Laptop Power

I have been attempting to determine the power requirements of living on my boat.  Most of it is fairly easy. The difficulty comes from my need to earn a living which I do using a laptop computer (software developer). I have been unable to determine exactly how much power is required for a laptop over the course of a business day so have been doing a lot of  guessing (which I really do not like to do).  I am hopeful that someone can provide some guidance and/or facts to help me figure this out.

I have a big, power hungry laptop (HP DV8).  The laptop battery lasts two hours and requires one hour to fully recharge (with laptop running).   I’ve measured the power draw of the laptop’s 110 volt power supply.  With the laptop running: When the battery is discharged and charging, draw is about 1 amp (115 watts). Somewhere in the charging process (near 90% capacity, I believe) draw drops to ~60 watts and then seems to steadily decrease until fully charged. Once fully charged, draw is 40 watts.

Based on that information, there is very little difference between keeping the laptop plugged in all day or only for an hour at a time to recharge (approximately 27 amp-hours versus 23).  Of course, this ignores the power required to run the inverter.

That got me to wondering if there is any difference in efficiency between using a 12 volt automotive power adapter to power and charge versus using than the 110 volt adapter running from an inverter – both powered from the battery bank?  Asked differently, is it more efficient to use an automotive adapter that bumps ~12 volts up to 14.4 volts OR an inverter that steps 12 volt DC to 110 volt AC which is then converted to 14.4 volt DC.  I assume the former based on the number of ‘conversions’.

My research found that AC-DC inverters have a standard loss of 10%-15%. Information on DC boost converters is harder to find (good reference document) but there are indications of similar losses. However; the specifics that I have found have been for 12 to 24 volt DC circuits.  This indicates that the difference between the two approaches is so slight that it is not worth worrying about.

I’d like to measure both but I do not have the automotive adapter nor an ammeter/gauge that is sufficient to connect between my battery and inverter.

So, perhaps it is all about simplicity?  Plugging  an automotive adapter into the 12 volt outlet is less complicated than connecting the inverter to the battery bank and plugging the laptop power adapter into the inverter.  Not by much, obviously, but I need something to work with!! 🙂