The math
Solar sizing combines three numbers: how much energy you use, how much sun you get, and how lossy your real-world system is. The third one is what catches first-time off-gridders.
array_W = daily_Wh ÷ (sun_hours × efficiency)
To turn 1 kWh per day into a panel size at 4.5 sun hours and 75% efficiency: 1000 ÷ (4.5 × 0.75) ≈ 296W of solar. Round up to the next stocked panel size.
Frequently asked
How do I know my peak sun hours?
The NREL PVWatts calculator gives location-specific monthly averages. For a rough number, use the regional preset here. For sizing a critical system, look up your worst month (typically December or January) and use that - anything you size based on annual average will fall short in winter.
Why such a big efficiency haircut?
Because panels are rated at ideal conditions. A 400W panel rarely produces 400W. Real output averages 70-80% of nameplate across a year. Cold winter days actually beat the rating; hot rooftop summer days fall well below it.
What size charge controller?
Match it to your array's maximum current. The calculator gives a minimum amp rating - go one size up for safety margin. MPPT controllers are 10-30% more efficient than PWM and are worth the cost for arrays over a few hundred watts.
Can I add panels later?
Yes if your charge controller and wiring have headroom. Plan for it: oversize the controller by 25-50% on initial install, run heavier wire than you think you need. Adding capacity later is much cheaper if the infrastructure supports it.