[[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 ====== 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. See [[opinion:solar:sizing.walkthrough|this article]] for a sample walkthrough of the numbers. ===== 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.