[ Solar Power Basics ]

Solar is the other half of an off-grid build: the battery stores the
energy, the panel refills it. This is the groundwork: what a panel's numbers
actually mean, how to wire several of them together, and the one component
between panel and battery that decides how much you keep: the charge
controller.

None of it is specific to a big roof array. It's the same physics for a single
100 W panel keeping a Meshtastic relay alive as for a cabin bank. Read the
label, wire two panels the right way, and pick the right controller, and you
won't cook a battery or throw away a third of your power.

A solar panel in daylight behaves like a current source: sunlight sets how
much current it can push, and it delivers that at whatever voltage the load
allows, up to a ceiling. The label's five numbers describe that curve.

  Voc    Open-circuit voltage — nothing connected, the highest voltage
         the panel reaches. This is the number that must stay under your
         controller's max input, and it rises in the cold (see Sizing).
  Vmp    Voltage at max power — roughly 80% of Voc; where the panel
         makes the most watts. A controller's whole job is to hold the
         panel here.
  Isc    Short-circuit current — leads shorted together, the highest
         current the panel can pass. Safe to measure; it won't hurt the panel.
  Imp    Current at max power — a hair below Isc.
  Pmax   Rated watts = Vmp × Imp, e.g. a “100 W” panel.

Two things the label won't shout. First, “12 V” on a panel is a
nominal label, not its real voltage: a 12 V panel has a Vmp around 18 V,
sized to charge a 12 V battery through a controller. Second, every number is
STC: a lab at 1000 W/m² and 25 °C. Real roofs are hotter and hazier
(heat drops the voltage, haze and dust drop the current), so a panel makes
25–30% fewer watts in the real world than its rated Pmax. Size around a
100 W panel behaving like a 70–75 W one. You can confirm Voc and Isc yourself
with a multimeter: Voc across the leads on volts, Isc across them on the
10A jack, in full sun.

Most panels terminate in a pair of MC4 connectors: keyed, click-locking,
weatherproof, and polarised so you can't cross + and − by accident. A panel's
positive lead ends in one MC4 gender and negative in the other, which is what
makes series chaining just plug-into-plug.

MC3 is the older predecessor you'll meet on panels from before ~2011: same
idea, but a single push-together contact with no locking latch. MC3 and MC4
do not intermate. Never force one into the other, and don't mix them in a
string. Old MC3 panels are often labelled “do not disconnect under load”,
since the unlatched contact can't break a DC arc safely; the clean modern fix is
to cut the MC3 ends off and crimp on proper MC4s with the right tool, minding
polarity.

  — Polarity matters. Not every controller protects against reversed
    panel leads; get + and − right before you connect. Red is +, black is −.
  — A lit panel is always live. There's no off switch on the sun. To work
    safely, cover the panel or wait for dark. Don't rely on “it's cloudy.”
  — Don't break a connection under load. DC arcs don't self-extinguish the
    way AC does. Disconnect at the controller first (or cover the panel), so
    no current is flowing when the MC4 pops apart.
  — Size the wire for the array current and the run length; long runs
    drop voltage, and a higher-voltage (series) array loses less to that drop.

The battery side of the controller connects to a pack with its own protection;
if you're building that pack, see the BMS guide.

This is the choice that trips everyone up, and it's just Kirchhoff. Wire panels
end-to-end (series) and the voltages add. Wire them side-by-side, all the
pluses together and all the minuses together (parallel), and the currents
add. Same total power either way. You're just trading volts for amps.

Series: each panel's − to the next panel's +. Voltage adds, current
stays the same. Three 20 V · 10 A panels become a 60 V · 10 A array.

Parallel: all + to one rail, all − to another. Current adds, voltage
stays the same. The same three panels become a 20 V · 30 A array.

Bigger arrays combine the two (strings of panels in series, those strings in
parallel) to hit a target voltage and current. But the choice between them
isn't free: it's dictated by the controller you're feeding, which is the next
section.

A charge controller sits between array and battery. Its job is to stop the
panel from boiling the battery (running the proper bulk / absorb / float
charge stages) and to get as much of the panel's power into the pack as it
can. There are two kinds, and the difference is real money and real watts.

PWM (pulse-width modulation) is, in effect, a fast switch that ties the
panel directly to the battery. Because they're connected, the panel is dragged
down to the battery's voltage, so an 18 V (Vmp) panel charging a 13.6 V
battery runs at 13.6 V, and the 4-ish volts of headroom are simply lost. That's
fine only when Vmp is already close to battery voltage. PWM is cheap, simple,
and reliable for a small, voltage-matched system; its rating is a current limit,
so your array's Isc must sit under it, and the array's nominal voltage must
match the battery's.

MPPT (maximum power point tracking) is a DC-DC converter. It holds the panel
at its Vmp and converts the surplus voltage into extra current on the way to
the battery, the same trick a buck converter does. Volts you'd have thrown
away with PWM come back as charge current, typically 20–30% more harvest, and
more than that in the cold or when the array voltage sits well above the
battery. It also lets you run a high-voltage series string into a low-voltage
battery, so you get thin wire and long runs for free. It costs more and has a
max input voltage you must respect (see Sizing).

The short version:

  — small, cheap, panel Vmp already near battery voltage → PWM.
  — want the most harvest, a series string, long runs, or cold winters
    → MPPT.

On anything bigger than a trickle-charger the MPPT pays for itself in panel you
didn't have to buy.

Three things have to agree: the array, the controller, and the battery.

  — Battery voltage is the anchor: 12, 24, 48 V. You set it in the
    controller (or it auto-detects).
  — Array voltage vs the controller. On PWM, the array's nominal
    voltage must equal the battery's, so you add panels in parallel. On
    MPPT, you add panels in series to raise voltage, up to but safely
    under the controller's max input.
  — Controller current. Size it for the array. On MPPT the output
    current can exceed the array's Imp, because watts in at high volts come
    out at higher amps: roughly array watts ÷ battery volts.

The cold-Voc trap. A panel's Voc climbs as it gets colder, about
+0.3%/°C below 25 °C. A series string reading 90 V on a warm afternoon can push
past 100 V on a clear, freezing morning before any load is drawn, and if that's
over the MPPT's max input it kills the controller. Size the string on its
cold-adjusted Voc, not the number you measured in summer.

  — Treat a lit panel as always on. You can't switch off the sun; cover
    the panel to work on it.
  — Break connections at no load. DC arcs sustain themselves, so open the
    circuit at the controller, or cover the panel, before unplugging.
  — Respect the controller's max input voltage, cold-adjusted. Over it is
    a dead controller.
  — Fuse it. A fuse per parallel string, and a fuse on the battery lead.
    A battery can dump enormous fault current into a short.
  — The battery has its own rules. Charge voltages and protection belong to
    the pack; the BMS guide covers that side.

Read the label, wire for your controller, respect the cold, and fuse both
sides. The rest is just sunshine.

PWM or MPPT: which should I get?
MPPT for almost anything: it harvests ~20–30% more and lets you run
series strings on thin wire. PWM only when the system is small, cheap, and
the panel's Vmp already sits near battery voltage.

Series or parallel for my panels?
Series adds volts (thin wire, MPPT, long runs; but shade-sensitive).
Parallel adds amps (shade-tolerant; needs a fuse per string and fatter
wire). PWM wants parallel at battery voltage; MPPT wants series.

What's Voc vs Vmp?
Voc is the no-load ceiling voltage (keep it under the controller's max).
Vmp is the voltage at peak power, ~80% of Voc, where a controller holds
the panel.

Is a “12V” panel really 12 V?
No, it's a nominal label for charging a 12 V battery. Its Vmp is around
18 V and its Voc higher still.

Why did my MPPT die in winter?
Almost always the cold-Voc trap: Voc rises as temperature drops, so a
string that was fine in summer can exceed the controller's max input on a cold
morning. Size on cold-adjusted Voc.