Solar rooftop calculator
Work out the rooftop solar system size, panel count and roof area you need to cover your monthly electricity use in Nepal, from your average units per month.
See the install cost, annual savings from NEA net-metering, and how many years the system takes to pay for itself. Every assumption is editable, computed entirely in your browser.
Your usage
From your NEA bill, the units consumed in a typical month.
Usable, unshaded rooftop area for panels.
Assumptions
Nepal averages 4.5–5.
Wiring, inverter, dust, heat losses.
0.4 kWp = a 400W panel.
Roof area used per panel.
Cost & tariff
All-in cost per kWp: panels, inverter, mounting and labour.
Blended NEA rate your export offsets. NEA bills by ampere-band slabs, so pick the rate closest to your bill.
Recommended system size
3.51 kWp
9 panels, 194 sqft of roof
Install cost
Rs 3,85,965
Panels
9
Roof area needed
194 sqft
Annual savings
Rs 52,800
Simple payback
7.3 yrs
| System size formula | monthly kWh ÷ 30 ÷ peak sun hours ÷ loss factor |
| System size | 400 ÷ 30 ÷ 4.75 ÷ 0.80 = 3.51 kWp |
| Panel count | ceil( 3.51 ÷ 0.40 kWp ) = 9 panels |
| Roof area needed | 9 × 21.5 sqft = 194 sqft |
| Install cost | 3.51 kWp × Rs 1,10,000 = Rs 3,85,965 |
| Annual generation | 3.51 × 4.75 × 365 × 0.80 = 4,867 kWh |
| Annual bill offset | min(4,867, 4,800) kWh × Rs 11.00 = Rs 52,800 |
| Simple payback | Rs 3,85,965 ÷ Rs 52,800 = 7.3 years |
A planning-level estimate using standard 400W-panel and per-kWp-cost assumptions, all editable above. Actual panel efficiency, shading, roof orientation, inverter losses and NEA net-metering approval terms vary by site. This is informational only, not a substitute for a site survey or a licensed solar installer's quotation.
From monthly use to a sized, priced system
Each step feeds the next: your electricity use sets the system size, the system size sets the panel count and roof area, and generation versus consumption sets the payback.
System size
kWp = monthly kWh ÷ 30 ÷ peak sun hours ÷ loss factor. This is the capacity that meets your average daily need.
Panels & roof area
Panel count = ceil(system kWp ÷ panel rating). Roof area needed = panel count × each panel's footprint.
Cost & payback
Install cost = system kWp × Rs/kWp. Payback years = cost ÷ annual bill offset from net-metering credits.
Rooftop solar, answered
What does net-metering actually mean?+
Under NEA's net-metering directive, a bidirectional meter tracks both the power you draw from the grid and the excess your panels export. Excess export is credited against your future bills as units, not paid out as cash, so any surplus you don't use rolls forward rather than being refunded.
Why is there a 0.8 system-loss derating factor?+
A panel's rated output is measured under lab conditions. In real installations, wiring resistance, inverter conversion loss, dust and soiling, heat de-rating and shading all cut into actual output, typically by 15–25%. The 0.8 factor (a 20% loss) is a common planning assumption; it is editable if your installer quotes a different figure.
Are the panel count and roof-area assumptions fixed?+
No, every assumption above the results, panel rating, panel footprint, peak sun hours, installed cost per kWp and the electricity rate, is editable. The defaults (400W panels, roughly 21.5 sqft per panel) reflect common industry sizes, but actual panel dimensions vary by manufacturer, so adjust them to match your installer's quotation.
Why does Nepal's peak sun hours matter, and what range should I use?+
Peak sun hours estimate the equivalent hours per day of full-intensity sunlight a site receives, the key driver of how much energy a given system generates. Nepal typically averages 4.5 to 5 peak sun hours depending on region, altitude and seasonal cloud cover; the Tarai and Kathmandu Valley sit toward the higher end, while hill and mountain districts can see more haze in summer.
How is the simple payback period calculated?+
Simple payback = total install cost ÷ annual bill offset. It divides what you spend upfront by the yearly savings the system's net-metering credits generate, without discounting for inflation, financing cost or panel degradation over time, so it is a rough guide rather than a full financial return calculation.
Sources & data note
Based on NEA's net-metering directive and standard 400W-panel, per-kWp installed-cost industry assumptions, all editable above. Actual panel efficiency, shading, orientation, inverter losses and site-specific NEA approval terms vary, this is an informational planning estimate, not a substitute for a site survey or a licensed solar installer's quotation.