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opinion:solar:sizing [2022/03/18 14:22] frater_secessus [special case: cargo trailers] |
opinion:solar:sizing [2022/05/24 15:13] frater_secessus [overall economics] |
[[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)) | [[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)) |
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====== solar sizing - the Big Picture ====== | ====== power system sizing - the Big Picture ====== |
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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. | 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. |
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| See [[opinion:solar:sizing.walkthrough|this article]] for a sample walkthrough of the numbers. |
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===== overall economics ===== | ===== overall economics ===== |
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Solar is relatively expensive when used alone. Since solar is dependent on local conditions one has to size the system to **the worst conditions**: winter, high latitudes, bad weather. [[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. | 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. |
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| In general, bigger systems (higher wattage panels, bigger controllers, bigger battery banks) cost less per-watt-harvested. |
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In general bigger systems (higher wattage panels, bigger controllers, bigger battery banks) cost less per-watt-harvested. | |
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==== days of reserve ==== | ==== days of reserve ==== |
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. | 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. |
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| ===== battery bank ===== |
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===== solar panels ===== | 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. |
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| Having an **undersized bank** means //running out of power// at night. Sitting in the dark at night with no fan. |
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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 usually 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! | 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. |
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| 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** |
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| * if you have undersized solar |
| * you have lead-chemistry battery bank |
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| You will need **somewhat less Ah capacity** |
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| * if you run loads in the daytime instead of at night |
| * if you have [[electrical:solar:overpaneling|oversized solar]] |
| * 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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| ===== solar charging ===== |
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| ==== solar panels ==== |
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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! |
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===== solar charge controller ===== | ==== solar charge controller ==== |
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==== controller choice ==== | |
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| === controller choice === |
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Most people do fine with a PWM controller. Folks who live off-grid with lead-chemistry batteries can get by with [[electrical:solar:shunt_tweaking|a $10 cheapie]]. | 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//. |
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| 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. |
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You may **want an MPPT controller**: | You may **want an MPPT controller**: |
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==== controller sizing ==== | === controller sizing === |
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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). |
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===== battery bank ===== | |
Having an **undersized bank** means //running out of power// at night. Sitting in the dark at night with no fan. | |
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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. | |
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IMO, 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** | |
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* if you have undersized solar | |
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You will need **somewhat less Ah capacity** | |
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* if you run loads in the daytime instead of at night | |
* if you have [[electrical:solar:overpaneling|oversized solar]] | |
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===== alternator charging ===== | ===== alternator charging ===== |
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For most people living in vehicles [[electrical:12v:alt_and_solar|alternator charging + modest solar]] will be the best-performing system for the dollar. | 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. |
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You will need **some kind of alternator charging** | You will need **some kind of alternator charging** |
* you have a voltage-sensitive battery like gel or lithium | * you have a voltage-sensitive battery like gel or lithium |
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| ===== generator charging ===== |
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| You may need a generator if: |
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| * 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]]. |
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* 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. | * 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. |
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RV travel trailers have additional challenges | **RV travel trailers** have additional challenges |
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* roofspace often lacks contiguous open spots to mount panels | * roofspace often lacks contiguous open spots to mount panels |