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[[lifestyle:words_of_wisdom|Words of Wisdom]]:  "You have to build for winter, and figure out what to do with the extra electricity in summer." -- timselectric((https://diysolarforum.com/threads/less-than-perfect-off-grid-system.35869/post-450655))

====== power system sizing - the Big Picture ======

This article focuses on designing a DIY power system from scratch.   See [[opinion:solar:sizing.walkthrough|this article]] for a sample walkthrough of how it might be applied for a given use case. 

Also see [[opinion:more_power|making more power from an existing system]].


===== overall economics =====

Since solar is dependent on local conditions one has to size the system to **the worst conditions** the system is expected to experience:  winter, high latitudes, bad weather.  This means solar is relatively expensive when used alone where solar conditions are poor. [[electrical:12v:alt_and_solar|Adding in another form of charging]] not affected by local conditions means you can downsize your solar to fit **average conditions**, saving money and space. 

In general, bigger systems (higher wattage panels, bigger controllers, bigger battery banks) cost less per-watt-harvested.


==== days of reserve ====


Your **reserve requirements** will profoundly affect the size, complexity, and cost of the system.  
Sizing for an overnight camping trip is easy;  sizing for a three day trip a challenge;  sizing for a long trip or full-time vandwelling is serious business.  See the bottom of this article for example configurations. 

===== battery bank =====

Once you know your daily power and reserve requirements you can spec out a battery  bank.  The capacity and chemistry of the bank will drive much of the charging section below. 

Having an **undersized bank** means //running out of power// at night or [[electrical:12v:battery_capacity|limits on charging current]]. Sitting in the dark at night with no fan is less fun than it sounds.  

Having an **oversized lead bank** for your charging ability results in [[electrical:batterycide|battery murder]] and early replacement.  Because lead batteries need frequent and full charging, it may be better in the long run an "undersized" bank you can charge rather than a bigger bank you cannot. 

Having an **oversized lithium bank** is $$$ and can [[electrical:12v:alternator_details#current|strain the alternator]] if [[electrical:12v:b2b|a DC-DC charger]] or other [[electrical:12v:directcharginglfp#tweaking_current_with_resistance|current-limiting approaches]] are not taken.((this can also happen with large lead banks))



After figuring your rough capacity requirements, consider these factors:

You will **need somewhat more Ah capacity**

      * if you have undersized solar (lithium banks only)
      * you have lead-chemistry battery bank (50% usable capacity rather than 80% usable)
      * if charging is time-limited and you want maximal harvest from the alternator, shore, or other high-current charging source.  **Example: ** If battery specs say you can constant-charge at [[electrical:12v:battery_capacity|0.2C]] (20A per 100Ah of capacity) then((all other things being equal)) a 100Ah bank could take 20A, 200Ah could take 40A, and 300Ah could take 60A.  

You will need **somewhat less Ah capacity**

    * if you run loads in the daytime instead of at night
    * if you have [[electrical:solar:overpaneling|oversized solar]]
    * if you drive often and have [[electrical:12v:alternator|altenator charging]]
    * you have lithium-chemistry battery bank((can be ~0.62% the size of the lead bank, due to deeper [[electrical:depth_of_discharge|DoD]].))


===== solar charging =====


==== solar panels ====



The absolute minimum for solar, assuming everything goes exactly right,((shallow discharge, excellent solar conditions, well-designed system))  is often said to be [[electrical:solar:panel-bank_ratio|1:1]] panel-to-Ah.  e.g. 150w for a 150Ah battery bank.  In reality, solar that small is often insufficient unless one has unusually small power needs or adds in another form of charging (see below).  [[opinion:frater_secessus:beginner_mistakes|Newbies typically think their power needs are small]] until they sit down to read those power labels on the stuff that want to run.  D'oh!  


Most people will do best with much more panel-to-battery depending on battery chemistry, [[opinion:frater_secessus:panelsizesforinsolation|geography]] and use patterns. 

You will **need somewhat more solar**

    * if you live in an area with relatively little sun, like the American Northwest, Northern Europe, etc.
    * if you want to run more [[electrical:12v:loads|loads]]
    * if you live offgrid full time (FT) or spend long periods [[camping:dispersed|boondocking]]
    * to run things off [[electrical:inverter|inverter]] rather than 12v((due to inversion losses, typically at least 10%))
    * to charge a bigger [[electrical:12v:deep_cycle_battery|battery bank]]
    * to charge lead-chemistry (FLA, AGM, Gel) banks

You will **need somewhat less solar**

    * if you live in an area with a great deal of sun, like the American Southwest.
    * if you camp recreationally mainly in the summer when solar harvest is easier
    * if you [[electrical:12v:alt_and_solar|augment solar]] with [[electrical:generator|generator]],  [[electrical:12v:alternator|isolator]], [[electrical:converter|shore power]] etc
    * if you live in the vehicle part time (PT) and can charge consistently from [[electrical:converter|shore power]] when not camping. 
    * if you voluntarily reduce your power consumption
    * if you time-shift loads to periods like the afternoon when [[electrical:solar:nonessential|excess power]] is available
    * to charge lithium banks

=== calculating real numbers ===


**Accurate calculations** would involve:

  * the Ah/Wh to be replaced
  * the charging efficiency of the battery chemistry (We might ballpark, 99% for Lithium, 80% for FLA, and 90% for AGM.  
  * overall efficiency of the solar setup (we can ballpark 85% for MPPT setups, 70% for PWM)
  * [[electrical:solar:pvwatts|average insolation sun power available at the time/place]];  in the northern hemisphere full-timers base this on December since it's the lowest-yield month.  For part-timers, it will be the month of lowest insolation you will camp in.
  * the contribution of any other charging sources
  * minimum charging current requirements, if any.   (we can ballpark 0.2C for AGM and 0.1C for FLA.  Lithium has no minimums)

Let's assume a 200Ah AGM bank depleted to 50% SoC, wintering in [[camping:snowbirding:quartzsite|Quartzsite]] with an MPPT-based solar config and flat-mounted panels.  

  - 200Ah x 50% = **100Ah to be replaced**
  - converting to Wh, 100Ah x nominal 12v = **1200Wh to be replaced**
  - battery charging efficiency of 90% means we need **1333.33Wh of actual charging power** to replace the 1200Wh (1200Wh / 0.90) 
  - the solar install operates at a 85% efficiency, so we need **1568.63Wh of harvestable sun** power landing on the panels (1333.33Wh / 0.85)
  - [[electrical:solar:pvwatts|In December in Quartzsite]], a panel will receive an daily average of **3.08 hours of full sun equivalent**
  - So we need **590W of panel** (1568.63Wh / 3.08 hours)

In practice you probably won't be drawing your bank to the lowest allowed level each day;  substitute your [[electrical:12v:dailypowerrequirements|actual daily power requirements]].  You may find it easier in the long-term to model this kind of thing in a spreadsheet. 


**Note**:  the numbers are for //average// yields, including average seasonal weather.  Individual days may be better or worse.  **If you have non-negotiable power requirements** you may want to oversize your array to account for days of locally-poor harvest. 
==== solar charge controller ====


=== controller choice ===

The [[electrical:solar:charge_controller|controller]]'s job is to sit in between the panels and battery bank and regulate charging.  Counterintuitively, its most important job is to //prevent overcharging//.


Most people do fine with a PWM controller.  Folks who live off-grid with lead-chemistry batteries can get by with even [[electrical:solar:shunt_tweaking|a $10 cheapie]].  Nicer PWM have staged ("smart") charging, more configurabilty, and likely better reliability. 

You may **want an MPPT controller**:

  * if you discharge your batteries deeply overnight((low battery voltage hamstrings PWM))
  * if you have a lithium or AGM bank((they have lower resistance and so come up to voltage more slowly;  see point above)) 


You will **need an MPPT controller**:

  * if your panels are higher voltage than your battery bank (e.g. 24v panels and 12v bank)


=== controller sizing ===

[[electrical:solar:charge_controller|Controllers]] are rated by the Amps they can pump out.  A 20A controller can handle up to 20 Amps (about 250w incoming power, depending on battery voltage).  

A common **rule of thumb for sizing PWM controllers** is to divide [[electrical:solar:panels|panel]] wattage by 10;  300w((rated power)) of panel on a 30A PWM controller. They are cheap enough that a little oversizing is not a big deal, and they need a bit of headroom since they do not throttle incoming current to protect themselves.((they do use PWM switching to throttle current to hold a given setpoint))   See [[https://mouse.mousetrap.net/blog/2023-06-13-backchannel---comments-on-solar-advice.html#fn:amps|these examples]].

MPPT sizing is less straightforward. These tend to cost 2-3x as much for a given rating as PWM, so oversizing can get $$$.  MPPT have the ability to [[electrical:solar:overpaneling#vs_charge_controller|clip power]] during unusually-high harvest to limit current to their rated capacity.  For this reason they are often **sized to the power the panels make under normal circumstances** rather than the panels' lab rated power. 

Examples:  

  * A 200w panel might really only make 166W in actual use under good conditions.  This panel might be used with a 15A MPPT controller. 
  * 300w of panel might make 249w under good conditions.  25A controllers are rare, so they might be put on a 20A mppt controller
  * MPPT smaller than 10A are rare, so 100w-150w of panel are usually put on 10A mppt.
  * [[https://mouse.mousetrap.net/blog/2023-06-13-backchannel---comments-on-solar-advice.html#sizing-an-mppt-controller|more examples with explanation]]















===== alternator charging =====

For many people living in vehicles [[electrical:12v:alt_and_solar|alternator charging + modest solar]] will be the best-performing system for the dollar.   The ratio of camping //vs// driving((and to some extent what time of day you are driving)) will affect the solar/isolator/[[electrical:generator|generator]] balance. 

You will need **some kind of alternator charging**

  * if you have no solar
  * if you have insufficient solar <-- very common
  * if you have AGM, which craves a ton of current in Bulk charging

You will **not need alternator charging**

  * if you have [[electrical:solar:overpaneling|a ton of solar]]
  * if you stay on shore power much of the time
  * if have solar and most of your driving happens after noon((the isolator contributes best during Bulk, which typically occurs in the morning))

More expensive [[electrical:12v:b2b|DC-DC chargers]] are warranted when

  * you have no solar
  * current-hungry battery like AGM or lithium threatens your alternator's longevity, or you need to restrict charging rate for lithium. 
  * you have a voltage-sensitive battery like gel or lithium


===== generator charging =====



You may need a generator if:

  * you need to run big loads like A/C
  * you have a lithium battery bank but camp in long stretches with minimal solar.  In this model you would charge the batteries full blast((up to [[electrical:12v:battery_capacity|0.5C]])) for a few hours every few days. 
  * you have enough solar to [[electrical:12v:charging|Absorp/Float your lead batteries but not enough for bulk]].


===== special case:  trailers =====

**Cargo trailers**((aka utility trailers)) have unique advantages and disadvantages

advantages

  * flat roof space for panels, especially cheaper-by-the-watt large panels
  * relatively low, making tiltable arrays more practical since you can reach them

disadvantages

  * charging from alternator can be challenging.  See [[https://diysolarforum.com/threads/ricks-charge-from-the-tv-solution-for-owners-of-mppt-charge-solar-in-the-trailer.20730/|one way]] around the charging limitations of the 7-pin harness.

**RV travel trailers** have additional challenges

  * roofspace often lacks contiguous open spots to mount panels
  * rooftop accessories [[electrical:solar:shading|shade]] panels
  * large parasitic [[electrical:12v:loads|loads]], due to the common assumption they will be hooked up to [[electrical:shore_power|a power pedestal]] at a paid campsite


===== example setups =====


==== overnight camping ====

  * advantages - only 24 hours away from home (and grid power), only need to store power for 24 hours of use.  No need to generate power. 
  * challenges - need to keep battery bank on [[https://amzn.to/3x94mZA|a small charger/maintainer]] at home to avoid sulfation and [[electrical:batterycide|early death]]

The battery is used occasionally and briefly then put back on the maintainer at home.  The battery doesn't have to be particularly good((it will die within a couple years no matter how good or bad we treat it));  it can be [[electrical:12v:deep_cycle_battery#chain_store_batteries|a chain store battery]], often labeled "marine" or "hybrid" or similar.  

It is used to run small fans, LED lights, recharge phones. 


==== weekend camping ====

This is like the scenario above, only with more battery capacity required. 

  * advantages - as above, only stretched to 72 hours
  * disadvantages - more capacity required((200Ah?)).  [[https://amzn.to/2SvH5lT|Bigger charger/maintainer]] required((unless [[electrical:12v:alternator|isolator]] installed))

For a longer discussion Also see blog article [[https://mouse.mousetrap.net/blog/2020/07/04/agm-for-weekenders/|AGM Charging for Weekenders]].


==== overlanding ====

In this scenario you are away from home for long stretches of time, driving daily to the next campsite.  The system needs to be self-sufficient


  * advantages - driving daily makes [[electrical:12v:b2b|DC-DC chargers]] highly effective.
  * disadvantages - rough roads increase likelihood of damage to flooded lead batteries.  Forest camping can make [[electrical:solar:shading|shade]] a challenge.  Fully-provisioned roof racks make mounted solar a challenge.

More expensive chemistries like AGM or lithium are warranted;  plain FLA batteries may experience plate damage from vibration and rough driving. DC-DC charging will do the majority of charging, with a portable panel or two supplementing charging/loads during the day.  

==== stealth camping ====

[[camping:stealth_camping|Stealth camping]] has some similarity to overlanding in that the stealth camper will likely drive every day.  

  * advantages - driving at least a short distance every day for relocation or work commute makes alternator charging effective. 
  * disadvantages - need to remain stealthy limits solar array mounting

The relatively short distances driven by stealthers means that a plain [[electrical:12v:alternator|isolator]] will likely outperform a fancy [[electrical:12v:b2b|DC-DC charger]].  These quick blasts suggest the need for a bank that can suck up power fast:  AGM or lithium. 

The need for stealth suggests smaller (or just more discreet) arrays):  higher-efficiency panels, perhaps with black frames to minimize visual impact to onlookers.  Mounting on OEM racks can help fool the eye. 

The possibility of [[electrical:shore_power|shore power]] charging, even rarely, means the stealther may want to adding a shore power port and [[electrical:converter|converter/charger]].((and carry a long, heavy,  [[https://amzn.to/35clxxG|outdoor-rated extension cord]]!))
==== fulltime boondocking ====

  * advantages - roofspace can be devoted to large solar arrays.  Can choose batteries based on long-term value. 
  * disadvantages - in-place camping makes DC-DC charging less cost-effective.  Entire life must be powered, not just recreational loads while camping. [[electrical:solar:sizing#your_reserve_needs|Days of autonomy]] == forever.

FT boondocking game or vacation anymore;  //this is your life//. You need power every day and under all conditions.  The most reliable way to do this is by [[electrical:solar:overpaneling|overpaneling]] (having massive solar to account for all weather conditions), although you could do it with smaller solar [[electrical:12v:alt_and_solar|combined with]] a [[electrical:generator|generator]]. 

Battery banks tend to be either lithium or flooded 6v golf cart((CG2)) batteries in series, both of which have lifetime $/kAh costs under $2.
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