Solar system sizing

“ I'd say 500 watts is the minimum for the [Pacific Northwest] in winter and you would be better off with [750 watts] if you can get it on the roof.^[1]”

Sizing a solar system for an off-grid house is relatively easy: calculate household power needs in the worst case scenario (sustained cloudy winter days in that specific geographic area)^[2] and pay for it.

Solar configurations for nomadic living have more variables:

• worst case scenario loads (the max loads you will apply in any 24hr period, or at any given time, if you plan to run heavy loads)
• worst case scenario sunlight, usually December in your geographical area due to shortest daylight hours, but also affected by tree canopies, mountains, "urban canyons", or other forms of shading
• days of reserve (with no solar charging) you'll need
• whether or not you have access to other forms of charging: shore power, alternator charging, generators
• the kind of batteries you have - lithium, for example, charges more efficiently than lead-acid
• the kind of camping you do. Full-timers will need more robust systems. Weekenders can "get by" then recharge batteries fully at home.

These factors will dictate the system capacity you will need. If the system you need will not physically fit on/in your vehicle, you will have to take a different approach.

The first step in (and difficult part of) designing a solar setup for your camper is to understand your real power needs. Most people running on solar power only (as when boondocking) will need to reset expectations to be more in line with reality:

• Necessities like charging phones, LED lighting, fans, etc. are easy to power.
• "Niceties" like electric coffee makers, toasters, hot plates, induction cooktops, and microwaves require big/costly setups.
• Luxuries like air conditioning require very large/expensive solar systems (or cheaper shore power or generator usage)

Your daily power needs

Before you dig in, consider that it is far easier and cheaper to use less power than it is to make more power.

Power requirements are usually expressed in Amp-hours needed every day (Ah/day), although thinking in Watt-hours (Wh) might be less complicated.

There are two different kinds of electrical loads nomads usually run, AC (alternating current, usually 110v or 220v) and DC (direct current, usually 12v). Home appliances and other devices designed for use with AC power will need to be run with an inverter, which will incur additional efficiency penalties.

Your reserve needs

“ ...one that really [effed] with me was running out of power or spending a few days in the dark cause the weather is bad.^[3]”

Your "solar harvest" (how much solar power you can get from the Sun each day) is highly dependent on local conditions like weather, season, and latitude. One cannot assume getting a full charge every day in every condition.

To account for this decide how many reserve days of power you need if there is little solar power available. solar power available. You may need few in desert regions and many more in the Pacific Northwest. Long stretches of little solar charging may necessitate rationing solar power.

It is safest and simplest for solar newbies to assume a day of poor solar harvest = zero solar harvest. In actuality zero harvest is highly unlikely; more experienced or risk-tolerant RVers can run with less reserve or no reserve at all.

Your battery bank

Now that we know the total amount of electrical capacity we need (from 24hr loads X number of days needed) it is time to figure out how that capacity is met by various battery chemistries.

It may not be obvious, but one cannot consistently use all the capacity in a battery without damaging it. Deep cycle batteries are usually cycled to 50% Depth of Discharge (DoD), so you will need a deep cycle bank 2x the size of your previously calculated needs. Starter batteries (those used to start your engine) may tolerate only 10% discharge. LiFePO₄ batteries are happy to go down to about 90% discharged.

Example: You need 100AH x 1 day, so your bank will be 200AH cycled 50% each day.

Example:LiFePO₄ can be drained down to 80%-90% DoD, so you would need only 110AH of rated capacity with that chemistry. 120AH if you are conservative.

A rule of thumb is flooded lead acid batteries in offgrid scenarios should be charged between C/10 and C/8. For a 200AH bank this would be 20A-25A. Charging at lesser rates may result in not getting fully charged each day. Charging at greater rates may result in increased outgassing and the need for a lengthier Absorption stage.

Solar panel array

Pick a solar panel array output on daily power requirements and battery bank capacity. A common rule of thumb for matching panels to battery bank is that at a minimum there should be 1 panel watt per amp-hour of nominal bank capacity.

“ ...the person draining their batteries to the 50% range regularly would do much better with a 2 watts to 1Ah ratio, or even 3 to 1.^[4]”

Here are some examples using this model:

• 1:1 - shallow cycling; or deep cycling with augmented charging in the morning
• 2:1 - deep cycling and charging from solar only
• 3:1 - deep cycling and charging from solar only in areas with low insolation like the Pacific Northwest (PNW)

A crude rule of thumb for panel output per day is a horizontally mounted panel on a sunny day will generate Ah (amp hours) roughly equal to Rated Wattage / 3.^[5] \\ Example: a 100W panel could output 33Ah/day.

Another rule of thumb for panel output per day is a horizontally mounted panel on a sunny day will generate Wh (Watt hours) roughly equal to Rated Wattage x 4.^[6] \\ Example: a 100W panel could output 400Wh/day.

Charge controller

After the bank and panels have been sorted one must size the charge controller to the panels.

There are two general rules:

• voltage from panels must not exceed controller voltage, ever.
• controller rating should be the panels' output plus a safety margin, unless you are overpaneling an MPPT controller on purpose.

Augmenting solar

Solar power often works best when paired with another power source. This is because the first half and second half of smart charging have different time/power ramifications

• high current / short duration during Bulk <-- generator, shore power converter and alternator charging do this best
• low current / long duration once Absorption gets underway, and during Float <-- solar does this best

simple system

Joe Vandweller needs 50AH of power each day. He wants to get all of it from solar.

If he lives in the desert southwest he might need

• 1 day of reserve, totaling 100AH of power
• 200AH of lead acid (2p1s GC2 golf cart batteries) or 100AH of LiFePO4
• 200W of panel
• a 20A charge controller

if he lives in the Pacific northwest he might need

• 3 days of reserve, totaling 200AH of power
• 400AH of lead acid (2p2s) or 200AH of LiFePO4
• 400W - 800W of panel
• one or more 40A charge controllers

If Joe is willing to augment his solar with another charging source he can significantly reduce panel and charge controller requirements. The battery bank would stay the same; it just gets filled by more than one source.

maxxed system

In nomad solar configurations the order of sizing calculations may be different due to space or budget limitations. Few of us will be able to fit all the solar panels we need to run every load on our wishlist. In this case we size our systems for the maximum amount of panels we can fit in the space allowed. This might be what fits in open space on our roof, or how much portable panel we are willing to deploy.

• max solar wattage you can install (or afford)
• whether your have access to other forms of charging: shore power, alternator
• max battery capacity you can charge fully/consistently (or afford, or install)
• these factors will tell you what kind of loads you can run.

real solar installations

See the self-reported capabilities of real life configurations.

“ You have to build for winter, and figure out what to do with the extra electricity in summer.^[7]”

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.

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. 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.

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.

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^[8] 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 chemistries^[9] 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

• if you have undersized solar
• you have lead-chemistry battery bank

You will need somewhat less Ah capacity

• if you run loads in the daytime instead of at night
• if you have oversized solar
• you have lithium-chemistry battery bank((can be ~0.62% the size of the lead bank, due to deeper DoD.))

solar charging

alternator charging

For many people living in vehicles alternator charging + modest solar will be the best-performing system for the dollar. The ratio of camping vs driving^[14] will affect the solar/isolator/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 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^[15]

More expensive 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 0.5C)) for a few hours every few days.
• you have enough solar to Absorp/Float your lead batteries but not enough for bulk.

Cargo trailers^[16] 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 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 shade panels
• large parasitic loads, due to the common assumption they will be hooked up to a power pedestal at a paid campsite

example setups

Yes, we can use a battery monitor to see how much power is going into the battery bank. And we can look at the solar charge controller to see how much power it is making.

But how can we tell how much untapped power does our solar have in reserve? Put another way: how much more stuff could we be running off the solar?

TLDR

• it is easy to test whether or not you can run a specific load
• it takes a lot of "seat time" with the system to predict how much excess power you have at a given moment
• it requires some math to predict whether or not you can run a load on average at a given place/time

about these summaries

direct testing

Plug it in and see if the solar can support it without affecting the battery bank. Easy peasy.

The downside is this is a YES/NO answer; it tells you a specific load will run but does not tell you about how much power is actually available. If you want to see the Bigger Picture then read on...

prediction

  panel wattage x system efficiency x cosine

Solar panel sizing for different geographical areas

This page is a rough attempt to show how geographical location and meteorological conditions affect the amount of solar wattage one needs.

Reminder:** available sunlight (insolation) is expressed in Hours of **Full Sun Equivalent (FSE)** compared to lab conditions. So an entire cloudy day might provide only 2 hours of FSE, meaning it would be the same amount of sunlight as 2 hours of full sunlight under lab conditions (1000w/sq meter). Therefore **2 hours of FSE = 2kWh.((and 7.2MJ (megajoules) for our metric friends)) Multiply [FSE for the time/place] x [rated panel wattage] to see what see how many watts your system would harvest on average if it were perfectly efficient.((hint: it's not)). Example: 4 hours of FSE x 200w = 800Wh, or 0.8kWh, before various losses.^[21]

Historical weather info is also used to calculate FSE, so good/bad weather is already baked into the average.

representative areas

These locations represent the least, average, and most sunlight available in the Continental US. The data is taken from this table.

Worst/lowest FSE tends to occur in December, and most/highest FSE in July.

^ Location ^ Average FSE ^ Worst FSE ^ Best FSE | | Seattle, WA | 3.53 | 0.90 | 6.27 | | Salt Lake City, UT | 4.53 | 1.97 | 6.98 | | Phoenix, AZ | 5.38 | 2.75 | 7.7 |

Note that during periods of poor insolation Phoenix has over 3x the sunlight as Seattle. In summer though, it only has 1.2x as much sunlight:

> "The difference in bright sunlight between the least efficient and the most is little. What a panel does in cloudy conditions can make or break your system." -- jimindenver^[22]

Amount of solar wattage required per 100Ah of Pb

These are crude estimates about the panel wattage required to fully charge 100Ah of AGM from 50% DoD each day. See the list of assumptions below the chart.

^ Location ^ MPPT only ^ PWM only ^ w/ isolator ^ w/ DC-DC charger or generator ^ | Seattle - avg | 280w | 329w | 140w | 103w | | Seattle - Dec | 921w | 1056w | 528w | 388w | | Seattle - Jul | 163w | 192w | 82w | 60w | | | | | | | | SLC - avg | 238w | 281w | 119w | 88w | | SLC - Dec | 473w | 556w | 236w | 174w | | SLC - Jul | 156w | 183w | 78w | 57w | | | | | | | | Phoenix - avg | 200w | 235w | 100w | 73w | | Phoenix - Dec | 369w | 434w | 184w | 135w | | Phoenix - Jul | 149w | 175w | 74w | 55w |

Assumptions:

• panel output is temperature-derated according to daily average high at each time/place
• no other loads other than charging are present
• AGM deep cycle battery is discharged to 50% DoD nightly and fully charged the next day. Charging efficiency 90%.
• MPPT controller is 95% efficient
• PWM yields, overall, 85% of MPPT yield. PWM yields will be quite close to MPPT yields when other forms of charging is present, as PWM will no longer be hamstrung by low Vbatt.
• isolator charging is done in early morning, every morning, at ~C/3((~33A, AGM max charging current)), and discontinued when alternator no longer raises voltage.((~14.4v?)) The chart assumes at that point ~50% of charging will be done.
• DC-DC (or generator) charging is is done in early morning every morning, at C/3,((~33A, AGM max charging current)) and discontinued when Vabs is achieved^[23]. We assume at this point 63.3%% of charging will be done. The practical difference between alternator charging and DC-DC or generator charging is the latter will charge all the way to Absorption voltage.
• temperature-related derating baked in based on monthly average high temperatures
• due to lower internal resistance lithium will accept more charge from alternator/generator than AGM by the time it reaches the charging source's voltage setpoint, thereby requiring somewhat less panel wattage. Likewise, flooded batteries will accept less than AGM and then require more panel.
• solar-only charging the same number of amps into lithium would require about 10% less panel than AGM due to better charging efficiency. Lithium can be charged successfully off alternator/generator only so no panel would be required.