How the calculation works
Solar sizing starts from your energy budget, not from the panel. A panel rated at 400 W produces that figure only under laboratory conditions; in the field, output is set by how many hours of strong sun your location receives and how much of the harvest survives the trip through wiring, charge controller and battery.
Peak sun hours are not daylight hours. They are the equivalent number of hours at full-strength sunshine (1,000 W/m²) your location receives per day. A summer day may be 14 hours long but deliver only 6 peak sun hours; a winter day may deliver 2. For year-round off-grid use, size for your worst month, not your best.
The efficiency factor covers what installers call system losses: voltage drop in cables, charge controller conversion, panel temperature (panels lose output as they heat up), dust and slight shading. With a quality MPPT controller and short cable runs, 85% is realistic; with a PWM controller or long runs, use 75% or lower.
Worked example
A cabin uses 2,000 Wh per day: a 12 V fridge (600 Wh), LED lighting (200 Wh), laptop and phone charging (300 Wh), water pump (400 Wh) and a margin for small loads (500 Wh). The location gets 5 peak sun hours on an average day. With an MPPT system at 85% efficiency:
Two 400 W panels (800 W) cover this comfortably and, more importantly, keep the batteries charging on overcast days when the array might only produce a third of its rating. Off-grid arrays are deliberately oversized for exactly this reason; grid-tied logic ("size to average use") leaves an off-grid system dark after two cloudy days.
Common mistakes
- Sizing from daylight hours instead of peak sun hours. This overestimates production by two to three times and is the single most common cause of undersized systems.
- Using summer sun figures for a year-round system. December sun hours in the northern US can be half the annual average. Size for the worst month you intend to occupy the system.
- Ignoring system losses. Nameplate watts never reach the battery. Budgeting 15–30% losses is not pessimism; it is measurement.
- Sizing panels before loads. Buying panels first and hoping they cover the loads works backwards. List every load, estimate its daily watt-hours, then size the array. Our battery bank calculator works the same direction.
Frequently asked questions
How do I find peak sun hours for my location?
Search for a solar insolation map of your country, or use NREL's data for US locations. Look up the figure for your worst intended month if the system runs year-round. As rough guides: northern US and UK winters run 2–4 hours, most of the continental US averages 4–6, and the desert Southwest reaches 6–7.
Should I round up to more panels?
Yes, almost always. Panels are the cheapest part of an off-grid system per watt; batteries are the most expensive. Extra array capacity charges the bank faster after cloudy spells and extends battery life by reducing deep discharges. Oversizing the array by 25–50% over the minimum is standard off-grid practice.
Does panel orientation change the result?
Yes. The calculation assumes panels tilted toward the sun at a reasonable angle. Flat-mounted panels (common on vans and boats) lose roughly 10–20% annually versus tilted ones. If that is your setup, use the conservative efficiency option or add a panel.
What if I have a generator as backup?
A generator changes the economics: you can size the array closer to the minimum and let the generator cover rare dark stretches. Without one, oversize the array and battery bank both, because solar is your only recharge source.