How Much Energy Does a Solar Panel Produce?

The short answer

A single solar panel produces roughly 1 to 3 kilowatt-hours (kWh) of electricity per day, depending on its wattage and how much sun your location gets. Using the working assumptions in this guide — 14% total system losses from the inverter, wiring, and soiling — here is what common panel sizes deliver at 5 peak sun hours per day: a 400W panel makes about 1.72 kWh per day, a 450W panel about 1.94 kWh, and a 550W panel about 2.37 kWh. Over a full year, that is roughly 630, 710, and 860 kWh per panel respectively. Fewer sun hours or higher losses move every one of those numbers down; better conditions move them up.

The entire topic reduces to one formula:

Daily kWh = panel wattage ÷ 1,000 × peak sun hours × (1 − system losses)

Everything else in this guide — the tables, the yearly math, the list of things that move the number — is this formula worked out in different directions. The wattage is printed on the panel's datasheet. Peak sun hours depend on where you live. Losses are the efficiency tax the real world takes between the panel and your meter. Every figure below is arithmetic from stated assumptions, so you can re-run it with your own numbers.

The formula, piece by piece

Panel wattage is the nameplate rating: the power the panel produces under Standard Test Conditions — 1,000 W/m² of light, a 25°C cell temperature, and clean air. A "400W panel" makes 400 watts only in those lab conditions; in the field it almost always makes less, which is exactly what the other two terms account for. Residential panels sold in 2026 typically range from 400W to 460W, while larger 550W panels are common on commercial and ground-mount installs. The rating itself is honest — it is the test conditions that are optimistic.

Peak sun hours are the equivalent number of hours per day when sunlight hits the panel at full rating intensity (1,000 W/m²). They are not daylight hours. A 12-hour summer day with morning haze, passing clouds, and low-angle evening light might deliver only 5 peak sun hours — the day's total solar energy, compressed into hours of "full" sun. This is the single biggest variable in the formula. As rough guides: much of the southern and southwestern US averages 5–6 peak sun hours per day, the northern US around 3–4, and the UK and northern Europe roughly 2–3. Moving from 3 to 6 peak sun hours doubles a panel's daily output with no change to the hardware.

System losses are everything that eats energy between the panel's cells and your usable AC electricity: the inverter converting DC to AC (roughly 3–5%), resistance in the wiring (about 2%), dust and soiling on the glass (2–5% depending on how often panels are cleaned), and mismatch between panels plus a small allowance for age (1–2%). Real residential systems typically land somewhere between 10% and 20% total losses. This guide uses 14% as the working figure — a middle-of-the-road assumption — which appears in the formula as × 0.86. If your system is cleaner, better-wired, and newer, use 10% (× 0.90); if it is dusty and aging, use 18–20%.

Worked examples: daily energy per panel

Daily kWh = (wattage ÷ 1,000) × peak sun hours × 0.86, using the 14% loss assumption. Read across for your panel size, down for your location's sun hours:

Panel size	3 sun hours	4 sun hours	5 sun hours	6 sun hours
400W	1.03 kWh/day	1.38 kWh/day	1.72 kWh/day	2.06 kWh/day
450W	1.16 kWh/day	1.55 kWh/day	1.94 kWh/day	2.32 kWh/day
550W	1.42 kWh/day	1.89 kWh/day	2.37 kWh/day	2.84 kWh/day

Two things jump out. First, output scales linearly with both wattage and sun hours — a 550W panel makes about 37% more than a 400W panel in the same spot, and any panel makes twice as much at 6 sun hours as at 3. Second, the absolute numbers are modest: even the biggest panel here makes under 3 kWh on a good day. That is the whole reason rooftop systems are arrays of many panels rather than one big one. If you want these numbers for your own wattage and loss assumptions, run them through the solar panel output calculator instead of doing the arithmetic by hand.

From one day to a month to a year

Daily kWh is useful for sizing; monthly and yearly kWh are what matter for savings. The extrapolation is simple multiplication — a 30-day month and a 365-day year — but state the assumptions, because real months are not average months:

Panel size (at 5 sun hrs/day)	Daily	Monthly (× 30)	Yearly (× 365)
400W	1.72 kWh	~52 kWh	~628 kWh
450W	1.94 kWh	~58 kWh	~706 kWh
550W	2.37 kWh	~71 kWh	~863 kWh

This gives a handy rule of thumb: 1 kW of installed panels produces roughly 1,250–1,900 kWh per year, depending on sun hours (1 × 4 × 0.86 × 365 ≈ 1,256 kWh at 4 sun hours; ≈ 1,570 kWh at 5; ≈ 1,883 kWh at 6). Installers often quote "about 1,400–1,600 kWh per kW per year" for average US conditions, which sits squarely inside this arithmetic — now you know where their number comes from.

One honest caveat: the yearly figure smooths over seasons that are anything but smooth. In temperate climates, a panel's December output can be one-half to one-third of its June output — shorter days, lower sun angle, more cloud. The yearly total is still correct as an average, but if you are sizing a system to cover winter bills or an off-grid load, size for the worst month, not the average one.

What changes the number

The formula gives the headline figure; the real world then negotiates. These are the factors that move a panel's actual output away from the table values, roughly in order of impact.

Tilt and orientation. Panels produce the most when they face the sun directly — true south in the northern hemisphere, tilted at roughly your latitude. A roof facing east or west typically yields 10–20% less than an ideal south-facing one, and a flat or steeply pitched roof costs a few percent more. The losses are gradual, not cliff-edge: an east-west roof still produces plenty of energy, just less per panel.

Shading. This is the harshest penalty on the list. A panel string wired in series is limited by its weakest panel, so shade on one panel — a chimney shadow, a growing tree branch — can drag down the whole string's output far out of proportion to the shaded area. Partial shade that covers 10% of a string can cut its output by 30% or more. Microinverters or power optimizers isolate the damage to the shaded panel, which is why they are worth considering on partially shaded roofs.

Temperature. Solar panels are strange devices: they love light but dislike heat. Every panel's datasheet lists a temperature coefficient, typically −0.3% to −0.4% per °C above the 25°C rating temperature. Work it through: on a hot afternoon with cell temperatures at 65°C — 40°C above rating — a panel with a −0.35%/°C coefficient loses about 14% of its rated power (40 × 0.35%). Paradoxically, a cool sunny spring day can out-produce a scorching summer afternoon. Good airflow beneath the panels and a light-colored roof both shrink the penalty.

Age and degradation. Panels lose a little power every year — modern panels degrade roughly 0.3–0.8% annually, which is why most carry 25-year performance warranties guaranteeing 87–92% of original output at year 25. A panel making 1.72 kWh per day when new makes roughly 1.50–1.58 kWh per day after 25 years at those warranty bounds. The decline is slow and predictable, not a failure — panels routinely keep working for 30+ years.

Soiling, snow, and wiring. Dust, pollen, bird droppings, and snow all block light before it reaches the cells — the 2–5% soiling allowance in the loss figure assumes occasional cleaning, and a panel caked in desert dust or under snow produces a fraction of its rating. Long, thin DC cable runs add resistive losses too; this is an installer-quality issue, and it is why the loss assumption deserves a pessimistic value on older or DIY-wired systems.

Inverter clipping. If the array's DC capacity exceeds what the inverter can convert, output gets clipped at the inverter's ceiling on the sunniest hours. Mild oversizing (an array 10–20% larger than the inverter) is deliberate and economical — the extra morning and evening production outweighs the clipped midday peak. Heavy clipping, though, is just wasted panels.

Panel technology. This matters less than most buyers expect. A high-efficiency monocrystalline panel produces more energy per square meter, but a 400W mono panel and a 400W polycrystalline panel produce the same energy under the same conditions — wattage is wattage. Efficiency buys you fewer panels for the same output, not more output per panel, so it matters most where roof space is tight.

What this means for your home

One panel's output is trivia; a roof's output is a plan. A typical US home uses roughly 29 kWh per day (around 870 kWh a month), so at 5 peak sun hours a 450W panel covering 1.94 kWh per day is about one-fifteenth of that home's needs — hence the standard 15–25 panel residential array. To turn this guide's math into your roof's numbers: first work out how many solar panels your home needs for your usage and sun hours, then size the full array in the solar system calculator, and finally translate the kWh into dollars in the solar ROI calculator. The formula on this page is the engine inside all three — now you have seen it with the cover off.