This calculator helps determine the right solar system size in kilowatts for your monthly electricity consumption. It accounts for Pakistan's location-specific sunlight hours and the panel wattage you're considering, producing a system-size recommendation in kW and an estimated number of panels.
Calculate Required Solar System Size
How solar system sizing actually works
The fundamental sizing equation is straightforward: required system size (kW) = monthly kWh needed ÷ (peak sun hours × 30 × system efficiency factor). For a household using 600 kWh per month in central Punjab (5 peak sun hours daily), the calculation runs 600 ÷ (5 × 30 × 0.85) = 4.7 kW. The 0.85 efficiency factor accounts for real-world derating — temperature losses, inverter conversion losses, dust accumulation between cleanings, and minor wiring losses. Solar panels rarely produce their nameplate wattage in ambient Pakistani conditions; the derating reflects that physical reality.
The sizing changes substantially with location. Southern Punjab and Sindh average 5.5 to 6 peak sun hours daily — a household with the same 600 kWh demand would need a slightly smaller 4.2 kW system. Northern areas average 4 peak sun hours daily, pushing the required system to 5.9 kW for the same demand. The calculator applies these location-specific factors automatically based on your selection.
Number of panels — the second half of the calculation
Once the system size in kilowatts is known, dividing by panel wattage gives the panel count. A 5 kW system using 400W panels needs 13 panels (rounded up); the same system using 550W panels needs 10 panels; using 600W panels needs 9 panels. Higher-wattage panels mean fewer total panels, less roof area, and typically lower installation labour cost. Lower-wattage panels may be cheaper per panel but the total installation cost usually ends up similar or higher because labour costs scale with panel count.
The roof area requirement is roughly the panel count × panel area × 1.3 (for installation spacing). Ten 550W panels occupy about 25 sq feet each × 1.3 spacing factor = 325 sq feet of usable roof. Most Pakistani houses with 4-bedroom and larger configurations have sufficient roof area; smaller houses or apartment buildings with limited roof access may need to size around available area rather than ideal consumption.
Grid-tied versus off-grid sizing implications
Grid-tied systems can be sized exactly to monthly consumption because the grid acts as a virtual battery — daytime excess exports back, night-time imports balance it out. The calculator's recommendation works for grid-tied installations without modification. Off-grid systems need to account for energy storage in battery banks, plus oversize the solar generation by 20–30% to keep batteries charged through cloudy days. An off-grid version of a 600 kWh household typically needs a 6 to 7 kW solar system plus a 15 to 20 kWh battery bank — significantly more upfront investment than grid-tied.
Hybrid systems split the difference — grid-tied with a smaller battery for backup during outages. A 5 kW solar + 5 kWh battery hybrid is a common configuration for Pakistani households dealing with load shedding while still benefiting from net metering. The sizing of the solar component matches grid-tied calculations; the battery is sized to support critical loads (lights, fans, refrigerator, internet) for the expected outage duration.
System sizing questions worth thinking through
Why do some installers recommend systems much larger than what this calculator suggests?
Installer recommendations sometimes assume future consumption growth — adding an EV in the next few years, finishing the upper floor of the house, adding a second AC. Some recommendations also assume you'll switch off the grid entirely and need full backup capacity. The calculator here estimates the system size needed to match your current consumption with grid-tied net metering — the most economically efficient configuration for typical Pakistani households. If your future plans include significant new consumption, oversizing by 20–40% may be justified; if your consumption is stable, the calculator's recommendation is the cost-optimal size.
How much roof space does a 5 kW solar system actually need?
A 5 kW system using 550W panels needs nine panels (5,000 ÷ 550 ≈ 9.1, rounded up to 10 for slight oversizing). Each modern panel is approximately 2.3 meters × 1.1 meters (about 25 sq feet). Ten panels occupy roughly 250 sq feet of roof area. The panels need to be arranged with proper spacing for installation access — typical roof requirement is 1.3× to 1.5× the bare panel footprint, so about 350 to 400 sq feet of usable roof space for a 5 kW system. Lower-wattage 400W panels require more roof area (roughly 13 panels for 5 kW), and 600W panels need less (roughly 8 panels). Confirm available roof area before sizing.
What's the practical difference between 400W and 550W panels for the same total system size?
Same total kilowatts means same total generation. The differences show up in three places: roof space (higher-wattage panels need less area), cost per watt (550W panels are usually cheaper per watt than 400W panels because of manufacturing economies), and aesthetics (fewer larger panels look slightly cleaner than more smaller panels). For a system that has to fit in limited roof space, higher-wattage panels are essential. For systems with abundant roof space and tight budgets, lower-wattage panels can work, but 2026 market pricing typically favours the higher-wattage modern panels on a per-watt basis anyway.
Should I oversize my solar system to handle a future EV charger?
If you plan to buy an EV within 2 to 3 years, oversizing now usually makes more economic sense than upgrading later — adding panels to an existing installation is more expensive per watt than installing the larger system at once. An EV adds roughly 200 to 400 kWh of monthly consumption depending on driving distance, which translates to needing an extra 1.5 to 3 kW of solar capacity. Plan for this if your timeline is reasonably certain. If the EV plan is more aspirational than concrete, size for current consumption and accept the upgrade premium if and when an EV actually arrives — many households planning EVs in 'a year or two' have those plans stretch to 4 or 5 years.
How does shading from nearby buildings or trees affect required system size?
Shading is the most overlooked factor in residential solar sizing. Even partial shading of a single panel can reduce that panel's output by 70–90%, and in some inverter configurations the shading can cascade to affect multiple panels in the same string. For roofs with afternoon shading from neighbour buildings or nearby trees, two strategies work: install panels only on unshaded sections of the roof (smaller system, no compromise on output), or install panel-level power optimisers that isolate shading effects to individual panels. Site surveys by reputable installers should always include shading analysis throughout the day — if a quote doesn't mention shading, the analysis is incomplete.