How to Size a Solar System for Your Home: kW, Units, and Roof Space

A small plant leaves much of your daytime demand on the grid. A plant far beyond your annual use can leave you paying for panels whose surplus earns less than the electricity you use yourself. Start with the units on your bills, then test the result against your roof and connection.

Three limits shape a useful proposal: how much electricity your household uses through the year, how much clear roof is available, and what your DISCOM has approved for the connection.

If you are comparing a home on-grid system with a hybrid system with backup, use the same bill and roof checks first. The battery decision comes after you know the solar capacity your home can use.

Know the difference between kW and units

A proposal states the plant size in kilowatts, or kW. Your bill records energy in kilowatt-hours, or kWh. One kWh is one unit. Keeping capacity and energy separate makes the calculation easier to follow.

A 3 kW array has a rated DC output of 3,000 watts under standard test conditions. That rating describes the panels at a particular moment. It is not a promise that they will deliver 3 kW throughout the day.

Energy adds up over time. A 1 kW appliance running for one hour uses 1 unit. At a steady 3 kW for four peak sunshine hours, an array would produce about 12 units. Weather, temperature and system losses change the actual result.

Step 1: Average the units on 12 bills

Do not choose plant size from the highest summer bill. Air conditioners can push that month well above your usual consumption, while the same array will keep generating through milder months. The full year gives you a sounder starting point.

Collect 12 consecutive bills and add their billed units. Divide the total by 12. APSPDCL customers in Tirupati, Chittoor, Nellore and Kadapa can check the DISCOM portal or consumer app for billing history if a paper bill is missing. Check whether any month covers an unusual billing period before using it.

Suppose your home uses 520 units a month in summer from April to June, 320 units during monsoon and winter from July to January, and 380 units in spring. The example totals 4,560 units a year, so the monthly average is 380 units. Use that average to begin sizing, then discuss the seasonal pattern with your installer.

Step 2: Turn average monthly units into kW

Once you have the monthly average, compare it with local solar output. Our guide to rooftop solar generation explains how the panels, inverter and meter work together.

As a planning estimate, 1 kW produces about 120 to 135 units a month here. That comes from roughly 4 to 4.5 units a day over 30 days. Divide your monthly average by a figure in that range:

Eligible homes can apply under the central PM Surya Ghar: Muft Bijli Yojana scheme. Current assistance is ₹30,000 for 1 kW, ₹60,000 for 2 kW and up to ₹78,000 for a 3 kW home system. Do not add panels only to reach the subsidy cap. Compare your annual use, roof and expected credits under Andhra Pradesh net-metering rules before choosing the final size.

Step 3: Check usable roof, shade and structure

Your bill may point to 3 kW or 5 kW, but the roof has to carry it safely. Measure the clear area and check where shadows fall. An installer should also inspect the slab or shed before fixing the layout.

A typical 540 Wp to 590 Wp monocrystalline panel is around 7.5 ft by 3.75 ft, about 28 sq ft. Panels need more area than their own footprint because rows, tilt, walkways and parapet setbacks take space. For an early estimate, allow:

About 80 to 100 sq ft of roof without shade for every 1 kW.

On that basis, 3 kW needs about 240 to 300 sq ft and 5 kW needs about 400 to 500 sq ft. A large shed calls for a separate structural assessment. If you are evaluating a commercial site, start with its load and commercial electricity bill.

Walk the roof with the installer and check these points:

• Direction and tilt: A south-facing layout at about 12 to 15 degrees is a useful starting point in Andhra Pradesh. The final angle depends on the roof and surrounding shade.
• Sources of shade: Mark overhead tanks, stairwell rooms and neighbouring buildings. Ask for a shade check across the panel area from 9:00 AM to 4:00 PM, including changes through the year.
• Raised structures: On a flat RCC terrace used by the household, an elevated galvanised iron frame with 8 to 10 ft of clearance may keep the area below usable. Its height and foundations need a structural design.

Indicative sizes for different household loads

Use this table to compare monthly units with plant size, roof area and daily output. The subsidy figures are indicative for eligible residential connections; the actual proposal still needs a site audit.

For a closer look at the financial calculation behind each size, read our guide to solar ROI and payback.

Check sanctioned load with APSPDCL

Your electricity connection can set a lower limit than your roof. Ask the installer to read the sanctioned load on your bill before preparing a grid-connected proposal.

Under the APERC and APSPDCL rooftop rules described here, grid-interactive inverter capacity must stay within 100% of sanctioned connected load. A bill showing 2.0 kW sanctioned load cannot support a 3.0 kW or 5.0 kW on-grid plant until the utility approves a higher load. Confirm the current rule for your connection during the feasibility application.

Ray2Volt can prepare the feasibility submission, help with load enhancement paperwork and coordinate bidirectional meter testing in Srikalahasti, Tirupati and nearby mandals.

Avoid these two sizing errors

A good proposal should explain why its capacity fits your annual use. Watch for these two common mismatches:

1. Filling every available part of the roof

More panels do not always mean a better return. Surplus units may be banked and settled under the DISCOM's annual cycle. In the example here, excess export is valued at an avoided cost of about ₹2.50 to ₹3.00 per unit, while using a unit yourself could avoid a retail charge of ₹6.00 to ₹9.50. Verify the tariff and settlement rule for your account before paying for capacity that mainly exports.

2. Using only the May bill

Three ACs might take a household to 800 units in May while December use falls to 250 units. An 8 kW array sized only for May could export heavily for eight months. In this example, a 4 kW to 5 kW proposal may offer a better balance: it tackles the higher summer slabs while covering much of the winter use. Compare monthly generation and bills before deciding.

Questions homeowners ask

Subsidy figures and savings estimates are indicative. PM Surya Ghar guidelines and net-metering rules can change, so confirm your eligibility and the current terms before applying. The scheme period stated here runs to 31 March 2027. Have Ray2Volt check the roof and bills on site. Ray2Volt does not provide tax or investment advice.

Ray2Volt Solar Private Limited

We design, install, and service rooftop solar for homes and businesses across Tirupati district and Andhra Pradesh, from PM Surya Ghar residential systems to commercial and industrial plants. Every enquiry starts with a free power audit.