How to size a solar system for a cabin

Table of contents

What you’re sizing (and why cabins are different)

Most cabin solar systems are off-grid or “mostly off-grid,” which means you’re sizing for both energy (watt-hours per day) and power (peak watts at one moment). Cabins also have two common curveballs:

• Seasonality: winter sun can be dramatically lower than summer in many locations.
• Occasional heavy loads: tools, pumps, or a microwave can spike peak power even if daily energy is modest.

Step 1: Estimate your cabin’s daily energy use (Wh/day)

You don’t need perfect numbers to start. You need a realistic list of what you’ll run on a normal day, and roughly how long you’ll run it.

Watt-hours (Wh) = Watts × Hours per day

A quick cabin load list (common categories)

• Lighting (LEDs)
• Water pump
• Phone/laptop charging
• Refrigeration (often the biggest daily energy draw)
• Fans or small heater loads (season-dependent)
• Occasional tools (higher peak watts; not always high daily Wh)

If you want a fast baseline, start with “critical loads only” and expand later. You’ll make better choices when you size for what you truly need, not everything you might want.

Step 2: Choose autonomy and size battery capacity

Autonomy is how long you can run without meaningful solar input. Many cabin setups aim for 1–2 days of autonomy, then adjust based on weather patterns and how often the cabin is occupied.

Battery Wh ≈ Daily Wh × Days of autonomy ÷ DoD

DoD (depth of discharge) is how much of the battery you plan to use regularly. Using a more conservative DoD can improve longevity.

Step 3: Size solar panels to refill the battery each day

Panel sizing is about replacing what you use daily (plus losses). The most common sizing error for cabins is using “best case summer sun” when you actually need a system that works in shoulder seasons or winter.

Panel watts ≈ Daily Wh ÷ Peak sun hours ÷ Efficiency

Use an efficiency factor like 0.75–0.85 for real-world losses.

Step 4: Size the inverter (continuous + surge)

Your inverter needs to handle your maximum simultaneous AC watts, plus starting surges for some devices (motors, compressors). Oversizing can increase idle losses, so aim for a realistic peak.

Step 5: Pick a system voltage that fits your power level

Voltage choice affects current, cable thickness, and how easy it is to scale. If your cabin system will run higher power loads or longer cable runs, higher voltage can simplify the build.

Common cabin sizing mistakes (and how to avoid them)

• Using summer-only assumptions: if you use the cabin in winter, plan for lower sun.
• Sizing the inverter “just in case”: peak watts drives wiring and battery stress.
• Skipping autonomy planning: batteries are expensive; decide the reserve you actually need.
• Forgetting losses: controller and inverter losses reduce usable energy.

FAQ

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