Solar Panel Calculator

Sizing a Solar System From Electricity Use

A solar system is sized to the electricity a home actually uses, not to the roof, not to the budget and not to a round number of panels. Start with a full year of kWh from utility bills, because a single month misleads badly in either direction depending on the season.

Worked example

A household uses 900 kWh a month, which is about 29.6 kWh a day. In a region with 4.5 peak sun hours, the usable figure is 4.5 x 0.8, which is 3.6. Dividing 29.6 by 3.6 gives 8.2 kW. At 400 watts per panel that is 8,200 divided by 400, which is 20.5, so 21 panels. At 450 watts it is 18.2, so 19 panels.

The same home in a 5.5 peak sun hour region needs 29.6 divided by 4.4, which is 6.7 kW, or 17 panels at 400 watts. Same house, same consumption, four fewer panels, purely because of location.

Peak Sun Hours

Peak sun hours are not hours of daylight. One peak sun hour is one hour of solar irradiance at 1,000 watts per square meter, the standard test condition panels are rated under. A location with 5 peak sun hours might have 13 hours of daylight, most of it weaker than the rating condition, adding up to the equivalent of 5 full strength hours.

Across the continental US the annual daily average runs from roughly 3.5 peak sun hours in the Pacific Northwest and parts of the Northeast to about 6.5 in Arizona, Nevada and southern California. The middle of the country and the Southeast generally fall between 4.5 and 5.5. Those are annual averages, and the seasonal swing is large: a northern site can see half its summer figure in December.

• Pacific Northwest, upstate New York, New England: about 3.5 to 4.2
• Midwest, Mid-Atlantic, Northern Plains: about 4.2 to 4.8
• Southeast, Southern Plains, Mid-South: about 4.8 to 5.5
• Southwest, inland California, Nevada, Arizona: about 5.5 to 6.5

Why 20 Percent System Losses Are Normal

A panel rated 400 watts produces 400 watts under laboratory test conditions that no roof reproduces. Real output is lower, and the 0.8 derate is a practical allowance for the gap. It is made up of several independent losses that compound.

• Temperature. Panels lose efficiency as they heat up, typically around 0.3 to 0.4 percent per degree C above 25 C. On a hot roof this is often the largest single loss.
• Inverter conversion. Converting DC to AC costs roughly 3 to 4 percent even with a good inverter.
• Wiring and connections. Two to three percent lost as resistance in DC and AC runs.
• Soiling. Dust, pollen, bird droppings and pollution cost 2 percent in wet climates and considerably more in dry dusty ones.
• Shading. Trees, chimneys, vents and neighboring structures, worse on strings without module level electronics.
• Mismatch and tolerance. Panels in a string perform to the weakest one, and manufacturing tolerance adds a little more.
• Orientation and tilt. A true south facing roof at latitude tilt is the reference. East or west facing arrays give up roughly 10 to 20 percent.

Add those up and 20 percent is a normal, not pessimistic, total. Well designed systems on cool, clean, unshaded south facing roofs can do better. Shaded, east-west, or dusty installations do worse, and 25 to 30 percent is realistic there.

System Size Reference Table

Monthly usage converted to system size at two common peak sun hour levels, with 20 percent system losses applied and panel counts based on 400 watt panels at 4.5 peak sun hours.

The panel count column uses the 4.5 peak sun hour size. In a 5.5 hour region the counts drop by roughly 18 percent, and at 450 watts per panel they drop another 11 percent.

Roof Space

A typical residential panel in 2026 is about 400 to 450 watts and measures roughly 74 by 44 inches, which is close to 22.6 square feet of panel. Allowing for spacing, rails and code required setbacks, plan on about 18 to 20 square feet of usable roof per panel as a working figure for area estimates.

A 21 panel array therefore needs roughly 380 to 420 square feet of unshaded, structurally sound roof at a workable orientation. Fire code setbacks around ridges and eaves, plumbing vents, skylights, chimneys, dormers and hip roof geometry all remove usable area, so a roof plane that measures 500 square feet may only fit 350 square feet of array.

Where roof area is the constraint rather than budget, higher wattage panels buy back capacity: 450 watt panels deliver about 12 percent more output in the same footprint than 400 watt panels.

Net Metering

Net metering is the billing arrangement that lets exported solar energy offset imported energy. A system produces most at midday when the house uses least, and the house uses most in the evening when the system produces nothing. Net metering credits the midday export against the evening import.

The terms vary enormously by state and by utility, and the trend has been away from the most generous versions.

• Full retail net metering. Exports credited at the same rate as imports. The simplest and most favorable arrangement.
• Net billing or avoided cost. Exports credited at a lower wholesale or avoided cost rate, often well under half the retail rate. California's current arrangement is the best known example.
• Time of use rates. Credits and charges vary by hour, which usually favors west facing panels or battery storage that shifts output into the evening peak.
• Annual true-up. Most programs settle once a year, and surplus credits are often paid out at a low rate or forfeited, which is why oversizing beyond actual usage rarely pays.

Because the export rate drives the economics, check the specific utility's current rules before sizing. Under a low export credit, sizing to daytime usage plus storage often beats sizing to the full annual bill.

Degradation and System Life

Panels lose output slowly and predictably, typically about 0.5 percent a year, with a slightly larger drop in the first year. Most manufacturers warrant around 85 to 90 percent of rated output at 25 years. At 0.5 percent a year, a system produces about 95 percent of its original output after 10 years and about 88 percent after 25.

Panels usually outlast their warranties. Inverters do not. String inverters commonly need replacement at 10 to 15 years, microinverters and optimizers carry longer warranties but still fail individually. Budget for at least one inverter replacement over the life of the system, and factor a modest annual output decline into any long range payback estimate.

Practical Checks Before Committing

1. Pull 12 months of kWh from utility bills, not one month or an estimate.
2. Account for known changes ahead: an electric vehicle, a heat pump, a pool pump or an addition can move usage by thousands of kWh a year.
3. Check roof age. Replacing a roof under an existing array is expensive, so a roof within 5 years of replacement should be done first.
4. Assess shade across the full year, not on one sunny afternoon. Winter sun is low and shadows are long.
5. Confirm the utility's current export policy and any interconnection size caps.
6. Compare quotes on price per watt installed, not on total price, and confirm equipment warranties separately from workmanship warranties.