Building a dual-axis arduino solar tracker increases your panel yield by up to 30% compared to a fixed mount, according to NREL research on tracking photovoltaic systems. But moving panels requires continuous power for microcontrollers and high-torque servos, and if you are running an off-grid AC load alongside it, your energy storage system must be sized to handle both the continuous draw and the inductive surge of the AC load. To reliably run a dual-axis tracker and a 300W AC water pump off-grid, you need a 12V 100Ah LiFePO4 battery paired with a 500W pure sine wave inverter. Below is the exact sizing math, architecture, and component decision path to build this system without brownouts or battery degradation.

System Block Architecture: Source to Load

A standalone tracking system is not just a solar panel and an Arduino; it is a complete microgrid. Here is the power flow from source to load for a 12V nominal system:

  1. Source: 200W Monocrystalline Solar Panel (mounted on the tracker).
  2. Regulation: 20A MPPT Charge Controller (steps panel Vmp ~18V down to battery charging voltage ~14.4V).
  3. Storage: 12V 100Ah LiFePO4 Battery Bank.
  4. DC Load Tap: 12V-to-5V 10A Synchronous Buck Converter (powers the Arduino Uno and two MG996R servos).
  5. AC Load Tap: 500W Pure Sine Wave Inverter (powers the 300W AC water pump).

The MPPT controller manages the panel output, but the battery acts as the system's voltage buffer. The Arduino and servos draw directly from the DC-DC buck converter to avoid the 15-20% efficiency penalty of converting 12V DC to 120V AC and back to 5V DC.

Battery Sizing Math: Peukert, DoD, and Efficiency

Sizing the battery requires calculating the daily Watt-hour (Wh) demand and applying efficiency and chemistry derating factors.

Load Calculation

  • AC Pump: 300W running for 2 hours/day = 600Wh.
  • Arduino Tracker: 15W average (Arduino + LDRs + servo movement) for 12 hours/day = 180Wh.
  • Total Daily Load: 780Wh.

Efficiency and Peukert Derating

Inverters are not 100% efficient. A quality pure sine inverter operates at about 85% efficiency under typical loads. Therefore, the battery must supply: 780Wh / 0.85 = 917Wh.

Next, we apply Peukert's Law, which accounts for capacity loss at higher discharge rates. While Peukert's exponent ($k$) is typically 1.3 for lead-acid batteries (meaning heavy loads drastically reduce usable capacity), LiFePO4 chemistry has a Peukert exponent of roughly 1.05. This near-ideal flat discharge curve is exactly why lithium is mandatory for motor-driven loads.

A 12V 100Ah LiFePO4 battery holds 1280Wh nominally (12.8V × 100Ah). Applying a conservative 80% Depth of Discharge (DoD) limit to preserve cycle life yields 1024Wh of usable energy. Since 1024Wh > 917Wh, a single 12V 100Ah LiFePO4 battery is mathematically sufficient for one day of autonomy.

Series vs Parallel & Cell Limits

If you need more capacity or voltage, how you wire your batteries changes the system dynamics entirely.

ConfigurationVoltageCapacity (Ah)Total Energy (Wh)Consequence & Use Case
1x 12V 100Ah12.8V100Ah1280WhBaseline. High current draw on 12V wires requires thick 2 AWG cables.
2x 12V 100Ah (Series)25.6V100Ah2560WhDoubles voltage, halves current for the same wattage. Requires a 24V inverter and MPPT.
2x 12V 100Ah (Parallel)12.8V200Ah2560WhKeeps 12V system but doubles Ah. Massive current potential; requires strict busbar balancing.
Lithium Fire & Safety Warning: Never wire mismatched lithium cells or batteries in parallel. If two LiFePO4 batteries with different internal resistances or states of charge are paralleled, the higher-voltage battery will dump massive, unregulated current into the lower-voltage battery, potentially melting terminals and causing thermal runaway. If you must parallel, use identical batteries from the same batch, top-balance them to exactly 3.65V per cell before connecting, and use a BMS rated for the combined fault current.

Charge and Discharge Limits

LiFePO4 batteries are governed by strict C-rate limits to prevent lithium plating and BMS tripping:

  • Charge Limit: 0.5C maximum. For a 100Ah battery, your MPPT controller should be hard-limited to 50A of charge current.
  • Discharge Limit: 1.0C continuous. Your 100Ah battery can safely output 100A continuous (1280W), which easily covers our 917Wh daily draw and the inverter surge.
  • Temperature Cutoff: Charging must be disabled below 0°C (32°F) to prevent permanent anode damage. Ensure your BMS has low-temperature charge protection (LTCP).

Inverter and Charger Sizing for the AC Load

The 300W AC water pump is an inductive motor load. Motors draw a locked-rotor or startup surge current that is 3 to 5 times their running wattage.

The Math: 300W running × 4 (surge multiplier) = 1200W surge requirement.
Furthermore, the Victron Phoenix inverter lineup specifies that motor loads require pure sine wave output; modified sine waves will cause the pump motor to overheat and hum due to harmonic distortion.

To handle a 1200W surge and a 300W continuous load, you need an inverter rated for at least 500W continuous and 1000W+ peak. A 500W inverter operating at 300W is running at 60% load, which is exactly where high-frequency inverters hit their peak efficiency curve.

Arduino Tracker Power Tap: Avoiding Brownouts

The most common failure point in an arduino solar tracker is the 5V power rail collapsing when the servos move. Standard MG996R servos draw about 500mA running, but their stall current is 2.5A each. If both servos hit a mechanical bind simultaneously, they will pull 5A. The Arduino Uno draws another 50mA.

Do not use a standard LM2596 buck converter; they are rated for 2A and will overheat and drop voltage, resetting your Arduino. You must use a synchronous buck converter rated for at least 10A (like the DROK 12V to 5V 10A step-down module). Wire the servos directly to the 5V and GND outputs of the buck converter, and feed the Arduino's 5V pin directly. Bypass the Arduino's onboard linear regulator entirely to prevent it from melting under the servo load.

Wiring Tip: Place a 2200μF electrolytic capacitor across the 5V and GND rails right at the servo power input. This acts as a local energy reservoir to absorb the microsecond current spikes when the servo motors commutate, keeping the Arduino's logic voltage perfectly flat.

Decision Tree: Picking Your Exact Power Components

Use this decision matrix to select your exact hardware based on your final AC load profile. For the baseline 300W pump and tracker system outlined above, follow the default path.

Condition / Load ProfileBattery PickInverter PickMPPT Pick
IF AC Load < 150W (e.g., LED lighting, small router) AND no motor surges. 12V 50Ah LiFePO4 250W Pure Sine Inverter 10A MPPT Controller
IF AC Load is 200W - 400W with motor surges (e.g., 300W water pump, fridge). [DEFAULT RECOMMENDATION] Renogy 12V 100Ah Smart LiFePO4 Victron Phoenix 12/500 Inverter Victron SmartSolar 100/20 MPPT
IF AC Load > 800W continuous (e.g., large well pump, microwave). 24V 100Ah LiFePO4 (2x 12V in series) Victron Phoenix 24/1200 Inverter Victron SmartSolar 150/35 MPPT

The Default Pick: For the standard arduino solar tracker powering a 300W AC pump, buy the Renogy 12V 100Ah Smart LiFePO4 (approx. $279), the Victron Phoenix 12/500 VE.Direct Inverter (approx. $215), and the Victron SmartSolar MPPT 100/20 (approx. $115). This combination provides the exact surge headroom, Peukert-efficient chemistry, and Bluetooth telemetry required to keep your tracker aligned and your pump running without tripping the BMS or browning out the microcontroller.