A roof size, bill size, and appliance load framework for choosing solar capacity

Why generic sizing fails

A neighbour installs 5 kW and tells you to do the same. But your roof is smaller, your bill is lower, and you run only one AC instead of two. Following that advice means overspending and stretching your payback period by years.

Generic recommendations treat every Indian home as identical. They ignore that a 1 BHK in Pune and a 3 BHK villa in Bengaluru have completely different roof areas, consumption patterns, and appliance profiles. A proper framework considers all three variables before suggesting a capacity.

Step 1: measure your roof

Usable roof area is the first hard constraint. Not all of your terrace qualifies. You need shadow free space that receives direct sunlight for at least four to five hours daily. Subtract area blocked by water tanks, staircase heads, satellite dishes, and parapet shadows.

As a working rule, each kW of solar panels requires about 100 sq ft of unobstructed roof. Here is what that means in practice:

If your roof is small but you still want higher capacity, structures like elevated or floor mounted setups can help. We offer options through ArkaCore, ArkaSky, and ArkaScape that are designed for different roof constraints, from compact terraces to large villa rooftops.

Step 2: read your electricity bill

Your monthly electricity bill is the clearest signal of how much solar you actually need. It tells you two things: how many units (kWh) you consume, and how much you pay per unit at your current slab.

Higher slab rates mean faster payback. A household paying ₹8 per unit at the top slab recovers its solar investment much sooner than one paying ₹4 per unit at a lower slab. This is why bill size matters as much as bill amount.

Here is a rough mapping between monthly bill and recommended solar capacity:

Generation figures assume average Indian city conditions with about 4.5 peak sun hours. Actual output varies by city, season, and panel orientation. We verify expected generation during a technical site visit before recommending a final size.

Step 3: list your appliance load

Roof and bill give you a range. Appliance load narrows it. The goal is to identify which devices run during daylight hours, because on grid solar generates value primarily when the sun is up and net metering offsets the rest.

Start by listing your major daytime appliances and their approximate wattage:

ACs are the biggest variable. A home with zero ACs and basic appliances can manage comfortably on 2 kW. Add one AC used regularly, and 3 kW becomes the practical floor. Two ACs push you toward 5 kW. If you plan to charge an EV at home, 7 kW is the realistic starting point.

Think about future loads too. If you expect to add an AC or an EV charger within two to three years, sizing up now avoids the cost and complexity of expanding later.

The decision matrix

Now combine all three inputs. The matrix below shows how roof space, bill range, and appliance load intersect to suggest a capacity. Find your row for roof size, check your bill column, and then verify against your appliance profile.

Notice that the matrix sometimes recommends a smaller system than your roof can hold. That is intentional. If your bill is ₹3,000 and your roof fits 7 kW, installing 7 kW makes no financial sense. You would generate far more than you consume, and net metering credits in most Indian states do not fully compensate for surplus export. Match capacity to consumption first, then to roof.

When inputs conflict

Sometimes the three variables point in different directions. Your bill says 5 kW, but your roof fits only 3 kW. Or your appliance load demands 5 kW, but your bill is only ₹4,000.

Here is how to resolve each conflict:

Roof too small for your bill: Consider an elevated or floor mounted structure that uses vertical space more efficiently. Our ArkaSky elevated structures, for example, let you fit higher capacity on a compact terrace while keeping the space below usable.

Bill too low for your appliance ambitions: If you plan to add ACs or an EV charger soon, size for the future load rather than the current bill. The incremental cost of going from 3 kW to 5 kW at installation is far less than retrofitting later.

High bill but few daytime appliances: If most of your consumption happens at night (lighting, entertainment, cooking), on grid solar still helps through net metering. But your effective savings rate will depend on your state's net metering policy. We walk you through this during the site assessment.

What this looks like in practice

Consider a family in Bengaluru living in a 2 BHK independent house. Their roof has about 350 sq ft of usable space. Monthly bill runs around ₹4,500. They use a fridge, washing machine, TV, and one 1.5 ton inverter AC during summer afternoons.

Roof says: 3 kW maximum. Bill says: 3 kW is a strong match (360 units per month covers most of their 300 to 350 unit consumption). Appliance load says: one AC plus basic appliances fits within 3 kW comfortably.

All three inputs agree. A 3 kW system is the right call. Systems in this size typically run from ₹1.3 lakh to ₹1.6 lakh before subsidy. With PM Surya Ghar subsidy (₹78,000 for 3 kW and above), the effective cost drops significantly, and payback lands in the range of three to four years depending on tariff slab.

Now consider a villa owner in Hyderabad with 600 sq ft of roof, a ₹9,000 monthly bill, two ACs, a water pump, and plans to buy an EV next year. Roof says: up to 6 kW. Bill says: 5 to 7 kW. Appliance load (with future EV): 7 kW. Here, the appliance load is the deciding factor. A 7 kW system uses the available roof efficiently and future proofs the home for EV charging without a second installation.

How we help you decide

We built this framework into our consultation process. When you book a site visit (which happens within two days of your ₹20,000 booking), our engineer measures shadow free roof area, reviews your last three to six months of electricity bills, and maps your current and planned appliance load.

The recommendation that comes out of this visit is not a sales pitch for the largest system. It is a capacity that balances your roof reality, your consumption pattern, and your payback expectations. If a string inverter setup is the right fit for your roof, we recommend that, even though micro inverters carry a higher price. That is what honest sizing looks like.

Our product lines (ArkaCore for standard rooftops, ArkaSky for elevated structures, ArkaScape for gazebo style installations) exist precisely because different homes need different structural approaches. The framework above tells you how much capacity; the product line tells you how to mount it given your roof constraints.

Every system we install comes with serial tracked panels, a waterproof inverter monitored every five minutes, and our Arkashield generation guarantee. If the system underperforms, we pay you back at ₹7 per unit of shortfall. That guarantee only works because we size systems carefully in the first place.

If you are still unsure where you fall in the matrix, start with your electricity bill. Pull out the last three months, note the units consumed, and check your usable roof area. Those two numbers alone will narrow your options to one or two capacity choices. The appliance list confirms the final pick.