The most common question we get before a consultation sounds like this: "Someone told me a 5 kW system produces 20 kWh a day. Is that right?"
Sometimes. Often not. The number depends on five things, and once you understand them, the "will it work on my roof" question becomes much less mysterious.
The five numbers that decide your production
1. Panel rating (kWp)
This is the peak capacity of your system — what it produces under ideal lab conditions. A 5 kWp system has panels that could, theoretically, generate 5 kilowatts at a single instant in perfect sun. Real-world output is almost always lower.
2. Peak sun hours (your location)
This is the number of hours per day that effectively deliver full-strength sunlight for production purposes. It's not "hours of daylight" — it's hours weighted by sunlight intensity. Across Asia, peak sun hours range from roughly 3.5 in consistently cloudy regions to 5.5+ in sunny tropical and semi-arid zones.
3. Orientation and tilt
A panel facing due south (in the northern hemisphere) or due north (in the southern hemisphere) at a tilt roughly equal to your latitude will capture the most annual energy. East- or west-facing panels lose 10–20%. North-facing panels (northern hemisphere) can lose 30%+ — sometimes enough to make a system not worth installing.
4. Shading
Even a small amount of shading on a panel — a tree branch, a vent pipe, a neighbouring wall — can cut that panel's output dramatically, and sometimes impact an entire string of panels depending on the wiring. A 10% shaded panel doesn't produce 10% less; sometimes it produces 50% less.
5. System losses
Real-world systems lose 10–15% of the panel's rated output to the inverter, cabling, temperature derating, dust, and general age. Good designs account for all of these; cheap quotes pretend they don't exist.
The quick math
A rough formula that gets you within 10–15% of reality:
Daily kWh ≈ System kWp × Peak sun hours × 0.80 (system efficiency factor)
For a 5 kWp system in a region with 4.5 peak sun hours: 5 × 4.5 × 0.80 = 18 kWh/day, or about 6,500 kWh/year. If someone quotes you 25 kWh/day from the same system, ask them to show their working.
Seasonal variation is bigger than most people expect
Across most of Asia, monthly production can swing by 30–50% between the sunniest and cloudiest months. Monsoon regions see sharper drops. A good yield model — the kind that should come with any serious quote — shows monthly production, not just an annual total.
A quote that shows only annual production or "average monthly" output is hiding the seasonal range. Ask for month-by-month expected production. If they can't produce it, they didn't actually model the site.
What you should take away
- Panel rating is just the starting point, not the expected output.
- Your location's peak sun hours is the single biggest factor outside your control.
- Orientation, tilt, and shading can swing the number 20–40%.
- A legitimate proposal will show you month-by-month modelled production, not a single rosy annual number.
When you're comparing quotes, the fastest way to separate good ones from bad ones is to look at the production assumptions. The good ones show their work.