If you are running a typical 12V off-grid van, cabin, or workshop drawing 1,000Wh per day, you need 400 watts of solar panels, paired with a 12V 100Ah LiFePO4 battery and a 40A MPPT charge controller. This baseline assumes 4 peak sun hours and standard system inefficiencies. If your daily load is higher or lower, the exact array size scales linearly, but the math requires factoring in inverter losses, battery depth-of-discharge (DoD), and local solar irradiance.

The Source-to-Load Power Path

Before calculating wattage, you must understand the system block architecture. Power flows in one direction, and every handoff introduces losses:

  1. PV Array (Source): Generates high-voltage, variable DC.
  2. MPPT Charge Controller: Steps down PV voltage to battery charging voltage while maximizing current. Operates at ~95% efficiency.
  3. LiFePO4 Battery Bank (Storage): Stores DC energy. The internal Battery Management System (BMS) protects against over/under-voltage and thermal runaway.
  4. Inverter: Converts 12V DC to 120V AC for standard appliances. Operates at 85% to 92% efficiency depending on the load curve.
  5. AC/DC Loads (Destination): Your fridge, lights, laptop chargers, and power tools.

When sizing the solar array, we work backward from the destination (the loads) to the source (the panels), adding a buffer for every conversion step.

Sizing Math: From Load Watts to Solar Array

Let us run the exact math for a 1,000Wh/day load profile (e.g., a 50W 12V fridge running 24/7 = 1,200Wh, but cycling on/off 50% of the time = 600Wh, plus 400Wh of evening lighting and laptop charging).

Step 1: Account for Inverter Efficiency

If your loads are AC, the inverter wastes energy as heat. Assuming a conservative 85% inverter efficiency, the DC energy pulled from the battery is:

1,000Wh / 0.85 = 1,176Wh required from the battery.

Step 2: Battery Capacity and Peukert's Law

With lead-acid batteries, Peukert's Law dictates that drawing high current drastically reduces usable capacity (a Peukert exponent of $k \approx 1.3$ can halve your capacity at high draws). LiFePO4 chemistry has a Peukert exponent near 1.05, meaning you get virtually the full rated Amp-hour capacity regardless of draw speed.

For LiFePO4, we size based on Depth of Discharge (DoD). While lithium cells can physically discharge to 100%, capping DoD at 90% extends cycle life from 2,000 to over 4,000 cycles.

1,176Wh / 0.90 DoD = 1,306Wh required battery capacity.

At a nominal 12.8V, this equals 102Ah. A standard 12V 100Ah LiFePO4 battery (1,280Wh total) is the perfect fit.

Step 3: Calculate Required Solar Watts

You must replace the 1,176Wh drawn from the battery in a single day. Using the NREL PVWatts Calculator, the US average yields about 4 Peak Sun Hours (PSH) per day.

1,176Wh / 4 PSH = 294W of raw panel output.

However, we must apply a 0.77 derating factor for real-world losses (dirt, heat degradation, wire voltage drop, and MPPT conversion).

294W / 0.77 = 381W.

Result: You need a 400W solar array to reliably sustain this load.

Battery Architecture: Series vs. Parallel and C-Rate Limits

When expanding your battery bank, how you wire the cells dictates your system voltage and capacity.

  • Series Wiring: Connects the positive of Battery A to the negative of Battery B. Consequence: Voltage adds up, Amp-hours remain the same. Two 12V 100Ah batteries in series yield 24V at 100Ah (2,560Wh). This is ideal for reducing current and allowing smaller AWG wire gauges on high-power systems.
  • Parallel Wiring: Connects positives to positives, negatives to negatives. Consequence: Voltage remains the same, Amp-hours add up. Two 12V 100Ah batteries in parallel yield 12V at 200Ah.
Lithium Fire-Safety & BMS Mandate: Never parallel mismatched cells, different brands, or batteries with different ages/internal resistances. If one cell group drops in voltage, the stronger bank will dump massive, unregulated current into the weaker bank, bypassing the BMS charge limits and risking thermal runaway. Always use identical, same-batch LiFePO4 batteries, and ensure every drop-in battery contains an internal BMS rated for your maximum fault current. For large custom banks, use a dedicated external BMS with active cell balancing.

Charge and Discharge Limits (C-Rates)

A 100Ah LiFePO4 battery is governed by its C-rate. The standard safe limits are:

  • Charge Limit (0.5C): Maximum 50A of continuous solar charge current. Pushing 100A (1C) into a 100Ah battery will degrade the anode and trip the BMS high-current cutoff.
  • Discharge Limit (1.0C): Maximum 100A continuous draw (1,280W). For a 12V system, a 1,000W inverter pulling 85A is safely within this limit, but a 2,000W inverter pulling 170A will trigger the BMS low-voltage or over-current disconnect.

Inverter and Charge Controller Sizing

With a 400W array and a 100Ah battery, your power electronics must be sized to handle the bottlenecks.

The MPPT Charge Controller

A 400W array at a 12V battery charging voltage (14.4V) generates roughly 27.7A of current (400W / 14.4V). However, MPPT controllers are rated by their maximum output current. You must buy a 40A MPPT controller. Furthermore, ensure the controller's maximum PV open-circuit voltage (Voc) rating exceeds your array's cold-temperature Voc. If wiring four 100W panels in a 2S2P configuration, the string Voc will be around 46V. A standard 100V max PV input controller handles this easily.

The Inverter

Size the inverter for your largest simultaneous AC load plus a surge buffer for inductive motor starts (like a fridge compressor or power tool). For a 1,000Wh daily load profile, a 1,000W Pure Sine Wave Inverter with a 2,000W peak surge is the correct spec. Wire it directly to the battery bus bars using 2/0 AWG stranded copper wire, keeping the run under 5 feet to prevent voltage drop.

The Decision Tree: Pick Your Exact Off-Grid Kit

Stop guessing. Use this decision matrix to select the exact hardware tier that matches your daily energy consumption.

Daily Load Profile Solar Array Size Battery Bank (LiFePO4) MPPT Controller Inverter Size
Light (500Wh)
LEDs, phones, small fan		 200W 12V 50Ah 20A MPPT 600W
Medium (1,000Wh)
12V Fridge, laptops, TV		 400W 12V 100Ah 40A MPPT 1,000W
Heavy (2,500Wh)
AC fridge, microwave, coffee maker		 1,000W (24V System) 24V 100Ah (2x 12V in Series) 60A MPPT (24V) 3,000W
The Default 'Weekend Warrior' Pick: If you are building the Medium (1,000Wh) system today, buy the Victron SmartSolar MPPT 100/40 for its superior Bluetooth logging and thermal handling, pair it with a Renogy 12V 100Ah Smart LiFePO4 (which includes built-in Bluetooth BMS monitoring), and use a Samlex PST-1000-12A pure sine inverter. Wire the solar array in 2S2P using 10 AWG UV-rated PV wire, and use 4 AWG THHN stranded copper for the controller-to-battery runs.

Sizing a solar array is not about buying the biggest panels you can fit on the roof; it is about mathematically balancing your daily watt-hour consumption against your battery's C-rate limits and local peak sun hours. Stick to the 400W / 100Ah / 40A baseline for standard 12V off-grid applications, and your system will run reliably without tripping the BMS or draining the cells past their safe DoD threshold.