Solar hours by ZIP code

"Peak sun hours" is the number of hours per day of full-strength sun (1,000 W/m²) your location averages. Multiply it by array watts to estimate daily production. Data is the long-term average for the centre of your ZIP code, computed by the PVWatts model from the National Laboratory of the Rockies (formerly NREL), for all three common tilts at once.

Solar hours by ZIP code

Look up once; fixed, summer-tilt and winter-tilt results appear together. Pick which to size for in the next step.

Array	Tilt	Year-round	Summer (Jun–Aug)	Winter (Dec–Feb)	kWh/yr per kW

Sun hours per day, facing south. Fixed ≈ your latitude; summer tilt ≈ latitude − 15°; winter tilt ≈ latitude + 15°. Adjustable racks use the summer tilt in summer and the winter tilt in winter.

Month by month

Month	Fixed	Summer tilt	Winter tilt

Panels to cover your daily load

Daily loadWh

Panel wattsW

System losses

Size for

Split the load between daylight and dark (optional)

A watt-hour used while the sun is up comes straight off the panels. The same watt-hour used after dark has to be charged into the battery and pulled back out again, and that round trip costs about 10%, so it takes more panel to produce. Saying how much of the day lands in daylight is what lets this size the array on what the load really costs, instead of pricing every watt-hour the same. Step 1 fills both of these in if you worked out the night split there, and leaving this closed keeps the plainer sizing.

Used in daylightWhEverything else is treated as running after dark.

Battery round tripWhat it costs to store a watt-hour and pull it back out. Only the share used after dark pays it.

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panels needed

–h

sun hours used

–W

array (DC)

Will that array put the battery back?

An array sized to replace the day's load has nothing left over at the end of the day, so a bank that went flat in an outage stays flat. Refilling it is extra energy on top of the load, and that means extra panels. Give it a target below and the panel count above covers both.

Bank to refillWhUsable energy, which is nameplate x depth of discharge. Step 1 fills this in.

Refill an empty bank indaysWhile still carrying the daily load. Set 0 to size for the load only.

Wh spare per day at the load-only size

days to refill at the load-only size

panels to carry the load and refill

← Back to the load calculator Next: can my inverter and roof take that? →

Sizing for the worst winter month is what keeps an off-grid system running in January; grid-tied and backup systems usually size for the annual average and let the grid fill the gap.

Planning figures, not a design. These calculators give you ballpark numbers to shop and plan with. They are not an electrical design, they cannot see your site, your loads or your local code, and they may be wrong. Wire sizing, over-current protection, grounding, rapid shutdown and interconnection must follow NEC 690 and your local code. Have any design checked by a licensed electrician before you buy or energize anything. You use these tools at your own risk.More on safety.