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opinion:solar:sizing [2022/05/24 15:13]
frater_secessus [overall economics] 		opinion:solar:sizing [2025/05/04 11:12] (current)
frater_secessus [power system sizing - the Big Picture]
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 ====== power system sizing - the Big Picture ====== ====== power system sizing - the Big Picture ======
  
-There are many [[electrical:solar:sizing#calculators|calculators where you can plug in the numbers]].  This page is a 35,000ft view of how choices affect what you will need. +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. 
  
-See [[opinion:solar:sizing.walkthrough|this article]] for a sample walkthrough of the numbers.+Also see [[opinion:more_power|making more power from an existing system]].
  
  
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 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.  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 **undersized bank** means //running out of power// at night.  Sitting in the dark at night with no fan.  +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 lead bank** for your charging ability results in [[electrical:batterycide|battery murder]] and replacement.  Having an **oversized lithium bank** is $$$ and can strain the alternator if [[electrical:12v:b2b|a current-limiting isolator]] isn't used.+Having an **oversized lithium bank** is $$$ 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))
  
-IMO, with lead chemistries((flooded, agm, gel)) it is better to have an undersized bank you can charge //fully and consistently// rather than a bigger bank you cannot charge. + 
 + 
 +After figuring your rough capacity requirements, consider these factors:
  
 You will **need somewhat more Ah capacity** You will **need somewhat more Ah capacity**
  
-      * if you have undersized solar +      * if you have undersized solar (lithium banks only) 
-      * you have lead-chemistry battery bank+      * 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** You will need **somewhat less Ah capacity**
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     * if you run loads in the daytime instead of at night     * if you run loads in the daytime instead of at night
     * if you have [[electrical:solar:overpaneling|oversized solar]]     * 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]].))     * you have lithium-chemistry battery bank((can be ~0.62% the size of the lead bank, due to deeper [[electrical:depth_of_discharge|DoD]].))
  
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-The bare-bones minimum for solar, assuming everything goes exactly right,((shallow discharge, excellent solar conditions, well-designed system))  is [[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!  +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.  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** You will **need somewhat more solar**
  
-    * if you live in an area with relatively little sun, like the American Northwest.+    * 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 want to run more [[electrical:12v:loads|loads]]
-    * if you live offgrid full time (FT) +    * if you live offgrid full time (FT) or spend long periods [[camping:dispersed|boondocking]] 
-    * to run things off [[electrical:inverter|inverter]] rather than 12v+    * 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 a bigger [[electrical:12v:deep_cycle_battery|battery bank]]
-    * to charge lead-chemistries (FLA, AGM, Gel) rather than lithium+    * to charge lead-chemistry (FLA, AGM, Gel) banks
  
 You will **need somewhat less solar** You will **need somewhat less solar**
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     * if you live in an area with a great deal of sun, like the American Southwest.     * 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 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]], etc+    * 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 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 voluntarily reduce your power consumption
     * if you time-shift loads to periods like the afternoon when [[electrical:solar:nonessential|excess power]] is available     * 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 ==== ==== solar charge controller ====
  
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 [[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).   [[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))  +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 $$$.  MPPT have the ability to [[electrical:solar:overpaneling#vs_charge_controller|clip power]] during unusually-high harvest to limit current to their rated capacity.  For this reason they are often **sized to the power the panels make under normal circumstances** rather than the panels' lab rated power.  MPPT sizing is less straightforward. These tend to cost 2-3x as much for a given rating as PWM, so oversizing can get $$$.  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. 
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   * 300w of panel might make 249w under good conditions.  25A controllers are rare, so they might be put on a 20A 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.   * 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]]
  
  
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   * 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.   * 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.
  
-This isn't a 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]]. +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.   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.  
opinion/solar/sizing.1653419617.txt.gz · Last modified: 2022/05/24 15:13 by frater_secessus

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