Off-Grid Solar Basics for a Small Cabin or Shop

Off-grid solar is arithmetic first, shopping second. The kits that skip the load list are how people buy 400 watts of panel and a microwave and then blame the sun. This guide walks a beginner through a small cabin or shop: lights, phones, a laptop, maybe a fridge. It is not a whole-house design and it is not permission to skip code.

Electricity can kill you and can burn a cabin down. Use listed equipment (UL or equivalent), put fuses or breakers on the right wires, install a DC disconnect you can reach, and hire a licensed electrician for anything that looks like a service panel, a ground rod decision you don’t understand, or a connection to a generator. Local rules apply. Nevada County’s public handout is a good example of how seriously jurisdictions take this: Off-grid solar handout (PDF).

The five pieces, in order

1. Loads — what you run, in watt-hours per day
2. Battery — stores night and cloudy-day energy
3. Panels — replace what you used, in your worst sun month
4. Charge controller — protects the battery from the array (MPPT for most modern arrays)
5. Inverter — makes AC if you need wall-outlet power; size for surge (fridge, tools)

Plus: wire, strain relief, fuses, a ground scheme the electrician signs off on, and honest expectations about winter.

Step 1 — Make a load table (do not skip this)

For each device write: watts × hours per day = watt-hours (Wh).

Example starter cabin (yours will differ):

Device	Watts	Hours/day	Wh
LED lights (total)	40	5	200
Phone + laptop charging	80	3	240
12V fan	20	4	80
12V fridge (average)	50	24	1,200
Total			1,720 Wh

Add 20–30% because you will forget something. Round to ~2,200 Wh/day.

A space heater or a big air compressor does not belong on a starter array. Efficiency (LEDs, a good fridge, not heating with electricity) is cheaper than extra panels.

NREL’s PVWatts tool helps you understand sun at your site later: PVWatts.

Step 2 — Size the battery from days you want to survive

Decide days of autonomy (often 1–2 for a weekend cabin you can leave; 2–3 if you live there). Decide chemistry.

LiFePO4 is common now: you can use more of the rated capacity (often designed around ~80% usable). Lead-acid is cheaper up front and hates being drained; usable capacity is much less of the sticker number.

A planning formula people actually use:

Battery Wh ≈ (daily Wh × days) ÷ (usable fraction × inverter efficiency)

Using 2,200 Wh, 2 days, 0.8 usable, 0.9 inverter:

2,200 × 2 ÷ (0.8 × 0.9) ≈ 6,100 Wh. That is a 48V 120Ah class bank, or a 24V 250Ah class bank—not a single “car battery.”

Place batteries in a dry, protected spot per the manufacturer. Vent lead-acid. Lithium wants a battery management system and usually a compatible inverter.

Step 3 — Size the array for the worst month, not July Instagram

Find peak sun hours for your location in December (north of the equator). PVWatts or a solar installer chart. Suppose winter gives you 3 peak sun hours.

Panels do not deliver sticker watts. A 0.70–0.80 derate for dirt, heat, wire, and controller loss is a sober beginner assumption.

Array watts ≈ daily Wh ÷ (sun hours × derate)

2,200 ÷ (3 × 0.75) ≈ 980 W. Add margin; a 1,200 W array is a reasonable conversation with a supplier, not a promise.

Face true south, unshaded from 9–3. Ground mounts are easier to clear snow. Roof mounts need structure and flashing done right.

Step 4 — Charge controller (usually MPPT)

MPPT controllers squeeze more out of a high-voltage array and are the default for cabin systems.

1. Array watts ÷ battery charging voltage × 1.25 (continuous margin) ≈ controller amps. Example: 1,200 W into a 24V bank → 1,200 / 24 = 50 A, × 1.25 = 62.5 A → buy a 60–80A controller as specified by the manufacturer, not a guess from a forum.
2. Check cold-weather Voc. Panel voltage rises in the cold. If winter Voc exceeds the controller’s max input, you let the smoke out. This is a real spec-sheet step, not optional.

PWM controllers can be fine on tiny, matched 12V toy systems. Don’t pair a big 60-cell array with a cheap PWM on a 12V battery.

Step 5 — Inverter: continuous and surge

• Add the watts of everything that might run at once.
• Then look at motor surge (fridge, well pump, circular saw)—often 2–3× running watts for a second.

A 2,000 W pure sine inverter that can surge higher is a common small-cabin size; a 700 W unit will not start many fridges. Cheap modified-sine inverters make tools and some electronics unhappy.

Idle draw matters. A big inverter left on 24/7 can eat your lights budget.

Step 6 — Wire, fusing, disconnects, ground

This is where beginners get hurt.

• Use the wire gauge the controller/inverter manual specifies for the current and the run length. Undersized cable heats up.
• Fuse at the battery on the positive cable, rated for the cable and the device.
• A DC disconnect between array and controller, and a way to isolate the battery, are not decorations.
• Strain-relief every cable through metal walls.
• Bonding and grounding: follow the equipment manuals and an electrician. Do not invent a ground rod story from a video.

Energy.gov’s homeowner solar pages are a sane overview: Solar energy at home.

Step 7 — A realistic first system

Many people should start DC-coupled: 12 or 24V lights and a fridge that can run DC or a small inverter only when needed. Every conversion to AC wastes energy.

Live on the load list for a week with a Kill-A-Watt meter on the shore power version of your devices if you can, then buy panels.

What usually goes wrong

• Shopping panels first
• No fuse at the battery
• Winter Voc surprise
• A 1,500 W heater “just in the evening”
• Unlisted Amazon specials with no manual

Further reading

• NREL PVWatts — sun at your address
• Energy Saver: solar at home
• Energy Saver: off-grid or stand-alone systems
• Verasol component-based off-grid design guidelines (PDF)
• Licensed electrician + your AHJ (building department) before you energize anything that looks like a dwelling service

If the load list is honest and the fusing is real, the sun is the easy part.