Solar PV System Sizing Tool

The tool incorporates temperature effects indirectly via the Performance Ratio (PR) input (default 0.8), which bundles module temperature losses, soiling, wiring losses, and inverter inefficiencies per IEC 61724-1. For high-temperature sites (>35°C ambient), we recommend lowering PR to 0.75–0.78 and selecting PV modules with low temperature coefficients (e.g., ≤ −0.35%/°C, per IEC 61215). Battery voltage stability is also affected: at −20°C, lead-acid capacity drops ~40%; lithium iron phosphate (LiFePO₄) maintains >85% capacity down to −20°C (UL 1973, IEEE 1625). Always validate final sizing with local 10-year TMY data—not just average irradiance—to capture seasonal thermal stress.

For remote livestock waterers requiring reliability and deep-cycling, LiFePO₄ is strongly preferred over flooded lead-acid (FLA) or AGM. At 50% DoD, LiFePO₄ delivers 3,000–5,000 cycles (IEC 62620), versus 500–800 for FLA—reducing lifetime replacement costs and maintenance. Its flat voltage curve (2.5–3.65 V/cell) improves pump motor efficiency and enables precise state-of-charge estimation. Crucially, LiFePO₄ operates efficiently from −20°C to +60°C without derating (UL 1973), unlike FLA, which loses ~1% capacity per °C below 25°C (IEEE 1188). Ensure BMS integration with low-temp charge cutoff and comply with NEC Article 706 for installation.

The Performance Ratio (PR) consolidates system-level losses into a single empirically validated metric per IEC 61724-1, enabling rapid, field-applicable sizing without over-parameterization. While granular inputs (e.g., 2% soiling, 1.5% wiring loss) are useful for detailed modeling (PVsyst), they introduce uncertainty in remote deployments where actual conditions—dust accumulation, connector corrosion, or shading—are hard to quantify upfront. A PR of 0.8 reflects typical real-world losses for well-maintained, pole-mounted arrays in semi-arid rangelands. Engineers should adjust PR downward (0.7–0.75) only if site-specific data exists—e.g., NREL’s NSRDB soiling maps or on-site pyranometer validation—avoiding speculative loss stacking.

The tool assumes constant load power (W) over specified hours—a simplification valid for duty-cycle pumps *if* average power (P_load) is calculated from measured duty cycle and peak power. For example: a 1.5 kW pump running 20 min/hour over 8 h yields P_load = 1500 × (20/60) = 500 W. Accuracy depends on representative measurement: use a clamp meter logging over ≥3 days (per IEEE 1459) to capture diurnal variation and startup surges. The output P_pv includes margin for transients via the PR and system loss inputs—but undersizing risk remains if surge current exceeds charge controller rating. Always verify controller max input current ≥ 1.25 × I_sc (NEC 690.8(A)(3)).

Yes—the tool’s outputs align with NEC 690.7(A) voltage calculations and 690.8(A) conductor ampacity requirements when used with engineering judgment. For instance, the recommended P_pv (Wp) must be converted to Isc using module datasheet values (not STC Pmp), then multiplied by 1.25 for continuous load (690.8(A)(2)). Battery capacity (Ah) informs overcurrent protection sizing per 690.8(B)(1): fuse rating ≥ 1.25 × max charge current. However, the tool does not auto-generate compliance documentation—engineers must manually verify conductor sizing (690.31), grounding (690.47), and rapid shutdown (690.12) per local AHJ requirements, especially for systems >30V DC in accessible locations.

Yes—the tool supports 12V, 24V, and 48V battery inputs, but 24V is strongly recommended for livestock waterers >300W. Higher voltage reduces current (I = P/V), cutting resistive losses (P_loss = I²R) by 75% vs. 12V for the same power—critical over long wire runs common in pastures. Per NEC 690.31(C), 24V systems allow smaller conductors (e.g., 10 AWG vs. 6 AWG for 50A), lowering material cost and voltage drop (<3% target per IEEE 1547). Note: 12V may suffice for small float-valve systems (<200W), but pump motors often stall below 10.5V under load—24V provides wider operational headroom and better cold-cranking performance, especially with lead-acid batteries.

Do not rely solely on the default 3-day autonomy—validate using local 10-year historical insolation minima, not averages. Download TMY3 or NSRDB data for your coordinates (NREL), then identify the longest consecutive period with daily irradiance <70% of annual mean—this defines realistic autonomy needs. For example, in Montana’s Big Horn Basin, 5+ cloudy days occur every 3–4 years; thus, 3-day autonomy may fail during winter storms. Adjust 'days_autonomy' to match this worst-case duration, and cross-check with battery C-rate: for 3-day autonomy at 50% DoD, ensure discharge rate ≤ C/6 (e.g., 100Ah battery → max 16.7A load) to avoid premature degradation (IEEE 1188).