Any website or maybe a simple formula to compute battery runtimes or sizing a battery to buy???

[narmenia]

Calculate the battery runtime.
Provide:
battery AH
inverter efficiency
load in watts

Or

Calculate the proper battery capacity to buy.
Provide:
-Runtime
-inverter efficiency
-load in watts

Thanks

 

[Hedges]

You've presented most of the parameters, maybe add maximum Depth of Discharge (DoD) for the battery.
Why don't you take a crack at it, propose the equation?

 

[Rocketman]

Actually you can do it yourself - once you know all the data...

Here is an example...

In the RV community everyone wants to run their air conditioners on their batteries - what does it take???

Invertor - Victron Multiplus 12/3000 90% efficient - subtracted a little because its a hot day - (fan's etc)).
Static Load of Motor Home - Lights, Fridge etc. - 50watts
AC 13,000btu Load in watts - Running 1250 *.90% efficiency = 1400watts
Runtime hours - We want A/C to run for 4 hours

So how much power does the AC use each hour 1400watts
How much static Load = 50 watts
Total Load/hour = 1450 watts / 12v = 121 amps per hour
We want to run for 4 hours = 484 amp hours

The EVE or Lishen 270-280ah cells I hope will have 250 ah useable in them in (You don't completely fill or discharge the cells except for testing).

So a 2p4s 560ah (500 useable) battery might barely make it... a 3p4s 840ah (750 useable) would be a better choice... Now we gotta refill the battery...

Hopefully I didn't make a dumb math mistake.

Now you create your formula like Hegges suggests.