Sizing a solar system requires knowing monthly electricity consumption (in kWh), the location's capacity factor (the fraction of theoretical maximum output the system actually delivers across the year, typically 16-18% in Pakistan), and per-panel wattage (modern panels typically 400-600W). The calculator computes required system capacity for 100% offset and approximate panel count. For partial-offset sizing, scaling down proportionally produces appropriate smaller systems.
The household wants to install rooftop solar but has received conflicting size recommendations from different installers — some quoting 8kW, others 12kW or larger — and the family wants to independently calculate appropriate sizing based on their actual consumption.
Where solar sizing estimates go wrong
Sizing from a single month's consumption misses seasonal variation; annual averages produce more realistic baseline.
Capacity factor varies by location — sunnier southern Pakistan slightly higher than cooler northern areas.
Roof area constraints sometimes prevent sizing to ideal-on-paper level.
Installer over-sizing (more panels than household needs) creates excess cost without proportional benefit if export credits don't fully match retail tariff value.
Calculate based on annual-average monthly consumption (sum 12 months bills, divide by 12). Use 17% capacity factor as Pakistan default; adjust if you have location-specific data. Result is system capacity and panel count for 100% annual offset; size lower for partial offset matching budget.
Estimate System Size
Pakistani average capacity factor ~17% (varies by location). Modern panels 400-600W typical.
The capacity-factor concept
Capacity factor expresses how much of theoretical maximum the system actually produces. A 10kW system at 17% capacity factor produces 10kW × 8760 hours × 17% = 14,892 kWh annually — substantially less than 10kW continuously (87,600 kWh) because solar only produces during sunlight hours, varies with weather, declines from peak output during non-peak sun, etc. The 17% factor is reasonable Pakistani average; local conditions affect specific cases.
The panel-wattage evolution
Solar panel wattage has progressed substantially: older panels were 250-300W typical, current generation commonly 400-600W with newer N-type panels reaching higher. Higher wattage means more capacity per panel and fewer panels per kW; useful for roof-constrained installations. For sizing calculations, using the wattage of panels you're actually buying gives accurate count; the calculator's 550W default represents current mid-market mainstream.
Roof-area implications
Each modern panel occupies approximately 2 square metres of roof area including spacing. A 10kW system at 550W per panel needs about 18-20 panels, occupying ~36-40m² of usable unshaded roof area. For roof-constrained installations, smaller systems use available area; for sufficient-area installations, sizing matches calculation result rather than roof minimum. The roof suitability guide covers physical assessment.
Average 12 months of consumption rather than picking one month.
Use ~17% capacity factor for Pakistani average; adjust if location-specific data available.
Higher-wattage panels reduce count for given capacity, useful where roof area is tight.
For savings estimation, the solar savings calculator applies. For broader solar planning, the sizing guide covers sizing methodology in detail.
The honest size-vs-budget framing
System sizing involves both technical calculation and budget reality. Calculator output represents 100% offset target; actual installation may size to 70-100% of that target based on budget and preferences. Both are reasonable approaches; the right choice depends on financial situation and offset priorities.
The future-consumption-growth dimension
Solar sizing for current consumption may not suit future patterns. EV adoption could substantially increase household consumption over coming years. Family growth or income improvement may produce additional cooling-load. Heat-pump heating replacing gas heating shifts load to electric. For households planning for known coming changes, sizing modestly above current consumption (10-20% growth allowance) accommodates expected growth without requiring system expansion later. For uncertain future, sizing to current consumption with awareness that expansion options exist later supports flexibility.
The roof-orientation effect
Calculator assumes good orientation (typically south-facing in Pakistan). East-facing or west-facing roofs produce less per panel — roughly 80-85% of south-facing equivalent. North-facing typically insufficient for primary installation. For households with sub-optimal roof orientation, slightly larger system sizing compensates for reduced per-panel output to deliver target generation. The orientation factor isn't in the calculator but affects practical sizing; the roof suitability guide covers orientation in more detail.
The capacity-factor variation across Pakistan
Capacity factor varies by location. Southern Pakistan (Karachi, Sindh) tends to have slightly higher than 17% baseline due to more sun-hours. Northern Pakistan (Lahore, Islamabad) approximately matches the 17% default. Higher elevation or mountain locations may differ. For initial sizing, the 17% default works as Pakistani average; specific locations may justify adjustment by 1-2 percentage points based on local conditions. Installer experience in your specific area provides location-calibrated estimates.
Frequently Asked Questions
5-15 kW common for residential depending on consumption. Specific size matches specific household pattern.
Calculation is for 100% offset target; sizing to 70-90% of consumption is also common and costs less.
It doesn't directly — assumes unshaded roof. Substantial shading reduces effective capacity factor; engage installer for shaded-installation specifics.
Slight oversizing (10-15%) may suit households with growth in consumption anticipated; substantial oversizing may not produce proportional returns if export credits don't fully match retail tariff.