Cabin Solar Sizing: Watts, Battery & Load Math Explained

How-To Guide · Updated July 2026 · SolarCabin Editorial Team

Every component in a cabin solar system is sized from one number: your daily energy consumption. Get that number right and the rest of the system falls out of four short calculations — array, battery, controller, inverter. Get it wrong and no amount of hardware spending fixes the mismatch.

This guide runs the complete math once, in order, on a single realistic cabin, so you can substitute your own numbers line by line. No shortcuts, no “rules of thumb” that hide the reasoning — just the actual arithmetic.

Step 1: The Load Audit

List every electrical load, its wattage (from the nameplate or a plug-in meter), and honest daily hours. Our example cabin — part-time, three-season, two people:

Two audit rules: use average watts for cycling loads like fridges (they run the compressor maybe a third of the time), and add 10% at the end for the loads you forgot. Ours: 2,010 × 1.1 ≈ 2,200Wh/day design load.

Step 2: Array Watts

Array = Daily Wh ÷ Peak Sun Hours × 1.3.

Our cabin sits in the Midwest and gets used April through October — shoulder-season sun of about 4.5 hours is the conservative basis. So: 2,200 ÷ 4.5 × 1.3 ≈ 635W. Standard hardware rounds that to a 600W array (acceptably close) or 800W (comfortable margin, and margin always gets used). We'll spec 800W: eight 100W panels or four 200W panels.

Step 3: Battery Bank

Bank Wh = Daily Wh × Days of Autonomy ÷ Usable Depth of Discharge.

Autonomy is how long the cabin runs with no sun. We'll choose 2 days — a sane Midwest minimum. On LiFePO4 (90% usable): 2,200 × 2 ÷ 0.9 ≈ 4,890Wh. In 12V terms that's about 400Ah: two 200Ah LiFePO4 batteries in parallel. The same target on AGM (50% usable) would demand nearly 9,800Wh of rated capacity — four times the batteries — which is the arithmetic behind lithium's takeover of cabin storage.

Step 4: Charge Controller

Controller amps = Array W ÷ Bank V × 1.25.

800W into a 12V bank: 800 ÷ 12 × 1.25 ≈ 83A — which is past common controller sizes and a hint that the bank wants to be 24V. At 24V: 800 ÷ 24 × 1.25 ≈ 42A — a 40–50A MPPT fits cleanly, cables shrink, and losses drop. This is the classic decision point: systems above ~500W of array are usually happier at 24V. (Our battery pick simply becomes two 200Ah 12V units in series instead of parallel.) Controller picks by size are in the MPPT guide.

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Step 5: Inverter

The inverter sizes to peak simultaneous load, not daily consumption. Our cabin's realistic worst moment: fridge compressor start (~400W surge) + microwave (1,000W) + lights and electronics (200W) ≈ 1,600W momentary. A 2,000W pure sine inverter with 4,000W surge covers it with headroom. Confirm the inverter accepts 24V input to match the bank.

The Finished Spec Sheet

Substitute your own load audit into line one and the whole sheet recalculates. If you'd rather buy the answer than assemble it, the complete kits guide maps these system sizes to matched packages, and the setup guide takes over on install day.

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Stress-Testing the Design: Three Questions Before You Buy

A finished spec sheet deserves three adversarial questions. What does the worst week look like? Run the bank math against two fully dark days followed by a half-production day — if the model ends with the bank below 20%, either add storage or accept that a generator is part of the design. What happens when loads grow? Re-run the sheet at 130% of your design load, because that's where most cabins land within two years; if the answer requires replacing the controller or inverter rather than adding panels and batteries, buy the bigger controller now. What single failure hurts most? In the example system it's the controller — one unit between the entire array and the bank — which is why experienced off-gridders keep a cheap backup PWM controller in a drawer: crude, but it keeps the fridge cold while the real replacement ships. Ten minutes of pessimism at the spreadsheet stage is the cheapest resilience you'll ever buy.

From Spec Sheet to Shopping List

Translating the finished spec into orders has its own small pitfalls. Panels: buy the array as one order of one model — mixed panels complicate wiring and warranties. Battery: confirm the model's BMS continuous discharge rating exceeds the inverter's maximum draw at your bank voltage, the compatibility check most often missed. Controller: verify both the amp rating from the sheet and the PV input voltage window against your planned string layout, cold-adjusted. Inverter: match input voltage to the bank and confirm surge covers the worst motor start. Then add the unglamorous line items the sheet implies but doesn't name — battery cable and lugs, fuses and holders at every rating the wiring guide specifies, a disconnect switch, and a shunt monitor. The accessories run a tenth of the budget and determine whether the other nine-tenths performs.

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