Solar Power Ideas

(Note that ideas expressed in this post are from my own understanding.  More like brainstorming on the concept of an off grid system.)

INPUT

200W panel capacity for practical purposes is actually 100W.  So that is 50% off the rated value.  Based on Wikipedia, the Philippines gets an annual sunlight of 2,000 to 2,4000 hours.  Choosing the mean of 2,200 hours we get therefore an average of about 6 hours sunlight daily.  A 200W panel therefore can supply about 600W-hr of energy per day.

200W * 50% * 6hrs = 600W-hrs

STORAGE

Batteries rated capacities are specified in Ampere-hours (Ah).  Therefore for a 100Ah battery with a load that draws 100A, it should be able to last for an hour.  If the load is 50A, 2 hours...so on.  I believe that is the ideal case with no losses and other factors.  But draining the battery is not a good idea since it will shorten the usable life.  50% DOD or less is oftentimes recommended. DOD or depth of discharge is the amount of energy drawn from the battery.  In the case of the 100Ah battery, instead of drawing 100A for an hour, we should should cut the time or the current to half to satisfy the 50% DOD criteria.  The energy we can extract from the battery is 600W-hrs.

12V * 100Ah * 50% = 600W-hr

12V * 100Ah * 40% = 480W-hr  ;if 40%DOD

*Learned this late, that batteries are normally 20hr rated.  Therefore a 200Ah battery could supply about 5A for 20 hours.

OUTPUT and LOAD

Load calculation is quite tricky since there are factors that needs to be managed.  Basically it is just the Time and Power but consumption is almost always varying for a 24 hour period.  For example during the day when people are awake, they could be charging their gadgets, watch TV or playing on their PCs.  During the night when there could be less human activity, perhaps one or two bulbs is ON or the electric fan is providing some needed cooling so that people are comfortable in their sleep. Therefore to be able to manage the available and stored energy, it is necessary to have an energy usage budget.

Ideally during the day, we should have enough power from the panels to keep our appliances and gadgets working while charging our batteries for night usage.  If we draw more power than what the panels can supply, the battery is there to the rescue.  But then remember this is the opposite of what we should be doing and if we are do this long enough such that the battery has no chance to charge, we end with no power left to consume during night.

At 40% DOD wherein we got 480W-hr available out of the 100Ah battery (that is assuming we have a full charge before nightfall) we can have a load of 48W from 6PM to 4AM the next day.  Say we've got two LED bulbs of 9W each that is continuously ON the whole night, the battery has more than enough juice for this purpose.  That is 18W from 6PM to 6AM is 216W-hr or less than half of the stored capacity at 40% DOD of the 100Ah battery :).
As an extreme example, if we watch a 100W TV from 6PM to 12PM, we consume 600W-hr and although the battery at that point could still be alive, we run the risk of over discharging it which is detrimental to its health.

I believe that knowing the amount of power available and having an energy budget and load management plan is a must specially on an off-grid solar power setup.