Before you can answer "is solar worth it?" you need to know one thing first: how much energy your system will actually produce. That single number drives everything downstream — how much of your electricity bill you can offset, how many batteries you'd need to store a day's energy, and how long the system takes to pay for itself. This calculator gives you that number in the unit that matters: kilowatt-hours.
The formula
Solar production estimation comes down to one core equation:
Daily kWh = System size (kW) × Peak sun hours × (1 − losses)
So a 5 kW system in a location with 5.5 peak sun hours and 20% losses produces: 5 × 5.5 × 0.8 = 22 kWh per day. Multiply by 30 for a monthly estimate, and by 365 for a yearly figure. The calculator does all three at once.
Why "peak sun hours" is the number that matters
A common mistake is to assume that more daylight means more production. What actually matters is irradiance — how strong the sunlight is, not how long the sun is visible. Peak sun hours package this into a single figure: one peak sun hour is one hour of sunlight at 1,000 watts per square meter, the standard intensity panels are rated against. Early morning and late afternoon sun is weak and contributes only a fraction of a peak sun hour, while midday sun contributes close to a full one. A place with 12 hours of daylight might deliver only 5 peak sun hours of usable energy.
Typical peak sun hours by region
| Region | Annual daily average peak sun hours |
|---|---|
| Middle East & North Africa (Jordan, Gulf, Egypt) | 5.5 - 6.5 |
| Southwestern USA (Arizona, Nevada) | 6.0 - 7.0 |
| Southern Europe (Spain, Italy, Greece) | 4.0 - 5.0 |
| Central & Northern Europe (Germany, UK) | 2.5 - 3.5 |
| Southeast Asia (humid tropical) | 3.5 - 4.5 |
These are annual averages. Summer figures run higher and winter figures lower — sometimes by half in high-latitude regions. For the most accurate value for your exact location, consult the Global Solar Atlas or NASA POWER irradiance database.
Where the losses come from
The rated power printed on a panel is measured under perfect laboratory conditions. In the real world, a chain of small losses adds up: the inverter loses a few percent converting DC to AC; panels lose output as they heat up above 25°C; dust and soiling block some light; wiring has resistance; and no two panels are perfectly matched. Individually each is minor, but stacked together they typically remove 18-25% of theoretical output. A 20% loss factor — equivalent to a performance ratio of 0.8 — is the standard planning assumption used across the industry.
What to do with this number
Once you know your expected production, two next steps are common. To find out whether that production is worth the investment, run the figure through our ROI calculator to get payback period and lifetime savings. To size a battery bank that can store a day's worth of this energy for night-time or backup use, use the battery calculator.
Frequently asked questions
What are peak sun hours and why do they matter?
Peak sun hours are not the number of hours the sun is up. One peak sun hour equals 1,000 watts of sunlight per square meter for one hour. A location that gets 5 peak sun hours receives the same total energy as 5 hours of full-strength noon sun, even though the sun may be visible for 12+ hours. This is the single most important number for estimating production, and it varies by location and season.
Why is the system loss factor around 20%?
Real systems never convert 100% of rated panel power into usable energy. Losses come from inverter inefficiency (2-4%), heat reducing panel output (5-12%), dust and soiling (2-5%), wiring resistance (1-3%), and module mismatch. Added together these typically total 18-25%, which is why a default derate of about 20% (a performance ratio of 0.8) is the industry-standard starting assumption.
How do I find the peak sun hours for my location?
Peak sun hours depend on latitude, climate, and season. Most of the Middle East and North Africa averages a high 5.5-6.5 daily peak sun hours annually; central Europe averages closer to 2.5-3.5; the southwestern United States can exceed 6. For a precise figure, check a solar irradiance database such as the Global Solar Atlas or NASA POWER data for your exact coordinates.
Does this calculator account for winter versus summer?
The calculator uses an average daily peak sun hours figure, which gives you an annual average. Real production is higher in summer and lower in winter. If you want a worst-case winter estimate, enter your location's lowest-month peak sun hours instead of the annual average.