How long will an inverter run?

I am not a learned one, but a 400W appliance, without any inefficiencies would be a 400W/12V = 33 A drain on your battery.
50AH at 33A drain would only take 1.5 hours to completely drain the battery.

Probably less than that at such a high current draw, leaving your battery feeling pretty sorry for itself (ie stuffed!).

Col

I have found the following helpful at times. Sorry its a bit long winded.
Calculating the right sized battery for your needs.

Item of equipment Loading Watts x Estimated hours use = Watt Hours
x =
x =
x =
x =
Total Watt Hours =

Watt Hrs ÷ by volts (system voltage) = Ampere Hours

Ampere Hours already estimated =

Plus 10% allowance for cable loss =

Plus 25% over-capacity allowance =

Plus 50% vehicle starting margin =
(if applicable)

Total Ampere Hours Required =

STEP 1
Calculate the ‘Watt Hours’ by multiplying the loading of each piece of equipment (this is expressed in watts and is stamped into the compliance plate attached to the item) by the number of hours you intend to use it between charges.

STEP 2
Determine the Ampere Hour (AH) requirement that the battery must accommodate.
A cable loss margin of 10% is usually appropriate. In addition, it is nice to have a little more capacity than you need. Hence an over-capacity margin of 25%.
If the battery will be needed for vehicle starting you will need to increase your Ampere Hour estimate by 50% to ensure you still have starting power when the battery has been partially discharged.

How long will the Battery last running my Inverter?

AC Watts = figure written on AC (240 volt) appliance

AH = Ampere Hours capacity of the battery

(AH÷2)
(AC Watts÷10*) = TIME IN HOURS

Example: a 16” TV (67 watts) running on a 130AH battery

(130÷2) 65
(67÷10*) = 6.7 = 65 ÷ 6.7 = 9.7 HOURS


Note: *Use a figure of 20 if your battery voltage is 24V DC.
Only 50% of the batteries capacity will be used to allow it to recover.
Cheers Alan

Sorry the first part of the table didint copy. It is basically, loading in watts x the estimated hours of use = watt hours

Hi Jeff
Also remember for long battery life they should not be discharged below 50%of capacity ie 25amp hrs for a 50amp/hr dc battery[do not take below 11.6v]
You can get low voltage cutouts [operate @ about 11.6v]to give some protection to battery [most only rated @10 amps]

Thanks all for the info and advice.
I didn't think your first answer was too abrupt Col.
Hope this helped you as much as it helped me Barry.
Gotta love the collective wisdom of the forum.
Cheers