What Is The Off-Grid Solar Calculator?
An off-grid solar calculator estimates panel wattage, battery capacity, charge controller size, and inverter size from daily appliance loads. It is for early planning of cabins, sheds, backup systems, and small homestead electrical projects.
How To Use This Calculator
- Enter watts and daily hours for two representative loads.
- Enter local peak sun hours.
- Choose backup days, battery voltage, and battery type.
- Review panel watts, battery amp-hours, controller amps, and inverter estimate.
- Have a qualified electrician or solar designer review any real installation.
How Is It Calculated?
Formula
panel watts = daily watt-hours / (peak sun hours x system efficiency)What The Constants Mean
- Daily watt-hours multiply appliance watts by hours used per day.
- The 0.75 system-efficiency factor accounts for common real-world solar losses.
- Battery amp-hours divide backup watt-hours by battery voltage and usable depth of discharge.
- Depth-of-discharge assumptions vary by battery chemistry, such as LiFePO4 versus lead acid.
A 1,080 Wh/day load with 4.5 peak sun hours needs about 320 watts of panels after losses. With two backup days on a 24 V LiFePO4 bank, the estimate is about 113 Ah of battery capacity.
Why This Matters
Solar sizing mistakes can damage equipment, disappoint users, or create safety hazards. Daily watt-hours, depth of discharge, inverter surge loads, wire sizing, fusing, grounding, code, and local weather all matter. This calculator is only a first-pass estimate.
Homestead Math calculators are designed to make practical estimates visible. They are intentionally transparent: the inputs are labeled, the formula is shown, and the result is paired with cautions so you can decide what to verify locally before spending money or changing a setup.
Frequently Asked Questions
What are peak sun hours?
Peak sun hours summarize usable solar energy for a location and season. They are not the same as daylight hours.
How much battery backup do I need?
It depends on loads, weather, tolerance for outages, and battery chemistry.
Can this design my electrical system?
No. Use it for planning only and have a qualified professional design code-compliant systems.
Why does battery type matter?
Different chemistries use different practical depth-of-discharge assumptions.