6 to 8 hours of laptop use can be a significant amount of electricity. It depends on how much the laptop actually uses in the task it is performing, and this also ranges rather wildly, depending on task, and the laptop.
My laptop uses, on average, when the laptop's battery is already full, about 3.5 amps per hour which is about 28 AH total consumtion in those 8 hours. It can draw close to 8 amps when spinning a DVD and recharging the depleted laptop battery.
To replace 28AH per day, + 10% more for inefficiency, in summer, you will require 50 to 80 watts of solar and park in direct sun at least 3 hours either side of solar noon.
The alternator can provide a good amount of amperage, but not through a Stock Ciggy plug receptacle.
The main problem is lead acid batteries need to be returned to 100% fully charged as often as possible, but at a minimum every so often, otherwise they shrink quickly in electrical storage capacity, and get more and more difficult to actually recharge to this diminished, maximum remaining capacity. Secondary problem is that to get a lead acid battery from 80% charged to 100% charged, takes about 4 hours no matter how powerful the charging source might be. the battery simply cannot accept much current in these higher states of charge, and it needs to get up to these higher states of charge, often, to live a respectable lifespan. This requires holding the battery in the 14.2 to 14.8v range for around 4 hours when drawn to 80% charged.
Due to voltage drop on too thin wiring over too long a circuit, even if the vehicle's voltage regulator allows , and holds(most will not hold)14.4v, only 13.8v might be getting to the battery, and the amperage flow at 13.8v will be half or less than that if it were able to get 14.4v at the battery terminals. Voltage is electrical pressure. The result is often inadequate recharging and poor battery lifespan and an often heard low voltage alarm on the inverter.
You are likely going to need at least a 100Ah of battery capacity, and no less than 100 watts of solar for it, if you want to easily power the laptop for 8 hours overnight, and do not drive daily and are not able to plug in to recharge. These other charging sources, utilized more often, can lower, or even eliminate the solar required to return the battery to as high a state of charge as possible as often as possible to get an acceptable lifespan.
'Acceptable' is also subjective.
In the daytime, the solar ideally needs to produce enough juice to get the battery to absorption voltage, and hold it there for enough time, so consuming solar wattage directly to power the laptop, there is that much less to actually recharge the battery from its overnight discharge, however much that might be.
What you want to do is totally doable, but you will spend less money on individual parts rather than get some well marketed all in one kit that is actually decrepit, in its individual poorly matched components.
Look on the power brick on your laptop. It will list an output voltage and amperage, like 11.1v and 5.3a vDC. Multiply these two numbers to figure out the maximum wattage that the power supply, can provide. Actual consumption will be below this number. How much below is an unknown. For an exact answer, You can plug the laptop power transformer into this, opr plug in a power strip into it and measure all your electrical loads.
http://www.amazon.com/P3-International-P4460-Electricity-Monitor/dp/B000RGF29Q
and see how much it actually consumes in those 6 to 8 hours of use. But due to inverter inefficiency, one must add 20% to this consumption.
This KWH number the kill a watt meter will display, will allow us to make a recommendation on the battery size you need and how much recharging you need to provide it to keep it happy enough, Otherwise a general guess is all we can do without this data.
Also goto amazon electronics and plug in the make and model laptop and add 'car adapter' to the search.
These DC to DC converters can be significantly more efficient than using an inverter to power the original laptop provided power brick.
Do not get a universal laptop car adapter if your laptop can draw more than 60 watts. I use a 90 watt PWR+ brand on my dell. At 90 watts output the ciggy plug it comes with wears out quickly, as ciggy plugs provided with most devices are mostly terrible electrical connections which can heat up and waste battery power as heat. They might be Ok new, but after ~50 hours passing 60+ watts their resistance increases. the 12v ciggy plug receptacles can also be pretty prro quality, and are wires with too thin of wire over much too great a distance to pass 60 watts or more continuously, safely without excessive loss.
Last summer I built a portable power pack with a 400+$ 125 AH capacity AGM battery. It has a 400 watt pure sine inverter and a 45 amp converter/charger. 2 USB outlets and 2 12v outlets, and a 500 amp connector to hook directly to the alternator with thick copper cable. All top of the line components. Expandable for solar. Parts alone were 1400$
It has built in voltmeter and ammeter and an AH counter, so one could see how depleted the battery is, and know when recharging, via alternator, or Grid power is necessary. This power pack was for a friend and he got the bro deal on labor, and I went a little overboard on the woodworking, but actually designing it and wiring it all up to fit with oversized copper wiring, into the smallest package possible, safely, was very time consuming.
It also weighs 100 pounds. Very capable though.
As long as he plugs it in after an outing, so the internal charger can indeed fully charge the battery, and does not often cycle deeper than 50% charged, he should be able to get at least 800 deep cycles to 50%, from the high $$ high quality AGM battery.( Lifeline GPL-31XT)
http://www.powerstridebattery.com/lifeline-gpl-31xt-deep-cycle-agm-battery
A chronically undercharged lead acid batteryFlooded/AGM or GEL, might only have about 150 deep cycles before its capacity has shrunk too much to be useful. how many actual cycles achieved before this point is variable on the depth of discharge and how close to 100% charged it ever gets and how close it gets to stay near that ideal 100%. Lower for longer is more damaging.
Most people will not know how quickly the battery is degrading. for 100 days everything can be just fine, and by day 110, the 'stupid' battery is 'not longer taking a charge'. What happened was the battery capacity took 110 days to decline enough before it no longer had the capacity to power the users loads for the required time. No hail mary full recharge, or even a forced overcharge is going to restore capacity at this point, the battery is effectively murdered and needs to be replaced, AND a better recharge regimen needs to be found.
Would have been cheaper to recharge the first battery better in the first place.
But had the user known the battery needs a lot longer to recharge and not be overdischarged, the total number of deep cycles might be 350 or 400 before capacity declines to the point where the average 12v Newb would notice something is not right. About 500 deep cycles to the 50% range is the best one can do with the best 12v flooded marine batteries( Trojan, Crown, Eastpenn/Deka and USbattery. The other flooded marine batteries are made by Johnson Controls or Exide, and built to lesser standards for wally world and other autoparts stores, and these lesser marine 12v batteries are doing good to actually achieve 500 cycles cycled no deeper than 75% charged and when given a fairly good recharge regimen.
An Equal capacity 6v Golfcart batteries 'should' be good for at least 800 deep cycles to 50% and recharged properly. Very few wet/flooded 12v true deep cycle batteries exist. They are marine batteries, a cross over closer to a starting battery in internal design, than a true deep cycle battery
AGMS fall somewhere in the middle, and are less tolerant of improper recharging. AGMS are the whiny brat of the battery world, unless they get high amp recharged to 100% nearly each and every deep discharge. Then and only then, can they achieve that 'superhero' battery status, that most 12v newbs imbue them with, due to their 2x or 3x price tag over their flooded/wet counterparts.
So to recap, how much battery you need to power 8 hours of laptop daily, is likely much more than you think, and requires enough recharging sources to achieve an acceptable lifespan from the battery. When the battery capacity declines prematurely to uselessness, Warrantying batteries is a Pain, and when a battery is murdered, by improper treatment.............It is not the fault of the battery or its manufacturer, or its purveyor.
