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Off-Grid Power System Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Uses NREL PVWatts methodology and NEC 690 charge controller sizing rules. Last verified 14 June 2026.

Sizing an off-grid solar power system requires matching your daily energy load, available solar resource (peak sun hours), battery storage for autonomy, and solar array size. This calculator applies NREL's standard off-grid sizing methodology: divide your daily energy need by the system efficiency and peak sun hours to find the required solar array size, then size the battery bank for your desired days of autonomy. Enter your load, location data, and system specifications below.

Sum all appliance Wh/day. Include inverter losses.
From NREL PVWatts for your location. US range: 3.5-6.5 h/day.
NREL default derate: 86%. Off-grid systems: 80-85%.
DOE/NREL residential default: 3 days
LFP: 80-90%; Lead-acid: 50%
Typical: 12V (small), 24V (medium), 48V (large systems)
Nameplate wattage of each solar panel
Nameplate kWh per battery module
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Off-grid system sizing formulas (NREL methodology)

Array Size (W) = Daily Load (Wh) / PSH / (System Efficiency / 100)
Panels = ceiling(Array W / Panel W)
Battery Wh Needed = Daily Wh x Days / (DoD / 100)
Battery Modules = ceiling(Battery Wh / (Module kWh x 1,000))
Charge Controller (A) = Array W / Battery V x 1.25 [NEC 690.8 safety factor]

The NREL PVWatts tool (pvwatts.nrel.gov) provides peak sun hours and derate factors for any US location. NEC Article 690 (National Electrical Code, Solar Photovoltaic Systems) governs charge controller sizing and safety factors. The 1.25 multiplier is the NEC 690.8 continuous duty factor.

Off-grid system design guidance

  • Start with a detailed load analysis. Underestimating loads is the most common off-grid design error. Include all AC and DC loads with actual wattage and daily hours of use.
  • Orient solar panels at your latitude angle, facing true south (in northern hemisphere) for maximum year-round energy. Winter months have fewer PSH; size for worst-case month unless a backup generator is available.
  • A backup generator reduces the required battery and solar array size significantly. Size the generator to at least 20-30% of peak AC load.
  • MPPT charge controllers harvest 10-30% more energy than PWM controllers, especially in cold climates and with larger arrays. Required for arrays above 200 W per NREL guidance.
  • Fuses and disconnects per NEC Article 690 are required for safe off-grid installations. Consult a licensed electrician for any system above 50 V DC or 240 V AC.

Off-grid power system: frequently asked questions

How do I size an off-grid solar system?

Off-grid system sizing follows four steps: (1) Calculate daily energy load in Wh/day. (2) Determine peak sun hours (PSH) for your location from NREL PVWatts. (3) Size solar array: Array (W) = Daily Load (Wh) / PSH / System Efficiency. (4) Size battery bank for desired days of autonomy: Battery (Wh) = Daily Load x Days / Depth of Discharge. This is the NREL/SMA standard methodology.

What are peak sun hours (PSH)?

Peak sun hours are the number of hours per day that solar irradiance equals 1,000 W/m^2 (the standard test condition for solar panels). A location with 5 PSH receives 5 kWh/m^2/day of solar radiation on average. NREL's PVWatts Calculator (pvwatts.nrel.gov) provides PSH data for any US location. Typical US values range from 3.5 (Pacific Northwest) to 6.5 (Southwest).

What system efficiency should I use for off-grid sizing?

Overall system efficiency accounts for battery charge/discharge losses (92-95% for LFP), inverter efficiency (92-98%), wiring losses (2-5%), and partial shading losses (0-10%). NREL PVWatts uses a default derate factor of 0.86 (86%) for standard systems, accounting for all losses. For off-grid design, using 80-85% overall efficiency is conservative and appropriate.

How many days of battery autonomy should I design for?

NREL and SMA recommend: for non-critical applications 1-2 days; for residential off-grid systems 3-5 days of autonomy (to cover cloudy periods); for critical loads or remote locations with infrequent backup charging, 5-7 days. More autonomy requires more batteries, increasing cost. The DOE Small Wind and Solar program guidance recommends 3 days as a standard residential default.

What size charge controller do I need?

MPPT charge controllers are sized based on maximum PV array short-circuit current (Isc) at temperature. Controller Amps >= Array Watts / Battery Voltage x 1.25 (25% safety factor per NEC 690.8). For example, a 2,000 W array at 48V requires a controller rated at least 2,000 / 48 x 1.25 = 52 A. MPPT controllers are recommended for arrays larger than 200 W for maximum energy harvest.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.