-hours you need to power the load, double it so that your battery state of charge stays above 50%.

Consider 4-5x rather than 2x.... the Puekart effect will cut your AH rating in half or more quickly if you use 1x or 2x AH of any type lead acid batteries in your calcs....

If you are using Lio Ion, forget Puekart but still maybe go 3x (from experience).
 
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