Methodology & assumptions
This page lists the reference data and planning assumptions shared across the calculators. Each calculator page also shows its own formula.
Copper conductor data
Wire resistance uses the standard AWG table for annealed copper at 20 °C (ohms per 1,000 ft), adjusted for temperature by +0.393% per °C. Ampacity checks use the copper 75 °C column of NEC Table 310.16 as a general reference. Bundled conductors, high ambient temperatures, engine compartments and marine installations need further derating under the applicable standard (e.g. ABYC E-11).
| AWG | Ω / 1000 ft @20 °C | mm² | Ampacity 75 °C (A) |
|---|---|---|---|
| 18 | 6.385 | 0.823 | n/a |
| 16 | 4.016 | 1.31 | n/a |
| 14 | 2.525 | 2.08 | 20 |
| 12 | 1.588 | 3.31 | 25 |
| 10 | 0.9989 | 5.26 | 35 |
| 8 | 0.6282 | 8.37 | 50 |
| 6 | 0.3951 | 13.3 | 65 |
| 4 | 0.2485 | 21.2 | 85 |
| 3 | 0.197 | 26.7 | 100 |
| 2 | 0.1563 | 33.6 | 115 |
| 1 | 0.1239 | 42.4 | 130 |
| 1/0 | 0.09827 | 53.5 | 150 |
| 2/0 | 0.07793 | 67.4 | 175 |
| 3/0 | 0.0618 | 85 | 200 |
| 4/0 | 0.04901 | 107 | 230 |
Battery assumptions
Usable depth of discharge: LiFePO4 90%, lithium-ion NMC 85%, AGM, gel and flooded lead-acid 50%. Peukert exponents: LiFePO4 1.03, NMC 1.05, AGM 1.12, gel 1.15, flooded 1.25, referenced to a 20-hour rate. Power station usable share defaults to 90% of rated Wh, inverter efficiency to 88-90% and inverter idle draw to 10 W. All of these are editable on each page.
Battery voltage and charging data
The battery voltage chart uses rested (open-circuit) cell voltages: LiFePO4 values from EVE Energy's published state-of-charge chart, and flooded, AGM and gel values from Table 7 of the Trojan Battery User's Guide (6-24 hours of rest, 25 °C / 77 °F). Resting voltage varies with chemistry, brand, temperature and recent charging, and LiFePO4's curve is very flat, so voltage-based state of charge is an estimate. The charging time calculator assumes bulk charging ends at 95% (LiFePO4) or 80% (lead-acid), charge efficiency of 98% (LiFePO4) or 95% (lead-acid), and typical absorption times of 0.5 h for LiFePO4 and Trojan's maximum absorption times of 2 h for AGM and gel and 4 h for flooded batteries.
Appliance presets
Preset wattages are typical planning values drawn from manufacturer sizing guides and common nameplate ratings. Real appliances vary widely. For refrigerators and freezers, presets give a 24-hour average rather than the compressor's running draw. Always prefer your own appliance's nameplate or a measurement with a plug-in energy meter.
Solar
Peak sun hours are user inputs. NREL's PVWatts and solar resource maps give location-specific values. Default system losses are 25% for battery-charging systems. The solar payback calculator reflects the end of the federal Section 25D residential clean energy credit for systems installed after December 31, 2025, per IRS guidance on P.L. 119-21.
Testing
Each calculator ships with automated test cases that check its outputs against hand-calculated reference values. The site will not build if any test fails.
Last updated October 19, 2026.