To know what solar panel you need, calculate your daily Watt-hours (Wh), divide by your location's peak sun hours, and multiply by 1.3 to account for system losses. For a typical 12V off-grid cabin consuming 1,200Wh daily in a region with 4 peak sun hours, you need a minimum 390W solar array. The direct answer for this baseline setup is one 400W monocrystalline panel (such as the Renogy RNG-400D), paired with a 30A MPPT charge controller and a 12V 100Ah LiFePO4 battery.

Sizing an off-grid power system isn't about guessing based on panel marketing. It requires tracing the energy from the source to the load, applying real-world derating factors, and respecting the electrochemical limits of your battery bank. Here is the exact bench-tested methodology to size your array, battery, and inverter without overbuilding or suffering mid-winter brownouts.

The Core Sizing Formula: From Load to Panel Wattage

The most common mistake DIYers make is sizing panels based on the battery's amp-hour (Ah) rating. You must size the array based on your load. The fundamental equation for solar array sizing is:

Required Array Wattage = (Daily Wh Load × System Loss Factor) / Peak Sun Hours (PSH)

  • Daily Wh Load: The sum of all AC and DC devices used in 24 hours. (e.g., 60W fridge running 50% of the day = 720Wh; 20W LED lights for 6 hours = 120Wh; 60W laptop for 6 hours = 360Wh. Total = 1,200Wh).
  • System Loss Factor (1.3): This 30% buffer accounts for MPPT controller heat dissipation, voltage drop in DC wiring, panel dust, and high-temperature voltage degradation. Panels lose roughly 0.3% efficiency per degree Celsius above 25°C.
  • Peak Sun Hours (PSH): Not daylight hours, but the equivalent hours of full 1,000 W/m² irradiance. Use the NREL PVWatts Calculator to find your location's worst-month PSH. For a year-round cabin in Colorado, use 4.0 hours to survive the shoulder seasons.

Worked Example: (1,200Wh × 1.3) / 4.0 PSH = 390W. You would purchase a single 400W panel or two 200W panels.

System Block Architecture: Source to Load

Before wiring, visualize the energy path. A robust 12V system follows this strict block sequence:

  1. Source: Solar Array (400W, ~32V Vmp, ~12.5A Imp).
  2. Regulation: MPPT Charge Controller (Steps down 32V to 13.5V-14.4V charging voltage, steps up current to ~29A).
  3. Storage: Battery Bank (12V nominal, 100Ah LiFePO4, acting as the system's voltage buffer).
  4. Conversion: Pure Sine Wave Inverter (12V DC to 120V AC).
  5. Load: AC Breaker Panel and DC Fuse Block.
Pro-Tip: Always connect the battery to the charge controller before connecting the solar panels. The controller needs to read the battery voltage to auto-detect whether it is a 12V or 24V system. Connecting panels first can fry the controller's logic board.

Battery Sizing: Peukert, C-Rates, and Chemistry

Your battery bank must store enough energy to cover your daily load plus a reserve for cloudy days (autonomy). However, you cannot simply divide Wh by 12V to get Ah. You must account for Depth of Discharge (DoD) and Peukert's Law.

Peukert's Law dictates that the faster you draw current from a battery, the less total capacity it yields. The formula is t = H * (C / I)^k, where k is the Peukert exponent. For Lead-Acid/AGM batteries, k is typically 1.3. If you pull 50A from a 100Ah AGM battery rated at a 20-hour discharge rate, your usable capacity drops to roughly 60Ah. Lithium Iron Phosphate (LiFePO4) batteries have a k value near 1.05, meaning a 100Ah LiFePO4 battery will still yield ~95Ah even under a heavy 50A load.

Charge and Discharge Limits (C-Rates)

For our 1,200Wh daily load, assuming 2 days of autonomy, we need 2,400Wh of usable storage. At 12V, that is 200Ah usable. Because LiFePO4 allows an 80% to 100% DoD (unlike AGM's 50% limit), a single 12V 200Ah LiFePO4 battery, or two 100Ah batteries in parallel, is required.

  • Charge Limit: LiFePO4 cells should be charged at a maximum of 0.5C. A 100Ah battery can accept up to 50A of charge current. Our 400W array pushing ~30A is perfectly within this safe zone.
  • Discharge Limit: Standard BMS units allow 1C continuous discharge (100A from a 100Ah battery), yielding 1,200W of continuous 12V DC power.
⚠️ Lithium Fire-Safety Callout: While LiFePO4 is vastly more stable than NMC lithium-ion and highly resistant to thermal runaway, short circuits or BMS failures can still cause catastrophic venting and fire. Never parallel mismatched cells or batteries of different ages/chemestries. Internal resistance differences will cause one battery to dump massive current into the other, melting busbars. Always install a Class ABC fire extinguisher near the battery bank and adhere to NFPA 855 spacing guidelines for energy storage systems.

Array Wiring: Series vs. Parallel Consequences

If you opt for two 200W panels instead of one 400W panel, you must choose how to wire them. This decision fundamentally alters your Voltage (V) and Amp-hours (Ah), dictating your wire gauge and charge controller selection.

Wiring Method Voltage Consequence Current (Amps) Consequence Best Used When...
Series Voltages add (e.g., 2x 20V = 40V). Ah stays the same. Current remains low (~10A). Allows thinner, cheaper PV wire. Using an MPPT controller with long wire runs from roof to battery.
Parallel Voltage stays the same (20V). Ah (current capacity) adds. Current doubles (~20A). Requires thicker wire and fusing per panel. Panels face different directions or suffer from partial shading.

Crucial MPPT Limit: If wiring in series, always check the Open Circuit Voltage (Voc) of your panels at the lowest expected winter temperature. Voltage rises as temperature drops. If your series Voc exceeds the MPPT controller's maximum input voltage (usually 100V or 150V), you will permanently destroy the controller.

Inverter and Charge Controller Sizing

Sizing the inverter and charge controller requires looking at peak surge loads and NEC-style wire ampacity rules.

Inverter Sizing

Your continuous load might be 400W, but a refrigerator compressor requires a 3-second startup surge of 1,200W. You need a 2,000W Pure Sine Wave Inverter (like the Victron Phoenix 12/2000) to handle surges safely without tripping the low-voltage cutoff.

Wire Sizing Math: A 2,000W inverter pulling from a 12V battery at the low-voltage cutoff of 11.5V draws 173 Amps (2000 / 11.5). Applying the NEC 125% continuous load safety margin (173 × 1.25) equals 216 Amps. You must use 2/0 AWG copper welding cable for the inverter-to-battery run, kept under 5 feet long to minimize voltage drop.

Charge Controller Sizing

For a 400W array charging a 12V battery, the maximum output current is 400W / 13.5V = 29.6 Amps. A 30A MPPT Charge Controller (such as the Victron SmartSolar MPPT 100/30) is the exact mathematical fit. Do not use a PWM controller; PWM clamps the panel voltage down to the battery voltage, instantly wasting 30% of your array's wattage as heat. Consult the Victron Wiring Unlimited guide for exact busbar and fuse placement diagrams.

Decision Tree: Picking Your Exact Hardware

Stop guessing. Match your daily Watt-hour load to the table below to find your exact bill of materials. This decision path terminates in concrete, bench-tested part recommendations.

Scenario Daily Load Solar Array Battery Bank Controller & Inverter
A: Weekend Camper 600 Wh 200W Mono 100Ah AGM (50% DoD) 20A PWM / 1000W Inverter
B: Standard Cabin (DEFAULT PICK) 1,200 Wh 400W Mono 12V 100Ah LiFePO4 MPPT 100/30 / 2000W Inverter
C: Heavy Off-Grid 3,000 Wh 2x 400W (Series) 24V 200Ah LiFePO4 MPPT 150/45 / 3000W Inverter

For the vast majority of DIYers building a reliable, year-round 12V system, Scenario B is the gold standard. By pairing a 400W monocrystalline panel with a 12V 100Ah LiFePO4 battery and a 30A MPPT controller, you eliminate the Peukert penalties of lead-acid, keep your DC wiring safely under 40 Amps, and guarantee enough daily harvest to keep the lights on through the shoulder seasons.