any idea of how much you would save with DC to DC option?
I have never bothered to measure it but I suspect it is generally not much and depends -
The first thing is that people go wrong in assuming that the DC to DC step up power supplies are a "one conversion step" device and go direct from 12 v DC to the higher DC voltage whereas that is incorrect. This claim is made in every thread about these devices but, in fact, they actually convert from the 12volt DC to AC first using similar technology to a small inverter
(using a buck converter device) and then convert that AC back to DC at the required higher voltage.
So whether one uses an inverter
together with the notebook manufacturer's AC to DC power supply or one of the 12 volt step up supplies there are exactly the same number of conversion steps - three, being DC->AC->DC. So, in the end, the overall conversion efficiency will depend on the efficiency of each of the devices used, not on whether one goes the inverter/power supply or the DC to DC converter way.
As I say I have never bothered to do any checks, mainly because I too believe, as another has said, any difference between the two approaches is likely to be small in terms of efficiency. As was also said the DC to DC converters seem to be quite inefficient whereas inverters are generally very efficient (but that depending on size and type and how well they idle when left on with no load) - I would not be surprised if going the inverter way is often the most efficient, but in the end I don't fancy that either way there is enough in it to actually worry about and I never have.
The most efficient way is, of course, to run the notebook straight off the boat's 12v as others have suggested. If you look at the voltage of the notebook's battery you will usually find that it is a multiple of approx 3.8v (assuming Lithium Ion) and generally approx 11.8 volts (being 6 cells made up of 2 x 3 cells in parallel) or, less frequently but for high end machines, approx 15.2 volts (being 8 cells made up of 2 x 4 cells in parallel). These voltages are, of course, lower than the manufacturer's AC supply output voltage (usually around 18.5v or 20 something volts respectively).
In the few I've tried and all those reported back to me by friends, six cell notebooks (ie battery around 11.8v) work happily on 12v from the boat and if seen in the light of if the notebook is not on its power supply it has to work on around 12 v from its battery this seems common sense (but there are always exceptions to common sense
). The higher voltage from the notebook's power supply seems to be only relevant to charging the battery. Also not often appreciated is that the batteries are internally regulated (Lithium Ion always are so due to the risk of precipitating Li) so that as long as one stays within the manufacturer's charge voltage (ie the 18.5 volt or whatever) it is not likely possible to overcharge them. I suspect they are all, certainly in recent times, also regulated against complete discharge, maybe someone knows.
I would be careful if powered directly off a small house 12 v battery bank with large inductive loads on it though, or if powered from the cranking battery when cranking as there is not any isolation from spikes that could normally be provided by the external supply normally used by the notebook (ie the manufacturer's supply or the DC step up supply discussed). We have a simple and cheap
automotive surge protector installed across the +ve and -ve of our DC house supply and doing so may provide some protection from spikes.
As an aside, a friend and I have tested a number of notebooks (all HP/Compaq or Compaq) in the range around 1.2 to 1.9 GHz for current
draw. We found all the ones we tested drew around 1.8 to 2 amps when running a typical program with hard drive access and the battery already fully charged - this is much less than many claim (without, I assume, actually measuring it, but just quoting from the power supply rating). Of course a higher current is drawn if the battery is under charge but this is not energy lost
except the little from the losses in the charging process itself and if wished to avoid that little loss then one can always remove the battery.
Hope that is of some help.