Solar Panel Calculator
Find the solar system size, number of panels and roof space you need, what it makes in a year, what it saves and how fast it pays back. Advanced adds net metering, incentives, NPV, CO₂ and a backup battery.
Your bill shows the kWh used. An average month gives the best size.
Find your location's peak sun hours from a solar map such as the Global Solar Atlas or PVGIS: the yearly average of daily solar energy on your panels in kWh per m² is the same number.
The performance ratio is what reaches your meter after heat, dust, wiring and inverter losses. Hot, dusty places sit at the lower end; clean panels in a mild climate at the higher end.
Net metering sets what you send against what you take, up to your use in a year. Extra kWh are paid at the export price.
Your grid operator or government publishes this figure. It differs a lot from country to country, so nothing is assumed.
Leave the load blank to use your average load. The battery's cost is added to the system cost; backup does not change the savings here.
Year by year: output, savings and payback
| Year | kWh made | Saved | Net of upkeep | Running total |
|---|
How your solar system is worked out
Size = kWh a year ÷ (365 × sun hours × performance).
- 1
Energy to cover
900 kWh × 12= 10,800 kWh/yr
- 2
System size
10,800 ÷ 365÷ 5.5 h ÷ 80%= 6.72 kW → 17 × 400 W= 6.8 kW
- 3
Savings in year 1
10,800 kWh used× $0.15= $1,620
- 4
Payback
$6,800 cost− yearly savings= 4.2 years
Read more: the formula, a worked example, slab tariffs, net metering, batteries and tips
A solar system is sized from the energy you want it to make in a year. Each kW of panels makes about 365 × peak sun hours × performance ratio kWh a year, so dividing your yearly kWh by that gives the kW you need. Whole panels round it up. The savings are the kWh you no longer buy, and the payback is how many years of savings it takes to cover the cost.
The formula
kWh a year to cover = kWh a month × 12 × share to cover
kWh per kW a year = 365 × peak sun hours × performance ratio
kW needed = kWh to cover ÷ kWh per kW
panels = kW needed × 1000 ÷ panel watts, rounded up
system kW = panels × panel watts ÷ 1000
roof area = panels × size of one panel
kWh made in year y = system kW × kWh per kW × (1 − output lost)^(y − 1)
saved = kWh used at home (up to your use) × price × (1 + price rise)^(y − 1)
+ kWh exported × export price
with a slab tariff: saved = 12 × (bill before − bill after)
cost = system watts × cost per watt (+ battery) − incentive − rebate
payback = years until the savings, less upkeep, add up to the cost
NPV = each year's net saving ÷ (1 + discount rate)^year, added up, − cost
Worked example
You use 900 kWh a month, pay $0.15 per kWh, get 5.5 peak sun hours (example only), and choose 400 W panels at $1 per watt (example only). 900 × 12 = 10,800 kWh a year. Each kW makes 365 × 5.5 × 0.8 = 1,606 kWh a year, so you need 10,800 ÷ 1,606 = 6.72 kW. That is 6,724.8 W ÷ 400 W = 16.81, rounded up to 17 panels, 6.8 kW, taking 17 × 2.0 = 34 m² of roof.
In year 1 it makes 6.8 × 1,606 = 10,920.8 kWh. You use 10,800 kWh, so it saves 10,800 × $0.15 = $1,620; the other 120.8 kWh go to the grid, paid at an export price of 0 here. It costs 6,800 W × $1 = $6,800.
Losing 0.5% a year, it makes 10,866.196 kWh in year 2 and 10,811.865 kWh in year 3, still above your use, so each saves $1,620. Year 4 makes 10,757.806 kWh, saving $1,613.67. After 4 years the savings add to $6,473.67, leaving $326.33. Year 5 saves 10,704.017 × $0.15 = $1,605.60, so 326.33 ÷ 1,605.60 = 0.20 of a year: it pays back in 4.2 years (4 years 2 months). Over 25 years it saves $38,557.65, which is $31,757.65 after its cost, and the NPV at a 5% discount rate is $15,196.81.
Slab tariffs: why the Gulf presets save more than an average price
On a slab tariff the last kWh in your bill are charged at the highest rates, and solar takes those away first. So this page works out your real bill before and after solar with the same tariff code as its bill calculators, instead of using one average price. A Qatar villa on the Kahramaa tariff (checked 2026-10-13) using 6,000 kWh a month pays QAR 840.00. A 21 kW system at 6 sun hours makes 36,792 kWh a year, 3,066 kWh a month, leaving 2,934 kWh and a bill of QAR 341.42. That saves QAR 498.58 a month, QAR 5,982.96 a year, more than the bill's average price of QAR 0.14 per kWh would suggest (QAR 5,150.88).
The presets use the tariff tables from our Qatar, Dubai, Saudi, Bahrain and Oman (Dhofar) bill calculators, each checked on the utility's own page (Kahramaa: km.qa). Fixed charges, fuel surcharges and VAT are included where that calculator includes them. Every month is taken as an average month; in summer you use more and the panels make more, so the real split by month will differ. Whether your utility lets you connect solar, and what it pays for exported power, is set by its own rules.
Peak sun hours
Peak sun hours squeeze the day's sunlight into hours at full strength (1,000 W per m²). They depend on where you live, the season, and the tilt and direction of your roof, so this page does not guess them: look up your own on a solar map such as the Global Solar Atlas or PVGIS. The 5.5 filled in at first is only an example.
Net metering or use it and export it
- Net metering: the kWh you send out are set against the kWh you take in. Every kWh you make counts at your full price, up to what you use in a year; anything above that is paid at the export price.
- Use it or export it: only the share you use as it is made saves the full price (it is higher if you run AC or appliances in the daytime, or have a battery). The rest is paid at the export price, often far lower, or nothing.
Battery for backup
Usable kWh = load to keep on (kW) × hours of backup. Battery size = usable kWh ÷ usable share. Keeping 1 kW on for 10 hours needs 10 kWh usable; at a 90% usable share that is a battery of about 11.1 kWh. With no load typed in, the page uses your average load: yearly kWh ÷ 8,760 hours.
Tips
- Get at least two installer quotes. The cost per watt filled in at first is only an example; quotes differ a lot by country, roof and brand.
- Panels facing the equator, with little shade, make the most. A shaded panel can pull down a whole string.
- In hot, dusty places, clean panels regularly: dust and heat both lower the performance ratio.
- Check your utility's rules on connecting solar and on export before you buy: they decide how much each kWh is worth.
This is an estimate for planning. It assumes the same use and output every month, and that the panels' kWh replace the dearest kWh on your bill. Prices, incentives and tariffs change: use your own figures.
Common questions
How many solar panels do I need?
Divide the kWh you want to cover in a year by 365, by your peak sun hours and by the performance ratio (about 80%) to get the kW you need, then divide by the watts of one panel and round up. 10,800 kWh a year at 5.5 sun hours needs 10,800 ÷ (365 × 5.5 × 0.8) = 6.72 kW, which is 17 panels of 400 W.
What are peak sun hours?
The number of hours a day the sunlight would be at its full strength of 1,000 W per m² if you squeezed the whole day's light into them. It is the same number as the daily solar energy in kWh per m². Find yours on a solar map such as the Global Solar Atlas or PVGIS; it depends on where you live and on the tilt and direction of your roof.
How long does solar take to pay for itself?
Payback is the cost after any incentives divided by what the system saves each year, counted year by year as the panels slowly lose output and prices change. With 6.8 kW costing 6,800 and saving 1,620 a year, it pays back in about 4.2 years. Cheaper power, a higher cost per watt or poor sun make it longer.
Why does my slab tariff change the savings?
On a slab (tiered) tariff, the kWh at the top of your bill are the dearest ones, and solar removes those first. A Qatar villa using 6,000 kWh a month saves more by cutting 3,066 kWh than the bill's average price suggests, because those kWh were charged at the higher slabs.
What is net metering?
Under net metering, power you send to the grid is set against power you take, so every kWh the panels make counts at your full price up to what you use. Without it, only the share you use yourself saves the full price and the rest is paid at a lower export price, or not at all.
How big a battery do I need for backup?
Multiply the load you want to keep running in kW by the hours of backup to get the usable kWh, then divide by the usable share of the battery (about 90% for lithium). Keeping 1 kW running for 10 hours needs 10 kWh usable, or a battery of about 11.1 kWh.
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Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.