RainbowHome guide · India residential
What size solar system do I need for my home? (kW sizing guide)
"How many kW?" is the first real question in every rooftop solar decision. There are two honest ways to answer it — size from your electricity bill, or size from your roof — and the right system is usually where the two agree. This page walks through both, with city-specific ranges for Bengaluru and Chennai, so you can arrive at a number before anyone quotes you.
Coverage: Karnataka and Tamil Nadu. Figures below are typical ranges, not guarantees.
Short answer
As a rule of thumb, you need roughly 1 kW of solar for every ~120–150 unitsyou consume per month, which varies with your city's sunlight. By roof, plan for about 100–130 sq ft of shade-free roof per kWp(~10–12 sq m, per MNRE/SECI). A 1 kW system generates around 4 units a day (~120 units/month) in much of India.
For most Indian homes that works out to a 2–4 kW system. Bengaluru homes tend to sit at the lower generation end (~120 units per kW per month) and Chennai a little higher (~130–140), so the same bill points to a slightly smaller system in Chennai. These are ranges — confirm against local irradiance data and your own bill.
Want the number without the arithmetic? Run the calculator with your bill and city — it does both methods and shows the roof-fit and subsidy* caps in one place.
Method 1: size from your bill (units)
This is the method that matters most, because a solar system is really sized to offset consumption. Work in units (kWh), never rupees — tariff slabs make the rupee figure misleading.
- Read your average monthly units. Average the last 6–12 months from your bills so seasonal AC use evens out.
- Divide by your city's per-kW monthly generation. Use roughly 120 units/kW/month in Bengaluru and 130–140 units/kW/month in Chennai (ranges — confirm against local irradiance data). Example: 360 units ÷ 120 ≈ 3 kW.
- Round to your roof and sanctioned load. Make sure the size fits your shade-free roof and your DISCOM sanctioned load; round down when either is tight.
- Cross-check subsidy* caps. Central residential subsidy* support is capped at a system size, so confirm on the official portal before locking a number.
City generation figures are approximate and should be confirmed against local irradiance data; per-kW yield changes with tilt, shading, dust and panel efficiency.
Method 2: size from your roof area
Use this when roof space, not consumption, is the binding constraint — common in apartments, row houses and homes with water tanks and stairwell rooms eating into the terrace.
- Measure the shade-free roof. Count only the area that stays in clear sun through the day — exclude tanks, parapet shadows, stairwell rooms and walkways.
- Divide by area per kWp. Use about 100–130 sq ft per kWp (~10–12 sq m, the figure MNRE/SECI cite). Example: 300 sq ft ÷ 100 ≈ 3 kW at the efficient end. Older or lower-efficiency panels need the higher end.
- Sanity-check against Method 1.If your roof allows more than your bill needs, you usually don't fill it — export caps and subsidy* limits make oversizing wasteful.
Roof-area rule of thumb per MNRE (about 10–12 sq m per kWp). Usable roof is always less than total roof after walkways and tilt spacing. Last verified: July 2026.
Sizing table (typical ranges)
| System size | Typical home | Shade-free roof | Units offset / month |
|---|---|---|---|
| 1 kW | 1BHK / low-usage, no AC | ~100–130 sq ft | ~110–140 units |
| 2 kW | Small 2BHK, light AC use | ~200–260 sq ft | ~220–280 units |
| 3 kW | 3BHK, regular AC use | ~300–390 sq ft | ~330–420 units |
| 5 kW | Large home, multiple ACs | ~500–650 sq ft | ~550–700 units |
Ranges are indicative for Karnataka and Tamil Nadu and blend Bengaluru and Chennai generation. Your figures depend on tilt, shading, panel efficiency and local irradiance — confirm against your own bill and a site assessment.
Watch-outs before you fix a size
- Subsidy* size caps. Central residential subsidy* support is slab-based and capped at a system size — a bigger system does not scale subsidy up without limit. Size for value, then check what qualifies.
- Sanctioned load. Your grid-connected system size is bounded by your DISCOM sanctioned load. If you want a larger system you may need a load enhancement, which your installer coordinates with the DISCOM.
- Future EV / AC load. A modest buffer for a near-term EV or extra AC can be sensible, since adding capacity later means fresh paperwork. But size for realistic near-term load, not a hypothetical maximum.
- Oversizing vs export value.In Karnataka, KERC's DSPV framework (effective 1 July 2025) settles residential rooftop on net billing, not a one-for-one credit: units you import are billed at the retail tariff (~₹5.80/kWh) while any net surplus you export is paid at the lower KERC feed-in tariff (~₹3.86/kWh for domestic 1–10 kW). So self-consuming your own generation is worth more than exporting it, and a system far larger than your daytime usage earns you less on the excess. Match the system to consumption first. KERC Combined Tariff Order 2025
Net-metering caps, sanctioned-load rules and subsidy* slabs are set by regulators and revised periodically. Last verified: July 2026. Check the current terms on mnre.gov.in , bescom.karnataka.gov.in (Karnataka) or TNPDCL (formerly TANGEDCO) (Tamil Nadu).
Related reading
- Solar price per watt in India — what your chosen size should cost, ₹/W.
- Solar panel types explained — which panels fit your roof area and budget.
- How to finance rooftop solar — EMI, loans and subsidy* timing for the size you pick.
- Net metering in Karnataka (BESCOM) — how export caps affect sizing.
- Solar for apartments in Bangalore — sizing when roof area is shared or limited.
- Install timeline — when a system of your size would actually be running.
- How we rank and our methodology — how RainbowHome stays neutral.
Get an exact size for your home
The calculator runs both methods on your actual bill and city, fits the size to your roof, and shows the subsidy* caps and installers who serve your area — no sales call, no quote form.
Harvesting rain too? See the rainwater calculator and RWH installers.
Frequently asked questions
How many kW of solar do I need for a 3BHK house?
Most 3BHK homes in India land in the 2–4 kW range, depending on how much of the day the house is occupied and whether it runs ACs. As a starting point, take your average monthly units and divide by your city's per-kW monthly generation (roughly 120 units in Bengaluru, 130–140 in Chennai, treated as ranges), then round to what your roof and any subsidy* limits allow. A home billing 300–450 units a month typically points to about 2.5–3.5 kW.
How much solar do I need to cover a Rs 3,000 monthly bill?
Work back from units, not rupees. On BESCOM's flat residential tariff of about ₹5.80/kWh, a ₹3,000 bill covers on the order of 500 units before fixed charges (₹3,000 ÷ ₹5.80 ≈ 517), and somewhat fewer once fixed charges are deducted — very roughly a 350–500 unit range in Karnataka or Tamil Nadu. At ~120–140 units per kW per month that suggests about 2.5–4 kW — but confirm the actual units on your bill, since two homes with the same rupee bill can differ by 100+ units.
How much roof area does a 3 kW solar system need?
Plan for roughly 300–390 sq ft of shade-free, south-facing roof for a 3 kW system, using a rule of thumb of about 100–130 sq ft per kWp — roughly 10–12 sq m per kWp, the figure MNRE/SECI cite. Older or lower-efficiency panels need more; the space also has to allow walkways and tilt spacing, so usable roof is always less than total roof.
How many units does a 1 kW rooftop system generate per day in India?
A 1 kW rooftop system in most of India generates about 3.5–4.5 units (kWh) on a good clear day, which averages out to roughly 4 units/day or ~120 units a month over a year once cloudy days, dust and seasonal variation are included. Chennai's stronger irradiance can push the monthly figure a little higher than Bengaluru's; treat these as ranges and confirm against local irradiance data.
Should I oversize my solar system for future EV charging?
It can make sense if you're confident an EV or extra AC load is coming soon, because adding capacity later means a second round of paperwork and mobilisation. But oversizing has limits: your sanctioned load and net-metering export caps constrain how big a grid-connected system you can install, and subsidy* support is capped at a system size. Size for today's consumption plus a realistic near-term buffer, not a hypothetical maximum.
Does a bigger system qualify for a bigger government subsidy*?
Central residential subsidy* support is structured in slabs and is capped — beyond a certain system size the additional capacity gets no extra central subsidy. So a bigger system does not scale subsidy up indefinitely. Because scheme amounts and caps change, check the current structure on the official MNRE portal before sizing around a subsidy figure.
* Potential government subsidies can change. Amounts, slabs and size caps are revised periodically and are not guaranteed. Review the current residential rooftop-solar subsidy structure on the official MNRE portal before committing to an installation. Last verified: July 2026.