Okay, I drained my laptop battery until it read 22%, and I shutdown the laptop. Not standby or hibernate but shut down.
I plugged in the 12vDC to 19.5v DC to DC converter/ car adapter in to the laptop, placed the clamp on Ammeter next to the wires, zerod it, then clamped it over the (+) line and it read a Steady 3.31 amps.
I then turned the meter's dial so as to read voltage, and probed the Anderson connector feeding the DC to DC converter and read 11.92 volts.
11.92v x 3.31a = 39.45 Watts to charge the laptop battery with the laptop OFF.
So I unplug the DC to DC converter and turn on my Wagan 400 watt PSW inverter which has only the original power transformer/brick plugged into it, put the clamp on meter right next to the Inverter cabling, Zero the meter, then clamp the meter over the (+) wire and it reads a steady 4.02 amps. I then probe the thumb screws on the back of the inverter. 12.35 volts.
4.02 amps x 12.35 volts = 49.64 Watts, to charge the laptop battery, laptop OFF.
Now this laptop battery is old and only at half capacity or so, but over a full charge cycle that 10+ watt difference can make a good sized dent in the Lead acid battery's capacity.
So no doubt, the DC to DC adapter uses significantly less electricity to both power the laptop, and to just charge the battery, than the inverter does to power the original power brick and accomplish the same task.
Now I'll do the same test while the laptop is on, only window open task running, is this forum.
DC car adapter
Laptop battery is now reading 48%
6.86 to 5.94amps at 11.59 to 11.72 volts
79.51 to 69.61 watt range
Laptop battery reading 53%
400 watt PSW inverter
7.67 to 6.34 at 12.31v,
94.42 to 78.05 watt range.
So it holds true, the Inverter uses about 10 more watts on average than the DC to DC car adapter when performing the same tasks.
This is hardly insignificant, even if you have a solar surplus.
My compressor fridge consumes about 1 A/h per hour, usually less.
1a x 12.6v = 12.6 watts,
So basically the inverter powering the laptop, compared to the DC to DC adapter, uses nearly as much extra energy as my fridge requires during that same hour.
Now some might ask why the input voltage was less on my DC car adapter. It is much further away from the batteries over 10 awg, and on that same 10 awg feed, but not being read my the meter, amp wise, are a few LED lights and a muffin fan on a medium slow speed, which is adding to the voltage drop on this circuit.
The inverter is much closer to the batteries circuit wise, over heavier cable. My battery voltage is only about 12.4 at the time of the test as today is hot humid cloudy and drizzly and I have been lazy inside on this dang laptop with all my fans and TV running, so no solar surplus today.
Often we get on this forum, people with no secondary battery system, who ask what size inverter they need to charge their cell phone and laptop, when the answer is that they do not need an inverter for these tasks. In fact, for less money, they can get a car adapter for their laptop(most likely), AND a USB charger for Ipad or cell phone for less money, and they will use less energy to both power these devices, as well as to charge them. Most never think that they can actually charge USB devices from a laptop USB port too, negating the need for a Ciggy plug usb adapter.
For these people with little concept of how little energy a battery actually stores, how much devices draw, and who do not have an alternate method to jump start the vehicle or secondary battery, these DC to DC power supplies can mean the difference between a vehicle which starts and one which does not, and I believe that informing such people of this less expensive and more efficient option should be among the first responses given.
In addition, with DC to DC adapters, there is no risk of a MSW inverter causing issues to the provided transformers usually provided with the device, though admittedly this risk is slight. If they don't want to risk it and want a PSW inverter, well add some $$$.xx to the price tag of accomplishing that goal.
Now if somebody already owns an inverter, then there is no Need to get a DC to DC adapter, unless one is trying to save all the battery power they can. If one is not concerned about battery consumption, then do not waste money on a DC to DC adapter.
I could in theory use my 800 watt MSW inverter to do these same tests but the results would be even more in favor of the DC to DC adapter. Besides that inverter's fan noise is the annoying, and runs full speed all the time and was a primary motivation of why I sought a better path to power DC devices.
On the topic of car adapters for laptops, well those pulling 60+ watts often will reveal that the ciggy plug/ receptacle, while a convenient connector, is a very Substandard connector and Will fail at some point. Better Ciggy plug receptacles and plugs themselves exist, and these receptacles should be wired with fatter wiring than comes stock in most vehicles.
Improving the Ciggy plug and receptacle can be well beyond the skills of many who visit these forums, but the ciggy plugs themselves are likely responsible for any bad reviews given to a laptop "car adapter".
I have cut off the Ciggy plug receptacle on my car adapter and use a 30 amp Anderson powerpole connector and have no issues anymore with my car adapter. But the stock ciggy plug wore out, and so did its replacement.
Do note that an inverter designed to plug into a ciggy plug receptacle will have the same issue with the plug or receptacle. It is really a poor design for a convenient connector. It heats up and wastes power even when it is working as designed. I limit their use to devices which cannot draw more than about 4 amps.
Those seeking to eliminate all wasted power in their rides, should consider eliminating them, especially on devices which can draw more than 40 to 50 watts.
