Small Packages for Off Grid

A number of people have expressed a desire  for  a minimal  solar power system  intended only for a laptop and maybe one or two LED lights.

The  standard question is  how much power do I need for_____?

That is like asking how long is a piece of string.  Same answer applies. It depends!

Its hard to give an accurate answer  especially one that  can be used by several people.

Put the question another way  will give you something that can be applied  to a variety of situations.

Automotive stores sell several products containing a sealed battery some kind of charge indicator and short booster cables with clamp on grips at one end.

Most of these products  use a battery of 20-amp hour capacity and the larger models use a 40 Amp hour capacity.

20 amps multiplied by 12V give 240 watts and the battery is typically rates at the 10-amp rate meaning it can deliver approximately 2400-watt hours at most. In real life lets call it a conservat8ive 2000-watt hours.

If your laptop uses 25 watts it will consume 25-watt hours each hour. In theory this should allow you to run the laptop for 80 hours maximum. 

I say in theory and at maximum because I am hedging due to certain physical laws we cannot violate.

When the battery is fully charged it will read 12.6V and when it is fully depleted it will read 10.5V Caution depleting a battery to 10.5 v will quickly damage the battery beyond recovery and further use.

Batteries are rated by using an arbitrary test method using standard numbers for equivalency of comparison.

Inverters typically have a 10% conversion loss.  So a 25% load would on average use 10% more or 26.5 watts to drive a 25W laptop.

At 12.6V the load current for the laptop would be 2.1 amps.  When the battery drops to 11.0V   the load current rises to 2.4 amps.

Using close to average numbers we would take 2.25 amps and divide that into 20 amps hours to get slightly more than 8 hours run time before the battery needs a recharge.

Using the charge rule of 25% also expressed as C/4 we should recharge at no more than 5 amps so it will take more than 4 hours to recharge.

Fortunately AGM batteries are fairly robust and can accept a faster charge but at the sacrifice of shorter lifetime. AGM batteries can last 5 years of regular cycling but this decreases to just over a year if you use the maximum charge rate and deplete the battery to its fullest every cycle.  A realistic average is two years life when depleting to 80% depth of discharge and using a 30% recharge rate.

To deliver a 6 amp  charge rate would  require at least a 120watt panel used in  latitudes north of 45 degrees and the location averaging   20% cloud cover average. 

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