How to read your results
The calculator gives you six numbers that matter and a pile of supporting detail. Here is what each one is really telling you, and where it can mislead you.
System size in kWp
kWp — kilowatt-peak — is the array's rating under laboratory test conditions: 1,000 W/m² of light at a cell temperature of 25 °C. Your roof almost never sees those conditions at the same time. A 10 kWp array in Lahore or Dubai in June is lit hard enough but running at 60 °C, so it might deliver 7.5 kW. That is normal and it is already in the numbers above.
Specific yield
The single most useful figure for comparing locations: kilowatt-hours per year per kilowatt-peak installed. A sunny, dry site runs 1,600–1,900. A cloudy northern one runs 850–1,100. If an installer's projection is far above the figure here, ask which irradiance dataset they used and what losses they assumed.
Payback period
This is the year your cumulative savings cross your upfront cost, with the tariff rising and the panels slowly degrading. It is extremely sensitive to two inputs you should not accept the defaults on: the installed cost per kWp, and your tariff. In markets with steep subsidised tariff slabs, solar that shaves your peak slab off is worth far more per unit than the average rate suggests.
Inverter size
Two things fight for this number. The array wants an inverter around 80–85% of its DC rating, because an oversized array and a slightly clipped inverter is cheaper per unit generated. Backup wants an inverter that can carry your peak load. The calculator takes the larger of the two and tells you which one won.
The traps that wreck real installations
- Shade you did not account for. A single water tank shadow crossing one panel can pull a whole string down. Walk the roof at 9 am and 4 pm, not at noon.
- Dust. In dusty, low-rain regions, soiling can cost 10–20% between washes, not the 3–5% that datasheets assume. It is the cheapest loss to fix and the most commonly ignored.
- Lead-acid sold as "equivalent" to lithium. It is not. You only get about half the nameplate capacity out of it, and far fewer cycles. Compare on usable kWh across the warranty, not on sticker price.
- Net-metering rules changing. Export rates get cut. Build your case on self-consumption first and treat export credit as a bonus.
- An inverter with no headroom. If your inverter is exactly your peak load, the first time the AC compressor and the water pump start together, it trips.
Common questions
Does this use Google's solar data?
No, and deliberately so. Google's Solar API has excellent roof-level detail but only covers selected countries, and it needs a billed API key. This site uses NASA's POWER dataset, which covers the entire planet at no cost and needs no key — so it works just as well in Karachi, Lagos or Quetta as it does in California. The trade-off is that we do not see your individual roof planes, which is why you tell us the mounting and facing yourself.
How accurate is it?
The sunlight figures are long-term satellite-derived averages and are solid to within a few percent for a typical year. The uncertainty is almost entirely in the things only you can know: your real shading, your actual tariff slab, and what an installer will charge you. Expect the generation estimate to be good and the payback estimate to be a starting point for negotiation.
Should I get a battery?
If your grid is reliable and you have decent net metering, a battery usually lengthens your payback — it is bought for comfort, not for return. If you face regular load-shedding, the calculation changes completely, because the alternative is a generator burning fuel. Size it for the loads you genuinely cannot lose, not for the whole house.
Do you store my location or my bill?
No. Everything runs in your browser. Your coordinates go to NASA POWER and OpenStreetMap to fetch sunlight data and map tiles, and that is the extent of it. There is no account, no database and no lead form. See the privacy policy for the detail, including the cookies our advertising partners set.
Can I use this to check a quote I have been given?
That is the best use for it. Enter your real consumption and the installer's own price, then compare their promised annual generation against the specific yield here. If they are claiming materially more, ask them to show their working.