The single most important rule of solar sizing: size your system from your electricity bill, not from your roof. In Gujarat, 1 kW of rooftop solar generates roughly 4–4.5 units per day and about 1,400–1,600 units per year — so divide your average monthly consumption by ~120–135 units to get a starting kW figure, then sanity-check it against roof area, sanctioned load, and budget.
The short version
- Homes: 3 kW is the sweet spot — it covers ~360 units/month and captures the maximum PM Surya Ghar central subsidy of ₹78,000. Over-sizing is poor economics because surplus exported to the grid earns only ₹2.25/unit versus ₹7–8/unit you save by self-consuming.
- Factories & commercial: size to your daytime load and contract demand (kVA), not for heavy export. Self-consumption at ₹7–8/unit drives 3–5 year paybacks; commercial systems cost less per kW (~₹35,000–50,000/kW), and the residential PM Surya Ghar subsidy does not apply.
The core sizing figure for Gujarat
Gujarat enjoys 5.5–6.5 kWh/m²/day of solar irradiance and ~300 sunny days a year, among the best in India. The realistic, design-grade generation figure is 4 to 4.5 units (kWh) per kW per day, or about 1,400–1,600 units per kW per year. This is the AC output after losses, not the nameplate maximum. Use ~4 units/kW/day as a conservative planning number for full-year averages (including monsoon) and ~4.5 for optimistic, well-oriented, clean installations.
Step-by-step sizing method (from bill to kW)
- Find your average daily consumption. Take 6–12 months of electricity bills, average the monthly units (kWh), and divide by 30. Example: 300 units/month ÷ 30 = 10 units/day.
- Divide by per-kW daily generation. Using Gujarat's ~4 units/kW/day: 10 ÷ 4 = 2.5 kW to fully offset. (Equivalent shortcut: monthly units ÷ ~120 = kW; 300 ÷ 120 = 2.5 kW.)
- Adjust for target offset. If you want to cover 100% of consumption, keep 2.5 kW. Many homeowners round up to 3 kW to capture the full subsidy and allow for future load growth.
- Check roof area. 2.5–3 kW needs ~135–160 sq ft of shadow-free roof (32 sq. ft. per 600 Wp panel). If you don't have it, your roof caps your system.
- Check sanctioned load. Residential consumers in Gujarat can exceed sanctioned load, but staying within it avoids paperwork; systems ≤10 kW get auto-enhancement by the DISCOM.
- Check budget. Multiply target kW by cost/kW and subtract the subsidy.
Performance ratio & system losses
A rooftop system never delivers its nameplate output. After inverter conversion, temperature derating (Gujarat's heat is significant), dust/soiling, wiring, and shading, a well-designed Indian rooftop plant achieves a performance ratio (PR) of about 75–85% in year one, with designers typically assuming a PR of 0.80. This is already baked into the "4–4.5 units/kW/day" figure, which is why you should never size off the theoretical ~5.5 units. Installers also slightly over-size the DC array (e.g. 3.24 kWp of panels for a "3 kW" system) to compensate.
Roof area
Plan for roughly 54 sq. ft. of shadow-free roof per kW (32 sq. ft. per 600 Wp / 0.6 kW panel) with modern TOPCon modules. Older government guidance cited 10 sq. m (≈110 sq. ft.) per kW, which was based on lower-efficiency panels; today's 600 Wp TOPCon panels (21–22.5% cell efficiency) significantly reduce the footprint. Homes are usually constrained by limited terrace area; factories with large sheds and warehouses usually have far more roof than they need.
Regulatory cap (Gujarat, verified against GERC)
The operative rule is the GERC (Net Metering Rooftop Solar PV) Third Amendment Regulations, 2022 (Notification No. 02 of 2022, dated 31/05/2022). Key facts:
- Net metering is permitted for systems from 1 kW up to 1 MW (1,000 kW).
- Residential consumers may install rooftop solar irrespective of the sanctioned load — there is no 10 kW or sanctioned-load cap for homes (only the 1 MW project ceiling). The widely-repeated installer-blog claim that residential net metering is capped at "10 kW or sanctioned load, whichever is lower" is not supported by the GERC regulation; it conflates a separate deemed-approval threshold.
- For systems up to 10 kW, the application is deemed approved without a technical feasibility study, and the DISCOM itself carries out any sanctioned-load enhancement (Fourth Amendment, 2024).
- For systems above 6 kW, the consumer bears system-strengthening charges on the additional capacity.
- Captive / third-party / commercial consumers are capped at their sanctioned load / contracted demand (within the 1 MW ceiling); projects for RPO compliance are exempt from the load cap.
- A new GERC DRES Regulations, 2026 is still in DRAFT only (not yet notified). Per draft, net metering up to 1 MW and virtual net metering up to 4 MW (surplus compensated at ₹2.25 per kWh or a rate determined by the Commission), and — importantly for large factories — a battery storage (BESS) requirement for consumers with contract demand above 100 kW who install DRES capacity beyond their contracted demand. Treat this as forthcoming, not current law.
Subsidy (residential only, 2026)
PM Surya Ghar Muft Bijli Yojana central financial assistance, confirmed by MNRE: ₹30,000/kW for the first 2 kW, ₹18,000/kW for an additional capacity up to 3 kW, with total subsidy for systems larger than 3 kW capped at ₹78,000 (i.e. ₹30,000 for 1 kW, ₹60,000 for 2 kW, ₹78,000 for 3 kW+). A 5 kW or 10 kW system still receives only ₹78,000. As of 2026, Gujarat does not offer a verified additional state cash subsidy stacked on top of the central amount for residential rooftop solar (some installer sources claim a ₹10,000/kW Gujarat top-up — treat as unverified). Subsidy is paid by DBT after commissioning. Commercial/industrial consumers do NOT get this subsidy but can claim accelerated depreciation (40% in year one) and benefit from 5% GST on modules.
Cost per kW, 2026 Gujarat
- Residential: roughly ₹55,000–70,000/kW all-in before subsidy (smaller 1–2 kW systems higher, toward ₹65,000–85,000/kW; 3 kW around ₹55,000–70,000/kW).
- Commercial/industrial (larger systems, 25 kW–100 kW+): roughly ₹35,000–50,000/kW (≈₹32–45/watt), thanks to economies of scale. A 100 kW system runs about ₹40–50 lakh.
Export vs self-consumption economics
Surplus solar exported to the grid in Gujarat is compensated at only ₹2.25/unit for the first five years from commissioning (per the GERC Third Amendment 2022, for self-owned and SURYA Gujarat consumers), thereafter at 75% of the average GUVNL competitive-bid solar tariff, settled annually. Self-consumed solar instead saves you your retail tariff (₹4.15–5.20/unit on the top residential slabs; ~₹7–8/unit effective for commercial/industrial including demand-related costs). This roughly 3x gap is the single most important reason NOT to over-size.
Common sizing mistakes
- Oversizing: paying full price for kW whose surplus only earns ₹2.25/unit. A 7 kW system on a 350-unit/month home exports hundreds of units monthly at ₹2.25 instead of saving them at retail rates — years of extra payback.
- Undersizing: still paying the expensive top tariff slabs and leaving roof space and subsidy headroom unused.
- Ignoring future load growth: roof space, new ACs, or factory expansion. Leave modest headroom.
- Ignoring shading/orientation: even 5% shading on a panel can cut its output 30–50%. Always get a shade analysis.
- Using nameplate generation: never size off ~5.5 units/kW/day; use 4–4.5 after losses.
Worked example — typical Gujarat home
- Consumption: 300 units/month (10 units/day).
- Sizing: 10 ÷ 4 = 2.5 kW; round to 3 kW to capture full subsidy and headroom.
- Generation: 3 kW × 4–4.5 units = ~12–13.5 units/day = ~360–400 units/month.
- Cost: ~₹1.65–2.1 lakh before subsidy → minus ₹78,000 → net ~₹90,000–1.32 lakh.
- Payback: ~3–4 years; 25-year life.
Worked example — small/medium Gujarat factory
- Consumption: 12,000 units/month (400/day), contract demand say 80 kVA, single-shift daytime operation.
- Daytime baseload mostly falls within solar hours, so a high self-consumption system fits.
- Sizing: 400 units/day ÷ 4 = 100 kW would fully cover, but to avoid export, size to ~80–90 kW matched to daytime load. Many factories pick ~80 kW.
- Generation: 80 kW × 4–4.5 = ~320–360 units/day, ~9,600–11,000 units/month, nearly all self-consumed.
- Roof: ~6,400–8,800 sq ft of shed roof (factories usually have plenty).
- Cost (at ~₹40,000–45,000/kW): ~₹32–36 lakh before incentives.
- Savings: ~10,000 units/month × ₹7.5 = ₹75,000/month.
- Payback: ~3.5–4.5 years, helped by accelerated depreciation.
Our recommendations
- Homeowners with bills up to ~300–360 units/month: install 3 kW. It maxes the ₹78,000 subsidy and offsets most of your bill. Go larger only if you consistently consume more or plan an EV/more ACs.
- Homeowners with very high consumption (>400 units/month): size to consumption (e.g. 4–5 kW), accepting that kW beyond 3 are at full cost; economics are still strong versus top-slab tariffs.
- Factories/commercial: get a 12-month load profile and a daytime-load study first. Size to ~70–90% match daytime load to maximise self-consumption and minimise ₹2.25/unit export. Stay within contract demand to avoid load enhancement (and under the draft 2026 rules) a future BESS mandate above 100 kW.
- Always get a professional shade analysis and confirm shadow-free roof area before finalising.
- Benchmarks that change the plan: if the export credit rises materially above ₹2.25/unit (watch the 2026 DRES regulation), oversizing becomes more attractive. If your DISCOM caps net metering on your feeder, consider gross metering or self-consumption-only sizing.
Want your system sized properly?
Share your last few electricity bills and we'll calculate the right kW for your home or factory — sized to your actual usage, not guesswork.
Get a free sizing assessmentGeneration figures (4–4.5 units/kW/day) are climate averages; actual output varies with orientation, tilt, shading, dust, and season (lower in monsoon). Costs, subsidy, and per-kW figures are realistic ranges, not guarantees, and depend on site-specific conditions. The GERC DRES Regulations 2026 and BESS Regulations 2026 are drafts as of mid-2026, not yet law. Always confirm current rules with the DISCOM/GEDA before relying on them.