How many solar panels do I need for a house in Sri Lanka?

It depends on your monthly units and your zone. A typical Colombo home using 300 units/month needs about 2.8 kWp — six 550 W panels (≈3.3 kWp installed) on roughly 18 m² of roof. A 500-unit home in the dry zone needs about eight panels. Enter your own units above for an exact figure.

What size solar system do I need for a Rs 5,000 electricity bill?

At the current domestic tariff a Rs 5,000 bill is roughly 120–140 units a month. That points to a 1–1.5 kWp system — two to three panels. Because tariffs are tiered, check the actual units on your bill and enter that number rather than the rupee amount for an accurate size.

How much roof area is needed for a 5 kW solar system?

A 5 kWp array of 550 W panels is about nine to ten panels. At roughly 3 m² of usable roof per panel (panel size plus spacing and access), that is around 27–30 m² — about 300 sq ft of unshaded, well-oriented roof.

How many units does a 3 kW solar system produce per month in Sri Lanka?

A 3 kWp system generates roughly 3 × PSH × 0.78 × 30 units a month. In the Colombo wet zone (PSH 4.5) that is about 316 units; in the dry zone (PSH 5.4) closer to 380 units. The calculator shows the monthly generation for your exact size and zone.

What size inverter do I need for a solar system?

Inverters are sized a little below the panel DC capacity — a DC:AC ratio near 1.15 is normal because panels rarely hit their full rating. A 3.3 kWp array pairs with a 3 kW inverter; a 4.4 kWp array with a 4–5 kW inverter. The tool snaps to standard stock ratings (3, 4, 5, 6, 8, 10 kW).

Why does my climate zone change the recommended size?

Peak Sun Hours differ across the island. The wet zone around Colombo averages about 4.5 kWh/m²/day, the intermediate zone 4.9, and the dry and northern zones up to 5.4. More sun means each panel makes more energy, so a dry-zone home needs fewer panels than a wet-zone home for the same bill.

Does this calculator include battery or off-grid sizing?

No. This tool sizes a grid-tied system that offsets your CEB bill under net metering, net accounting or net plus. Battery and off-grid autonomy sizing is a different design problem (days of autonomy, depth of discharge) and is out of scope here. Use the net-metering calculator for payback.

How accurate is this estimate?

It uses standard PV sizing identities with irradiance from the Global Solar Atlas and a 0.78 performance ratio, so it lands within about 10% for a well-oriented, unshaded roof. Tilt, shading, dust and roof orientation shift the real figure. Treat it as a planning estimate, then confirm with an installer's site survey.

When was this calculator last verified?

The zone Peak Sun Hours, performance-ratio range and grid CO₂ factor were last cross-checked against the Global Solar Atlas and SLSEA sources on 2026-07-09. The figures are reviewed whenever the underlying solar-resource data or CEB schemes change.

Sri Lanka · Solar

Sri Lanka Solar Panel Size Calculator — how many panels do I need?

Enter your monthly CEB units and see the exact rooftop solar size for your home — recommended kWp, number of panels, roof area, inverter rating, generation and CO₂ offset. Tuned to your climate zone using Global Solar Atlas irradiance. No signup, sources cited below.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 9, 2026

Size your rooftop solar

Global Solar Atlas · PSH 4.5

Average monthly electricity (units / kWh)

units

Find this on your CEB/LECO bill as "units" consumed. Around Rs 40 per unit, so a Rs 12,000 bill ≈ 300 units.

Your climate zone

Colombo, Gampaha, Kalutara, Kandy, Galle, Ratnapura

How much of the bill to cover

100% aims to zero out your grid import over a year.

Panel wattage

Higher-wattage panels mean fewer panels for the same size.

Typical bills

Recommended system

3.3 kWp

Load needs 2.81 kWp

Panels

× 550 W each

Inverter size

3 kW

≈ 2.87 kW at 1.15:1

Roof area needed

18 m²

≈ 194 sq ft

Estimated generation

352 units/mo

Covers your 300-unit load (117.44%)

CO₂ offset

2,241 kg/yr

At 0.53 kg CO₂ per unit

Sizing breakdown

Daily consumption to offset	9.86 kWh/day
Specific yield (PSH 4.5 × performance ratio)	3.51 kWh per kWp/day
Exact system size required	2.81 kWp
Panels (550 W, rounded up)	6 panels → 3.3 kWp installed
Roof area (3 m² per panel, usable)	18 m² (194 sq ft)
Inverter (÷ 1.15 DC:AC, snapped up)	2.87 kW → 3 kW
Annual generation	4,228 kWh/yr

Estimates assume a well-oriented, largely unshaded roof. Real output varies with tilt, azimuth, shading and dust. Confirm with a licensed installer's site survey before ordering.

Sources cited: Peak Sun Hours from the Global Solar Atlas (World Bank / Solargis); grid CO₂ factor 0.53 kg/kWh from the Sri Lanka Sustainable Energy Authority. Full methodology and references are below the calculator.

How it works

Sizing rooftop solar comes down to one identity from PV engineering: each installed kilowatt-peak (kWp) of panels produces roughly Peak Sun Hours × performance ratio kilowatt-hours per day. Peak Sun Hours (PSH) is the number of hours of full-strength sun your roof effectively receives, and the performance ratio (PR) accounts for real-world losses — heat, wiring, inverter conversion, soiling and mismatch.

The calculator uses PSH bands from the Global Solar Atlas irradiance data for Sri Lanka, split into three zones:

Wet zone: PSH 4.5 kWh/m²/day — Colombo, Gampaha, Kalutara, Kandy, Galle, Ratnapura
Intermediate zone: PSH 4.9 kWh/m²/day — Kurunegala, Badulla, Monaragala, Kegalle
Dry / Northern zone: PSH 5.4 kWh/m²/day — Anuradhapura, Hambantota, Jaffna, Trincomalee, Puttalam

The steps it runs are:

Daily load. Multiply your monthly units by the offset target, then divide by 30.42 (365 ÷ 12) to get the kWh you want to cover each day.
Required size. Divide daily load by (PSH × PR). With the default PR of 0.78, this is the standard sizing formula kWp = daily kWh ÷ (PSH × PR).
Panel count. Round the required watts up to whole panels at your chosen wattage (450, 550 or 600 W). Rounding up gives the installed capacity, which is usually a little above the bare requirement.
Roof area. Allow about 3 m² of usable roof per panel — a large-format panel is roughly 2.6 m² plus a 1.15 spacing-and-access factor.
Inverter. Divide installed kWp by a DC:AC ratio of 1.15 and snap up to a standard stock rating (3, 4, 5, 6, 8 or 10 kW). Panels rarely hit their nameplate rating, so a slightly smaller inverter is normal and cost-effective.
Generation and CO₂. Installed kWp × PSH × PR × 365 gives annual kWh; dividing by 12 gives the monthly units shown against your load. Annual kWh × 0.53 kg estimates the CO₂ displaced, using the SLSEA grid emission factor.

The requirement is verified two ways: the daily method above and an annual method that divides yearly units by (PSH × PR × 365). Both return the same kWp to three decimals, which is why the recommended size is stable no matter how you slice the period.

Worked examples

Colombo home · 300 units/month · wet zone

Daily load: 300 ÷ 30.42 = 9.86 kWh/day
Required size: 9.86 ÷ (4.5 × 0.78) = 2.81 kWp
Panels: ceil(2,810 ÷ 550) = 6 → installed 3.30 kWp
Roof area: 6 × 3.0 = 18 m² (≈ 194 sq ft)
Inverter: 3.30 ÷ 1.15 = 2.87 → 3 kW
Generation: 3.30 × 4.5 × 0.78 × 365 = 4,228 kWh/yr ≈ 352 units/mo
CO₂ offset: 4,228 × 0.53 = 2,241 kg/yr

Anuradhapura home · 500 units/month · dry zone

Daily load: 500 ÷ 30.42 = 16.44 kWh/day
Required size: 16.44 ÷ (5.4 × 0.78) = 3.90 kWp
Panels: ceil(3,903 ÷ 550) = 8 → installed 4.40 kWp
Roof area: 8 × 3.0 = 24 m² (≈ 258 sq ft)
Inverter: 4.40 ÷ 1.15 = 3.83 → 4 kW
Generation: 4.40 × 5.4 × 0.78 × 365 = 6,764 kWh/yr ≈ 564 units/mo
CO₂ offset: 6,764 × 0.53 = 3,585 kg/yr

Edge case · exact panel boundary

If the required size lands on an exact panel multiple — say 2.00 panels' worth of watts —
the count rounds to 2 panels, not 3.
Panels: ceil(1,100 ÷ 550) = ceil(2.00) = 2 (no off-by-one).
Only a fraction above a whole panel bumps the count up, so you never buy a spare panel you don't need.

Frequently asked questions

Sources & references

Zone Peak Sun Hours, the performance-ratio range and the 0.53 kg CO₂/kWh grid factor were last cross-checked against these sources on 2026-07-09. The page is reviewed whenever the solar-resource data or CEB schemes change.

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