From sunlight to electricity
A solar panel, or photovoltaic (PV) module, turns sunlight straight into electricity. Panels wired together make an array.
Panels make direct current (DC), which flows one way. Homes mostly use alternating current (AC). An inverter converts DC to AC. Off the grid, a charge controller sits between the panels and the battery so the battery charges safely.
A panel’s rating, such as 400 W, is its output under standard test conditions: bright sun, a cool panel, pointed straight at the sun. Real conditions are almost always less.
Peak sun hours
The sun’s strength changes all day. To keep the math simple, solar designers use peak sun hours. One peak sun hour is one hour of sunlight at full test strength, 1,000 watts per square meter.
A day might have 10 hours of daylight but only 4 peak sun hours. The weak morning and evening light adds up to fewer full-strength hours.
Text description
A chart of sunlight strength through one day, from 7 in the morning to 5 in the evening, 10 hours of daylight. The real sunlight rises slowly in the morning, peaks around noon well below full strength, and falls in the afternoon; the area under this curve is shaded. A dashed box shows the same amount of energy as if it came at full test strength, 1,000 watts per square meter, for 4 hours around midday. The two areas are equal, so this day has 4 peak sun hours.
Peak sun hours vary a lot by place and month. Across the US, they often run from about 1.5 to 3 in a northern winter, and less in the cloudiest places. In the sunny Southwest they reach 6 to 7 or more in spring and summer. PVWatts, a free calculator from a DOE national lab, gives values for your address.
What an array makes
To estimate daily energy, multiply array size by peak sun hours. Then take off losses for heat, dust, wiring, the inverter and charging. These are called derates. Together they often take 15 to 30 percent.
array kW × peak sun hours × derate = kWh per day
Try a 2 kW array at 4 sun hours, then at 2 for a cloudy northern winter.
About 5.6 to 6.8 kWh per day.
That is 168 to 204 kWh a month.
Assumptions: Derate of 0.70 to 0.85 covers wiring, heat, dust, inverter and charging losses. Sun hours vary by month and place; PVWatts gives local values.
Notice how much the sun hours matter. Halve them and you halve the energy.
Winter is the hard month
In winter the sun is lower and the days are shorter. Clouds and snow add to the drop. In many northern places, December output is only about a quarter to two-thirds of June output. Cloudy places and flatter panels sit at the low end.
Many homes also use more power in winter: more lights, longer evenings, fans and pumps for heating.
So off-grid systems are usually sized for the worst month, often December or January. Grid-tied systems are often sized on the yearly total instead, because the grid covers the gap. Climate zone changes this answer a lot. A sunny southern site has a much smaller winter gap.
Tilt and direction
Panels make the most energy when they face the sun. In the US that means facing roughly true south.
Tilt is the angle of the panel from flat. A common rule of thumb is:
- Tilt about equal to your latitude for good output across the year.
- Steeper, roughly latitude plus 10 to 15 degrees, to favor the low winter sun. Steep panels also shed snow better.
- Flatter to favor summer.
Text description
A side view of a ground-mounted solar panel facing true south, to the right, at an example latitude of 40 degrees north. The solid panel is tilted at about the latitude, 40 degrees from flat, which gives good output across the year. A dashed steeper position, about latitude plus 10 to 15 degrees, favors the low winter sun and sheds snow better. A dashed flatter position favors summer. The high summer noon sun and the low winter noon sun are shown to the south, with sunlight reaching the panel.
Off grid, a steeper winter tilt often makes sense. You can try panel count and tilt in the course’s Homestead Lab and watch the battery through a year.
Shade is the enemy
Shade cuts output more than most people expect. In many arrays, panels are wired in series in a string. Shade on part of one panel can pull down the output of the whole string.
Look at your site at different times of day and year. A tree that is fine in June can shade the array in December, when the sun is low. Check for chimneys, other buildings, hills and trees that will grow.
Text description
A side view, not to scale, with north to the left and south to the right. A ground-mounted solar array faces south. A tree stands some distance to its south. The high June noon sun casts a short shadow that ends well before the array. The low December noon sun casts a shadow several times longer, which reaches across the array. Shade on part of a panel can pull down the output of the whole string, so check the site at different times of day and year.
Pro and permit
Check
A 3 kW array gets 5 peak sun hours in June and 2 in December. With an 80 percent derate, about how much does it make per day in December?
Summary
- Panels make DC. An inverter makes AC. A charge controller protects the battery.
- array kW × peak sun hours × derate = kWh per day. Derates often take 15 to 30 percent.
- Winter has far fewer peak sun hours. Off-grid systems are usually sized for the worst month.
- Face true south, tilt steeper for winter, and avoid shade at all seasons.
Go deeper: why heat lowers output
Solar cells make less power as they get hotter. A panel on a hot summer roof can run well above the air temperature and lose a noticeable share of its rated power. That is part of the derate. It is also why cold, bright days can give surprisingly strong output for a short time. Panel data sheets list a temperature coefficient that says how much power drops for each degree of warming. PVWatts includes these effects when it estimates output for your site.