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There are many calculators where you can plug in the numbers. This page is a 35,000ft view of how choices affect what you will need.
See this article for a sample walkthrough of the numbers.
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. 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.
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.
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.2). For shorter projects (traveling around the country for ≤3 years) in an RV with exterior battery trays the standard T-105 style FLA bank3) 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. Sitting in the dark at night with no fan.
Having an oversized lead bank for your charging ability results in battery murder and replacement. Having an oversized lithium bank is $$$ and can strain the alternator if a current-limiting isolator isn't used.
IMO, with lead chemistries4) 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 less Ah capacity
The absolute minimum for solar, assuming everything goes exactly right,6) is often said to be 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). 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, geography and use patterns.
You will need somewhat more solar
You will need somewhat less solar
The 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 a $10 cheapie. Nicer PWM have staged (“smart”) charging, more configurabilty, and likely better reliability.
You may want an MPPT controller:
You will need an MPPT 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 panel wattage by 10; 300w9) 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.10)
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 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.
For many people living in vehicles alternator charging + modest solar will be the best-performing system for the dollar. The ratio of camping vs driving11) will affect the solar/isolator/generator balance.
You will need some kind of alternator charging
You will not need alternator charging
More expensive DC-DC chargers are warranted when
You may need a generator if:
Cargo trailers14) have unique advantages and disadvantages
RV travel trailers have additional challenges
The battery is used occasionally and briefly then put back on the maintainer at home. The battery doesn't have to be particularly good15); it can be a chain store battery, often labeled “marine” or “hybrid” or similar.
It is used to run small fans, LED lights, recharge phones.
This is like the scenario above, only with more battery capacity required.
For a longer discussion Also see blog article AGM Charging for Weekenders.
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
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 has some similarity to overlanding in that the stealth camper will likely drive every day.
The relatively short distances driven by stealthers means that a plain isolator will likely outperform a fancy 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.
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 overpaneling (having massive solar to account for all weather conditions), although you could do it with smaller solar combined with a generator.
Battery banks tend to be either lithium or flooded 6v golf cart19) batteries in series, both of which have lifetime $/kAh costs under $2.