Every calculator here publishes what it assumes. This page collects the models, the constants,
and — more importantly — where each one stops being accurate. If a number cannot be defended,
it does not belong on this site.
Cable voltage drop
ρ(T) = ρ₂₀ × [1 + α × (T − 20)]
R = ρ(T) × L ÷ (A × n)
ΔV_dc = I × 2R ΔV_1φ = I × 2R × cos φ ΔV_3φ = √3 × I × R × cos φ
- ρ₂₀: copper 17.241 Ω·mm²/km, aluminium 28.264 Ω·mm²/km (IACS, 100 % annealed).
- α: copper 0.00393 /°C, aluminium 0.00403 /°C.
- Reactive drop (inductance) is neglected: the tool is resistive-only. Below roughly
25 mm² that is immaterial; above 95 mm² at poor power factor the true drop is a few
percent worse — run a reactance-inclusive tool before finalising large feeders.
- Standard sizes: IEC 60228 metric series and AWG/kcmil equivalents.
Not covered: ampacity (IEC 60364-5-52 / NEC Table 310.16 depend on installation method,
grouping, ambient), short-circuit withstand, protective coordination.
Solar string configurator
V_oc(T) = V_oc,STC × [1 + α_voc × (T − 25)]
N_s,max = floor( V_dc,max × (1 − margin) ÷ V_oc(T_min) )
N_s,min = ceil( V_mppt,low ÷ V_mp(T_max) )
- α_voc from the module datasheet, typically −0.25 to −0.30 %/°C for crystalline silicon.
- The margin on the cold-morning ceiling (default 3 %) covers the irradiance-driven
low-temperature edge case and inverter tolerance.
- Records cold-record temperature and hot-cell temperature as separate inputs; using the
same number for both is the classic string-sizing error.
Battery bank sizer
C_eol = E_load × N ÷ (η_inv × √η_rte × DoD × f_T × SOH_EOL)
- Round-trip efficiency split as √η_rte per direction; charge-side losses are already
paid upstream by the PV array.
- Temperature factors f_T: lithium interpolated from −20/−10/0/10/25 °C test points
(0.60/0.80/0.90/0.97/1.00 for LFP); lead-acid similar but with a slight high-temperature bonus.
- C-rate limits used for the check: LFP 1 C, NMC 1.5 C, AGM 0.2 C, OPzV 0.1 C — storage
duty, continuous. Match against the specific BMS and datasheet.
- Peukert effects on lead-acid are not modelled explicitly; the conservative C-rate
limit is the proxy.
PV energy yield
GTI = GHI × TF(tilt, lat) × AF(azimuth)
PR = [1 + γ(T_cell − 25)] × η_inv × (1−s)(1−m)(1−dc)(1−ac) × A
E₁ = kWp × GTI × PR × (1 − clip)
- Transposition: isotropic-sky model (Liu–Jordan) with 0.2 ground albedo, tabulated on a
latitude × tilt grid and bilinearly interpolated. Accuracy ±5 % vs hourly models for
equator-facing fixed tilt.
- Azimuth factor: empirical mid-latitude curve, 1.00 at 0° to 0.74 at ±90°.
- Cell temperature: T_cell = T_amb + 6 + GHI/280 + 0.8 × max(0, T_amb − 20) — an annual
irradiance-weighted shortcut that stays within ~1 % of hourly NOCT models at mid latitudes.
- Clipping: empirical DC/AC curve, 0 % at 1.0 to ~4 % at 1.6.
- Overall band: expect ±8–10 % vs PVGIS/PVSyst. This is a screening tool; it is not
bankable-grade and does not model shading, horizon, or snow.
City irradiance data
Built-in GHI values are approximate annual totals consistent with the ranges published by
the Global Solar Atlas
(World Bank / Solargis) and ESMAP. They are rounded and intended for option screening.
For any real project, pull site-specific data from
PVGIS
(free, hourly, Europe/Africa/Asia coverage) or purchase a Solargis or Vaisala prospecting report.
General limits of everything on this site
- All tools compute physics, not code compliance. Where a wiring code, grid code or product
standard governs — ampacity, overcurrent protection, earthing, anti-islanding — the tools
say so and stop.
- Constants are the open, published ones (IACS conductivities, IEC 60228 sizes, datasheet
temperature coefficients). No proprietary data is used or implied.
- Every calculator states its failure modes. If you find a result that contradicts its own
stated limits, that is a bug — report it.