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====== Gentle introduction, Pt 2 ======

This is a collection of illustrations that things are more complex than can be revealed in [[electrical:solar:gentle_intro|a one-page intro]].  Again, drill down for more info.

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**Solar panel output will almost never be as high as the claimed output.**  In fact it will probably hover around 75% of that number under the best conditions;  this is called "[[electrical:solar:output#panel_temperature|derating]]", or correcting output to actual conditions.  It sounds like fraud but it's not.  A panel's power is expressed as a "nominal" rating like 100W as when tested under //laboratory// conditions.  Since the roof of your van is unlikely to meet those same lab conditions, if you think you need 200W of panel you probably need 250W.

**//Usable// battery capacity is about half of nominal capacity.**  This is due to a quirk of lead-acid chemistry batteries ([[electrical:12v:deep_cycle_battery#flooded_lead_acid|flooded]] or [[electrical:12v:deep_cycle_battery#agm|AGM]]).  You can go lower than [[electrical:depth_of_discharge|the 50% mark]] but it generally leads to early (and $$$) [[electrical:batterycide|battery death]].   So if you think you need 100Ah of battery capacity you probably need 200Ah.

**PWM charge controllers may not be able to get even the derated power out of the panels**.  This is because the panel makes maximum power for conditions((Pmax)) at a particular voltage((Vmp)) which is higher than the voltage PWM controllers can use.((Vbatt))  You can work around this issue by using an MPPT controller, but they typically cost 2-3x as much as PWM.  

**[[electrical:solar:charge_controller|Charge controllers]] have specific current limitations**.  Regarding current they are rated by Amps of output:  10A, 20A, 40A, 60A, etc.  To protect the controller from damage their nominal output is derated by .80;  this assures "headroom" for power spikes due to odd solar conditions.   So a 10A controller is safely used at 8A, 20A controller at 16A, etc.  If you will be pumping 10A from the panels a 10A controller may not hold up.  A 15A or 20A controller may be required.  [[electrical:solar:charge_controller#mppt|MPPT]] controllers may be able to cope with excess current.

Controllers also have an **input voltage limit** which you exceed at your wallet's peril.  Going over this limit (usually by wiring too many [[electrical:solar:series_vs_parallel|panels in series]]) will usually kill the controller and void the warranty immediately.  So don't do that. 

**Vigorously charging a deep-cycled house battery using only solar is $$$**.  But it takes a much smaller and cheaper setup to charge in combination with [[electrical:12v:alt_and_solar|alternator]], [[electrical:shore_power|shore power]], or [[electrical:generator|generator]].

**12v systems are not really 12v**.  We call them 12v (("nominal 12v")) but deep cycle batteries are over-discharged at 12.0V;  they should probably never go lower than 12.2V.  During some stages of [[electrical:12v:charging|smart charging]] they might be over 15v.  It is very common to hold well over 14v for hours during the Absorption statge.

**Shade will clobber your panel output.**  Even a tiny amount of [[electrical:solar:output#shade|shade]] will usually have a drastic effect, losing up to 90% of what you might otherwise harvest.  This means no tree shade, no antenna shade, no roof vent shade, no roof rack shade, etc.

**Flooded lead-acid (FLA) batteries need to be "watered"**.  They lose water as part of their normal charging and need to be checked and [[electrical:12v:deep_cycle_battery#flooded_lead_acid|replenished with distilled water]] regularly.

**[[http://rvwiki.mousetrap.net/doku.php?id=electrical:12v:deep_cycle_battery#agm|AGM]] lead-acid batteries are not well suited to solar-only charging**.  They cost about 2x that of FLA, have less capacity, require vigourous charging and cannot be charged as high in voltage.  They remain popular because people think "$$$ is better" and are loathe to vent their banks.

It is possible to have one all-purpose battery for both house and starting.  This only works when [[electrical:solar:sizing#your_daily_power_needs|daily power requirements]] are quite low((40-50Ah/day)) or when [[electrical:solar:nonessential|opportunity loads]] are agressively utilized.





===== further reading =====
  * [[https://wiki.mikrotik.com/wiki/Solar_Power_HOWTO|MikroTik Solar Power HOWTO]]
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