What Size Solar System Does Your Home Need? (by Bill & Roof)

Almost every solar conversation we have at our Ashok Vihar shop starts with the same question: "How many kilowatts do I actually need?" It's the right question — and the answer isn't a guess. It comes from two numbers you already have: how many units your home uses each month (printed on your electricity bill), and how much shade-free roof you can give to the panels. Get the size right and your bill drops sharply without wasting money on panels you can't use. Get it wrong and you've either left savings on the table or paid for generation you can't sell back. This guide walks you through both, with a table you can match yourself before you ever call us.

Start with your bill, not the panel brochure

The single most useful number is your average monthly consumption in units (kWh). Pull out three or four recent BSES or Tata Power bills and look at the "units consumed" line. Take an average across the seasons if you can — Delhi homes burn far more in May and June, when the AC runs, than in February. A home that shows 250 units in winter and 650 in peak summer is really a 400–450 unit home on average, and that average is what we size to.

Why work from units rather than the size of your house? Because two identical 3BHK flats in Pitampura can have completely different bills — one with three ACs and a geyser, the other with one AC and a gas heater. Solar follows your electricity habit, not your floor area. The rule of thumb we use: in Delhi, 1 kW of well-installed rooftop solar generates roughly 4 to 4.5 units a day, or about 120–135 units a month, averaged across the year. Winter and monsoon days produce less; clear spring days produce more. From there, the maths is simple — divide your monthly units by about 120 to get a starting kW figure, then we refine it for your roof and your daytime usage.

The bill-to-size table for a Delhi home

Notice that we don't usually size the system to cover 100% of your units. A grid-tied home produces most of its solar in the daytime, while a lot of household use — lights, ACs, TV, geyser — happens in the evening and at night. Net metering bridges that gap: surplus daytime units are exported to the grid and credited against what you pull back after dark. So a 3 kW system on a 400-unit home can still knock out a very large share of the bill, because the day's surplus is banked for the night. We size to your usage pattern, not just the headline number.

Why 3 kW is the sweet spot for most homes

If there's one size we recommend more than any other, it's 3 kW — and it's not an accident. It lines up almost perfectly with both the central subsidy and the typical Delhi home's bill. Under the PM Surya Ghar Muft Bijli Yojana, the central subsidy is ₹30,000 per kW for the first 2 kW and ₹18,000 for the 3rd kW — which adds up to ₹78,000 at exactly 3 kW. That ₹78,000 is the ceiling: it's the maximum central subsidy a home can get, and it doesn't grow if you install 4, 5 or 10 kW. So at 3 kW you capture the entire central subsidy, and on a typical 3 kW system priced around ₹1.5–1.9 lakh, that ₹78,000 covers roughly 45–50% of the cost. (Subsidy figures are as of 2025–26; please confirm the current rates on pmsuryaghar.gov.in before you commit.)

On top of the central money, Delhi adds its own layer under the Delhi Solar Policy 2024: a state capital subsidy of ₹2,000 per kW up to ₹10,000 per home (credited via your first electricity bill after commissioning), plus a Generation Based Incentive of ₹3 per unit for domestic 1–3 kW systems — which for a typical home works out to roughly ₹700–₹900 a month, paid for five years. That GBI is one more reason the 1–3 kW band is so attractive in Delhi specifically: the per-unit incentive is highest in that range. The policy itself talks about near-zero bills for many homes and a payback of around four years — we'd frame that as the policy's stated aim rather than a promise, because your real payback depends on your usage and roof. Check the live state figures on solar.delhi.gov.in.

When it's worth going bigger than 3 kW

The 3 kW sweet spot isn't a hard rule — it's just where the incentives peak. There are good reasons to go larger, and we fit plenty of 4–5 kW and bigger systems across Delhi NCR. Consider going bigger if:

• Your average bill is genuinely high — 600+ units a month, often a large independent house in Gurgaon or Faridabad running multiple ACs through the summer.
• You're about to add load you know is coming: an EV charger, a new AC, or a borewell pump. Sizing for next year's usage saves a second installation later.
• You have plenty of shade-free roof and want to maximise net-metering exports — the central subsidy still caps at ₹78,000, but every extra unit you generate offsets grid power.
• Your sanctioned electricity load supports it. Net metering capacity is tied to your sanctioned load, so a larger system sometimes needs a load enhancement with your DISCOM first — we'll flag that during the survey.

The honest trade-off above 3 kW: you pay full price per kW for the extra capacity with no additional central subsidy, and Delhi's higher ₹3/unit GBI only applies up to 3 kW (it steps down for the 3–5 kW band). The panels still pay for themselves through bill savings — it just takes a little longer than the heavily-subsidised first 3 kW. For most Delhi flats and modest houses, 2–3 kW is the value pick; for big detached homes with real consumption, 5 kW upward earns its keep.

Check the roof before you fall in love with a number

Your bill suggests a size; your roof decides whether it fits. Plan for roughly 80–100 sq ft of shade-free roof per kW — so about 250–300 sq ft for a 3 kW system, and 500+ sq ft for 5 kW. But raw area isn't the whole story. What we actually check on a site survey:

• Shade through the day — a neighbour's taller building, a water tank, a parapet wall or a tree can shade panels in the morning or evening and quietly cut your generation. South-facing, unshaded roof is ideal in Delhi.
• Usable orientation and tilt — panels facing roughly south at a slight tilt generate best here; we work around tank platforms, mumty rooms and existing structures.
• Structure and access — the roof has to safely carry the mounting frame, and we need a sensible cable run down to your meter.
• Shared or DDA flats — if you're in a society or a builder floor, roof rights and society approval matter as much as the area. We've handled both; it's worth sorting early.

It's common for the roof, not the bill, to set the ceiling. A 600-unit home that only has 250 sq ft of clear terrace will land on a 3 kW system regardless — and that's fine, it still cuts the bill substantially. We'd rather tell you that on day one than after you've paid for panels that won't fit.

Add backup? Then you're sizing a hybrid, not just panels

Everything above sizes a grid-tied system — the type that maximises bill savings but shuts off during a power cut for safety. If you also want the lights and fan to stay on when the grid drops (and in parts of Delhi NCR, cuts are still a fact of life), you want a hybrid system with a battery. That changes the sizing in two ways.

First, the panel array can stay sized to your bill as above — its job is still to offset units. Second, the battery is sized separately, to the load you want to run during an outage and for how long, exactly the way we'd size a normal inverter-battery setup. A few essentials (lights, fans, TV, router, one fan-cooled room) need far less battery than running an AC through a long cut. A hybrid PCU manages all three jobs — running the home, charging the battery, and exporting surplus to the grid — but it does cost more than a plain grid-tied inverter, and the battery is a consumable you'll replace down the line. If your supply is fairly reliable and you only care about the bill, grid-tied is the cheaper, faster-payback choice. If cuts genuinely disrupt you, the hybrid premium buys real peace of mind. We'll lay out both options against your actual bill and your area's outage pattern.

How we size it for you

When you come to the shop or we visit, we do four things: read three to four of your bills to find your true average units, survey the roof for shade-free area and structure, check your sanctioned load and DISCOM (BSES Rajdhani, BSES Yamuna or Tata Power-DDL), and then recommend a size — usually landing at 2, 3 or 5 kW — with the central and Delhi subsidies applied so you see the real out-of-pocket number. We're a genuine multi-brand dealer, so we fit the panels and PCU that suit your roof and budget rather than pushing one label, and we install and service it on-site across Delhi, Gurgaon, Noida, Ghaziabad and Faridabad. Bring your bills — that's where every good solar decision starts.