🔆 Solar Array Sizing

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

How many solar panels it takes to cover a daily electricity need.

Sizing a solar array starts from demand, not supply: how much energy does the site actually use in a day? That daily energy need, divided by how much sun the site actually gets, tells you how large an array has to be to replace it.

"Peak sun hours" is the standard way solar output gets estimated without modeling the sun's path across the sky hour by hour — it's the number of hours of hypothetical full-intensity (1000 W/m²) sunlight that would deliver the same total energy as the site's real, variable-intensity sunlight over a full day. A location with 5 peak sun hours might see sunlight for 10+ actual daylight hours, but at varying intensity that adds up to the same total as 5 hours at full strength.

No system converts 100% of the sun hitting a panel into usable electricity at the plug — wiring resistance, inverter conversion loss, panel heating, and dust or shading all take a cut, which is why a system-losses derating factor is applied before sizing the array, not after.

Key formula

Required array wattage = (daily energy need) ÷ (peak sun hours × (1 − system losses))
Panels needed = required array wattage ÷ one panel's wattage

How to use the Solar Array Sizing calculator

Use this to work out how many solar panels it takes to cover a daily energy need: the array size in watts, and the number of panels of a given wattage. It suits an off-grid cabin, an RV or shed, and a first estimate for a grid-tied house before you request quotes.

It works from four things: the energy you use per day, the peak sun hours your site receives, the fraction of energy lost between the panels and the point of use, and the wattage of one panel. The most important decision is which month's sunshine to design for, because that choice moves the answer more than any other input.

Daily Energy Need (kWh)

The energy you want to generate per day, in kilowatt-hours. For a home take a monthly bill and divide by 30; for an off-grid site add up each appliance's watts times its hours of use per day, and include the phantom loads that run around the clock.

Peak Sun Hours (hr)

Equivalent hours per day of full sun (1 kW per square metre) on the panels at your site, tilt and season. A sunny climate averages 5 to 6 in summer and 3 or less in a northern winter. Solar maps and calculators from national labs list monthly values for any location.

System Losses (wiring, inverter, heat) (%)

The share of the panels' output lost before it reaches the load: heat, wiring, the inverter, dirt, mismatch and, off-grid, battery charging losses. About 14 percent is a common figure for a grid-tied system and 20 to 30 percent for an off-grid one.

Panel Wattage (W)

The rated power of one panel under standard test conditions, printed on its label. Real panels seldom reach that number, which is the reason the losses figure exists.

Worked example: an off-grid cabin

A remote cabin uses 6 kWh per day and is occupied from spring to autumn, when even the least sunny month at the site gets about 4.5 peak sun hours. The system will store energy in batteries and run through an inverter, so 25% losses are assumed. The panels available are 400 W each. How big an array is needed?

Worked by hand:

1. Energy in watt-hours. 6 kWh × 1,000 = 6,000 Wh per day.
2. Effective sun hours. Losses leave only 0.75 of what the panels make, so 4.5 × 0.75 = 3.375 useful hours per day.
3. Array power. 6,000 ÷ 3.375 = 1,778 W.
4. Number of panels. 1,778 ÷ 400 = 4.44, which rounds up to 5 panels, an installed 2,000 W.
5. Check the result. 2,000 W × 4.5 h × 0.75 = 6.75 kWh per day, comfortably above the 6 kWh needed.

Five 400 W panels, about 90 ft² of roof or ground mount, cover the cabin in its least sunny occupied month. Notice how much the sun figure matters: if the cabin were used through a northern winter with only 3 peak sun hours, the same load would need 7 panels. Many off-grid owners size for the shoulder months and let a generator cover the worst weeks rather than pay for panels that sit oversupplied most of the year.

Reading the result: choosing the design month

An array that meets the load on the average day fails on the worst one, and the gap between them is large at higher latitudes, where winter sun can be a third of summer sun. A grid-tied system can be designed to the annual average because the grid fills the gaps. An off-grid system must decide which month it is willing to fall short in, and that decision, not the panel choice, sets the cost.

• Tilt and orientation change the sun hours the panels see. A steeper tilt helps in winter, and a fixed south-facing tilt near the latitude is the usual compromise for the whole year in the northern hemisphere.
• Shade is the most damaging loss. A small shadow across one panel can cut the output of a whole string, so a site with midday shading needs a different layout or microinverters.
• A 400 W panel is roughly 18 ft² (1.7 m²), so array area is easy to estimate from the panel count.
• The array is only one part. The battery, charge controller and inverter must be sized to match, and the wire to the array needs its own voltage-drop check.

Notes & limitations

• Peak sun hours vary a lot by location and season — a rough site estimate for the worst realistic month (typically winter) gives a more conservative, dependable array size than an annual average would.
• This sizes the array for average conditions, not the worst single day — a few consecutive cloudy days is exactly what battery autonomy (a separate calculation) is meant to cover, not a larger array.

Common mistakes

• Using the annual-average sun hours for an off-grid system. The average hides the winter shortfall that leaves the batteries flat when you need them most.
• Multiplying panel wattage by daylight hours. Peak sun hours are not the length of the day; they are the equivalent hours of full-strength sun, always fewer.
• Ignoring losses. Panels seldom deliver their label rating, and heat, dust, wiring, conversion and battery charging together take a fifth to a third of it.
• Sizing the array and forgetting the rest. A large array with a small battery or inverter wastes energy, so treat the array, the storage and the conversion as one design.
• Not allowing for future load. Adding a freezer or an air conditioner later multiplies the daily energy, and adding panels is far easier when the mounting and wiring were planned for growth.

Frequently asked questions

Papers worth reading

PVWatts Version 5 Manual Dobos, A. P. (2014), National Renewable Energy Laboratory (technical report). Documents how NREL’s PVWatts estimates the energy a PV array delivers, including the system-loss assumptions a quick sizing estimate depends on.