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opinion:solar:sizing [2022/11/08 19:29]
frater_secessus [solar panels] 		opinion:solar:sizing [2024/09/26 12:55] (current)
frater_secessus [battery bank]
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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. 
  
-Broadly speaking, lithium chemistries are most cost-effective when the camping experience is expected to last for many years and the batteries stored inside.((Note that it is false economy to pay for 10 years of battery cycling (as with lithium) if one is going to wreck it in 2 years)).  For shorter projects (traveling around the country for  ≤3 years) in an RV with exterior battery trays the standard T-105 style FLA bank((like 2x 6v Trojan T-105 batts in series for 12v)) may be more cost-effective.  For very short installations ( ≤1 year) marginal batteries found at walmart may be fine. I encourage people to do the //$/kWh math for the situation// rather than relying on the latest trends or loudest voices. Horses for courses.+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.  
  
  
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 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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 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((due to inversion losses, typically at least 10%))     * 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]]
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     * to charge lithium banks     * 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]]
  
  
opinion/solar/sizing.1667953742.txt.gz · Last modified: 2022/11/08 19:29 by frater_secessus

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