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electrical:solar:sizing

Words of Wisdom: “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.” akrvbob1)

Sizing a solar installation

Sizing residential off-grid solar configurations is relatively easy: calculate power needs in the worst case scenario (December in a particular geographical area)2) and pay for it.

Campervan solar configurations have a more variables:

  1. worst case scenario loads (the max loads you will apply in any 24hr period3))
  2. 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
  3. days of reserve (ie with no solar charging) you want
  4. whether or not you have access to other forms of charging: shore power, alternator, generator
  5. the kind of battery chemistry - some4) charge more efficiently than others
  6. These factors will tell you size of the system needed. 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.
Some luxuries like A/C are impractical on typical solar installs and are therefore relegated to shore power or generator scenarios.

As a sanity check of what's actually being done out there, here is a list of some descriptions of working (rather than theoretical) solar setups.

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). There are two different kinds of loads 'dwellers usually run, AC (110v) and DC (12v).

how much power does stuff use?

Luckily there are stickers on most products that say how much power they take to run. Sometimes it will be expressed in Amps and Sometimes in Watts. That's ok, because the math is easy: Watts (W) = Volts (V) * Amps (I).5)

AC loads

Let's say you are a minimalist and the only 110v appliances you take are a hot dog cooker and lava lamp.

Hot Dogger: 110v * 5A * 02.5hrs = 137.5Wh
lava lamp: 110v * 1A * 4hrs = 440Wh

Combined this would be 577.5Wh. But inverter losses apply to all 110v loads in your van, so multiply by 1.1.
440Wh * 1.1 = 484Wh.

And since we are going to run it off a nominal 12v system we will divide by a typical “12v” voltage, which is usually stated as 13v.6)
484Wh / 13 = 37.23Ah

DC loads

DC appliances are usually labeled in Amps, so the math is easy.

Batman projector light: 2A * 1hr = 2Ah
Heated bunny slippers: 5A * 3hrs = 15Ah
Spinning disco ball: .5A * 3hrs = 1.5Ah

This is 18.5Ah/day.

total loads

37.23A + 18.5A = 55.73Ah/day7)

calculators

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. – hellexual8)

Solar power 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 power9) you need if there is little 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

usable capacity in Amp-hours

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

voltage

Most camper solar installs run at nominal 12v. Because of this most accessories are 12v.

You may want to consider running a 24v system if:

  • your panels are rated at 600w or higher - Ohm's law tells us that 600w / 12v = 50A but 600w / 24v = 25A. A 24v system can use a much smaller/cheaper charge controller and small/cheaper wiring to handle the same amount of power.
  • the run from panels to charge controller is lengthy
  • you will be using inverters heavily - inversion from 24vdc to 110vac is more efficient than inversion from 12vdc.

There are downsides to 24v, however. There are fewer 24v accessories, and you cannot use a chassis ground as it is already grounded to the coach's 12v system.

solar panel array

Pick a solar panel array output on battery bank capacity. A common rule of thumb for matching panels to battery bank is that there should be 1-2 panel watts 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.” – SternWake10)

Here are some examples using this model:

  • 1:1 -
  • 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 / 3.11)
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 4.12)
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:

  1. voltage from panels must not exceed controller voltage, ever.
  2. controller rating should be the panels' output plus a safety margin, unless you are overpaneling 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

  1. high current / short duration during Bulk ←- generator, shore power converter and alternator charging do this best
  2. 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 'dweller 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.

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

real solar installations

electrical/solar/sizing.txt · Last modified: 2020/06/18 08:03 by frater_secessus