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electrical:solar:gentle_intro
A good solar system is one that meets your needs. – jimindenver1)
The fastest way to bring beginner expectations of solar in line with reality is to assume you will run nothing off an inverter. – secessus

A gentle introduction to solar

This page aims to provide beginners with a basic overview of solar power for nomads. There are oversimplifications and concepts skipped for clarity. If you would like a refresher on basic electronics, see the AltE Intro to Electronics for solar video.

A quick definition: the word nominal will crop up again and again in your reading, both here and elsewhere. In your head you can replace the word “nominal” with the phrase “so-called”. Nominal refers to something's name, and the name for car electrical systems is “12v” even though it is very rarely literally 12.0 volts.

limitations of solar

Solar is not a magic bullet; it is a compromise like anything else.

Solar has very high startup costs for the amount of power you get out of it. It can cost several thousand dollars to get the same amount of reliable power one might might get from a residential (or campsite) outlet with a $20 extension cord. Over time, though, the power is “free”.

Minimizing power consumption is key to happiness with a simple, affordable solar setup. It requires a change in mindset for most folks. Electrical power becomes a precious resource to be carefully consumed. The path to frustration (or at least considerable expense) involves heating or cooling with electricity or trying to run household appliances in a van.

“…trying to take the comfort level you had in a house into your mobile life [is] tough to do for a boondocker.” – akrvbob2)

The amount of power a solar configuration outputs is greatly affected by things like latitude, season, shade, temperature, and even battery voltage.

Since it looks like magic, beginners tend to worry about whether or not their system is working. It usually is.

Given these limitations why do we do spend so much time and money on solar? Because having power off-grid, however expensive and limited, can make a huge difference in quality of life. Solar is silent, works for decades, and is always working when the sun is shining.3)

first things first: daily power requirements

It's not fun but the first step in getting solar is to assess daily power requirements. It's like writing down all your monthly expenses when making a family budget. Unpleasant but necessary.

Power requirements are assessed as daily because solar power feeds the batteries4) only in daylight. Then at night the 'dweller pulls power back out of the batteries. So a minimal solar configuration will meet your daytime needs then get you through the night without running out of power. No one but you knows how much power you will need daytime and nighttime.

AC, DC, Amps, and Watts

The basic idea is to tally up all the loads you want to run daily, the time in hours each will run, and the amount of power (Watts) each will consume while running.

It is useful to calculate power use in Watts, because “watts are watts” no matter if it's a 12v load or a 120vac load.5)

  • AC (household) devices will usually give consumption in watts on a label.
  • DC devices may give watts, but more commonly show Amps on a label. From this we can derive watts: Amps from the label times voltage (12v) == watts

First things first: Bill needed some idea of how much power he actually needed each day.

One way to figure it out is to see what configurations other people are running successfully in their own camper right now. The gamut runs from 100W to 1000W+, with 200W being the most common.

The details of Bill's logic and math has been moved to its own page, but for now we will stipulate he came up with a daily requirement of 91Ah of usable battery capacity for each 24hr period. On average he will consume that much power each day, and will have to replace that amount6) from the sun.

how solar power works

Solar power uses solar panels to convert sunlight into electricity. This power is usually stored in a deep-cycle "house" battery, which is seperate from the vehicle's starter battery. Electricity from the panels is fed to the battery by a solar charge controller, which keeps the battery optimally charged and powers any loads (things that consume power).

the easy answer

Kits are available that handle the basics at a reasonable price. 200W of solar is the most common campervan setup, and is sufficient for most campers that do not run inverter (residential appliance) loads.7)

Are kits the penny-pinching cheapest, or the highest tech, or the most tacticool military/marine grade option? No. But the average user doesn't need those attributes. And once users get comfortable with how their system works it can often be tweaked for better operation by adjusting controller setpoints.

For now, let's consider some common kit-based installations. The terms used will be discussed later in the page in case you don't recognize some of them.

  • 200W kit + a wally world learner battery is about $450 at late 2017 prices. This is about $1.75 per solar watt produced8) and would cost ~$750 over 5 years, including expected battery replacement.
  • 200W kit + 2 x 6v golf cart battery bank is about $550 at late 2017 prices. This is about $1.75 per solar watt, and would cost ~$550 over 5 years. Flooded lead acid golf cart batteries are the most tolerant of beginner mistakes, and will take the most abuse.
  • 200W kit w/MPPT controller + 2 x 6v golf cart batteries is about $675 at late 2017 prices. This is about $2.37 per solar watt, and would cost $675 over 5 years.9)
  • 200W kit w/MPPT controller + AGM battery bank is about $875 at late 2017 prices. This is about $2.37 per solar watt, and would cost $965 over 5 years~. Note: AGM batteries are not particularly well suited for solar-only charging.10)

200W is not a magic number, but it is the most common size of solar install for vandwellers and boondockers. It is probably more than what's needed for the basics and has enough power to add in a 12v compressor refrigerator if desired.11)

Going beyond the basics, handling the basics more frugally, or running non-trivial loads off an inverter will require more research, planning and patience. Read on, and follow links in the text to drill down to deeper explanations. Beware that some of the links are technical.

a typical DIY setup

BLM Bill across the wash over there built a system he successfully uses to run LED lights and roof vent, charge a phone/laptop, and run a 12v refrigerator. It is made up of the following components:

You've never seen him run out of power. He isn't wasteful with power but is not obsessed with conserving every watt, either. He has a functional, comfortable system suited to his needs.

So how did he pick those components? How did he know they would work? Where did he start? What are other people doing? How the heck does anyone untangle this stuff?12)

battery

Solar setups require a battery bank of some kind, even during the daytime.13) In simple installs the bank is one or two batteries.14) The battery also stores excess power. At night or any other time your demand is higher than your solar is pumping out, you will be pulling power from the battery.

For reasons related to the chemistry of deep cycle batteries, it takes twice the amount of battery bank Ah rating to deliver the daily Ah requirement:

total 91
Minimum battery bank15) 182

and that's just to get through a 24hr period with good sun. If you want to get through a day (or more) of terrible solar yield (rain, snow, fog) it requires even more battery capacity (reserve), money, and cargo capacity. These numbers are both very rough but will illustrate the pattern:

Rated Ah cost (flooded) cost (AGM) weight in lbs
Minimum battery bank16) 0 days of reserve 182 $182.00 $364.00 91
1 day of reserve 364 $364.00 $728.00 182
2 days of reserve 546 $546.00 $1,092.00 273
3 days of reserve 728 $728.00 $1,456.00 364

Don't panic! You don't need to spend thousands of dollars. In practice, many vandwellers do not design for extra days of reserve, planning on just not having power (or not much power) on rainy days. And there are other ways to compensate for rainy days:

  • voluntarily using less power. This approach has the added benefit of costing nothing but does require some study and discipline
  • overpaneling to harvest usable power even in challenging conditions like rain and overcast skies
  • staying at a RV park with hookups on bad days
  • buying a generator
  • investing in lithium or other exotic batteries

The most beginner tolerant and value-for-money battery bank is two 6v deep cycle flooded17)“golf cart” batteries wired in series to make 12v. This will provide 200+ Ah (amp-hours) of rated capacity.18) This bank gives Bill one normal day's use and and about a half/day of of reserve (ie, a day with no charging). A set of golf carts like this will be sufficient for most needs and does well with 200-500W of solar panel. Deep cycle batteries are typically discharged no lower than 50% as this is a good usability/lifespan compromise. 50% DoD is about 12.2v for lead-acid chemistries. A Low Voltage Disconnect can help prevent deeper, damaging discharge.

Yes, they should be vented. No, not everyone does.

Sternwake says:

“I don't think any battery is going to out cycle a 6v flooded golf cart battery. These are simply the most tolerant of the type of usage typically seen in this lifestyle.”19)

Because beginners commonly murder their first battery bank, a common beginner's battery is a Walmart "deep cycle" / marine battery, preferably one with removable cell caps for adding distilled water as needed. Often called a “learner battery”, they do not have long lives or much capacity but are widely available nationwide and often have pro-rated warranties. A walmart battery will usually have half the capacity of actual deep cycles, so Bill would get one nights's power with no reserve for rainy days.

Also common are AGM batteries, a sealed type. They do not have to be vented under normal conditions and have very high charge/discharge rates. On the downside they cost 2x as much, require vigorous charging one generally cannot get with solar charging alone, cannot be charged to as high a voltage, and have less capacity.20)

About these Sternwake says:

“While AGMs are considered a no maintenance battery if they were a person… they would be considered high maintenance… and only with a surplus of everything they want could they be kept happy.”21)

A single 12v AGM would get Bill through one night; two can be paralleled to double capacity to nearly match the capacity of the golf cart batteries.

panels

A common rule of thumb for solar-only charging is that one needs one watt of solar for every Ah of rated battery capacity. 2:1 is recommended for winter or other challeging conditions, and 3x is even better for off-grid folks.22)

So about 200w of solar is a good fit for Bill's 200Ah golf cart bank if he stays in a sunny area like the desert southwest.

Ratio W:Ah Panel watts
1:1 solar 182
2:1 solar 364
3:1 solar 546

Considering the solar panels themselves, there are framed panels (mono, poly), and semi-flexible (mono, poly, or amorphous) panels. Buy whatever:

  1. fits on your roof
  2. is the cheapest per watt

This usually means framed panels, as flex panels are usually 2+ times more expensive per watt and are less rugged. Flex panels do have legitimate uses but they are specialty panels and are not recommended as a default panel for most builds.

Arrays of two 100W panels are popular because 100w panels can be shipped individually by UPS/FedEx. Larger panels may require $$$ truck shipping; try to buy those locally if possible to avoid shipping fees.

100W panels are usually nominal 12v; there are also 20v and 24v panels one can source from the solar grid-tied market. The higher voltages panels require a MPPT controller (see below) but tend to be cheaper per watt.

The two main types of panel in 2018 are monocrystalline (mono) and polycrystalline (poly). The technical details are unimportant but you will see panels described as mono or poly when you are researching. Due to relatively small differences in their voltages, poly panels are slightly better suited to PWM charge controllers, and mono panels23) are slightly better suited to MPPT controllers. These controllers are discussed next.

charge controller

The charge controller stands between the panels and battery bank and takes care of charging. There are two types of controller commonly found in vans and RVs:

  • PWM - simple, inexpensive, cannot make complete use of the panels' capabilities.
  • MPPT - more complex, 2-3x more expensive, can run the panels at max power, optional with 12v panels and mandatory when nominal panel voltage is higher than nominal battery voltage.24)

PWM are cheap enough that it is common to start with PWM and upgrade to MPPT later only if necessary. After one upgrades from the PWM unit it can be kept as a spare/emergency part.

Folks in areas of limited sunlight (like the Pacific Northwest) or heavier power use might be better served by investing in MPPT right away.

Is my solar working?

One of the challenges of learning solar is that until you understand how it works it can be difficult to be confident that it does work.

This information will help you tell if it's working or not.

muddying the waters

Once you have a headlock on the information above it is time to introduce a bit more complexity.

1) paraphrased
3) portable solar only produces when deployed
4) and loads
5) inverter inefficiencies will be addressed later
6) and a little bit more due to charging inefficiencies
7) Yes, you can run inverter loads. The problem is not with the inverter; it is with users who do not realize how much power residential appliances consume. Planning to have no inverter will build in realistic expectations from the beginning. – secessus
8) actually going out of the controller into the battery
9) using a locally sourced 24v panel with the MPPT controller would bring the cost down to $1.25/solar watt
11) assuming good conditions like the desert southwest
12) by mastering small sections at a time!
13) this is to help the controller stay stable
14) not including your vehicle's starter battery
15) , 16) except lithium
17) you can/must add water periodically
18) 100+ Ah of usable capacity
22) 1:1 is usually sufficient if solar charging is augmented with alternator or other sources. 2:1 might still be required in areas lihe the Pacific Northwest.
23) and “high voltage” 20v/24v panels
24) e.g., 24v (72 cell) panels with a 12v bank
electrical/solar/gentle_intro.txt · Last modified: 2019/09/24 13:31 by frater_secessus