Why we built this
Sizing an off-grid or backup power system shouldn't take a spreadsheet and a week of forum threads. SunReckon puts the real field math — panel watts, battery amp-hours, inverter surge, charge-controller amps, backup runtime — in one place, in the US units gear is actually sold in, with the standard formulas installers and seasoned DIYers use.
How we calculate
Every tool uses an established sizing formula. The table documents the exact math and key assumptions behind each calculator so you can see precisely what each result represents.
| Calculator | Core formula | Key assumptions |
|---|---|---|
| Solar Array Sizing | Array W = daily Wh ÷ peak sun hours ÷ derate | Off-grid derate ≈ 0.75; sun hours by location. |
| Battery Bank Sizing | Battery Wh = daily Wh × days ÷ DoD; Ah = Wh ÷ V | DoD ≈ 0.5 lead-acid / 0.8 LiFePO₄. |
| Inverter Sizing | Continuous = running W × 1.25; surge ≥ largest starting W | Surge handles motor startup. |
| Charge Controller | MPPT A = (array W ÷ battery V) × 1.25 | 25% headroom for cold-weather Voc. |
| Backup Runtime | Hours = Ah × V × DoD × inverter eff ÷ load W | Inverter efficiency ≈ 0.9. |
| Watt-Hour / Amp-Hour | Wh = Ah × V | Definitional; rated (not usable) energy. |
| Solar Output | kWh/day = system kW × peak sun hours × derate | Derate ≈ 0.8 grid-tied. |
| Off-Grid Load | Daily Wh = Σ(watts × hours); peak = Σ watts | Measure real appliance draw where possible. |
| Solar Wire Size | cmil = (2 × K × A × ft) ÷ (V × VD%); map to AWG | K = 12.9 Cu / 21.2 Al; low-V DC is drop-limited. |
| Generator Sizing | Peak = running + largest surge; continuous = running × 1.2 | Size for the biggest motor startup. |
| Panel Tilt Angle | Year ≈ latitude; summer ≈ lat − 15°; winter ≈ lat + 15° | Rule of thumb; face true south (N. hemisphere). |
| Solar Payback | Payback yrs = (cost − incentives) ÷ annual savings | Estimate, not financial advice. |
The calculators run entirely in your browser in plain JavaScript. Nothing you type is sent to a server or stored — your numbers stay on your device.
Sources & how we keep this accurate
Formulas and constants come from published references and manufacturer data: NREL PVWatts for production, peak-sun-hours, and derate; the National Electrical Code (NFPA 70) Table 310.16 and Article 690 for DC wire/ampacity; battery depth-of-discharge and efficiency from lithium and lead-acid makers (Battle Born, Victron); and the U.S. federal solar tax credit (ITC) for payback. Each calculator carries its own cited source and a review date. We review the formulas periodically and welcome corrections via the contact page.
Methodology last reviewed June 2026
What this is not
SunReckon gives educational estimates to help you plan and shop. It is not engineering, electrical certification, or financial advice. Off-grid power involves real electrical and fire risk — always verify against manufacturer specifications, the NEC, and a qualified electrician or installer before building.
Get in touch
Want a calculator we don't have yet, or spot a number that looks off? Tell us on the contact page.