Buying an EV? Size your solar for the car too.

An EV is a second electricity bill. Your roof can take a share of it.

If you are weighing rooftop solar and also own, or plan to buy, an electric car or scooter, the two decisions are linked. Charging at home adds a steady load to your BESCOM bill, and that changes how big a system makes sense. It also changes when your solar units actually get used. This post is about doing that sum properly, without assuming the answer.

Start with how much the EV really uses.

Do not use the manufacturer's headline figure. Use your own driving. As an illustration only: if a car uses about 12 units (kWh) per 100 km in city traffic and you drive 1,000 km a month, that is roughly 120 units a month. A two-wheeler would be a small fraction of that. Your car's real consumption will differ, so check the trip computer or the app after a few weeks of normal use and take that number instead.

Then convert it to system size. As a rough rule of thumb, each kW of rooftop solar in Bengaluru produces something like 100 to 120 units a month on average, varying by season, shading and orientation, as our post on monsoon generation explains. On those illustrative figures, a 120-unit-a-month EV habit needs roughly one extra kW of panels. Ask your installer for a generation estimate for your own roof rather than trusting this rule.

Day charging versus night charging.

Solar produces in daylight. Most people charge at night, when they are home. Under net metering the two are connected through the meter, not through the car. Units you export to BESCOM during the day are credited, and units you import at night are set against those credits. How the credit is carried forward and settled is set by the rules and your agreement, so read our explainers on net metering and export credit settlement, and confirm the current terms with BESCOM or your installer before you size anything.

If you work from home or the car sits at home on weekends, daytime charging uses your own solar directly. If the car leaves at 9 and returns at 7, you are relying on the net-metering credit to do the balancing. Both can work. The point is to know which one you are, because it affects how much of the EV load you can count against solar.

The sanctioned load question.

A home charger draws real power, and a faster charger draws more. Your BESCOM sanctioned load, and the size of system allowed against it, are connected, as we cover in the post on sanctioned load and solar size. Before you add a dedicated charger, check that your sanctioned load, your wiring and your main breaker can take it. A slow charger running overnight is usually gentler on all three than a fast one, but have an electrician confirm for your home. An EV plus a bigger solar system may mean a load enhancement application, so ask early.

Do the payback in two versions.

Ask your installer for two quotes: one for your current consumption, and one that adds the EV units. Then compare the cost per extra kW. The extra panels are often cheaper per kW than the first ones, because fixed costs such as the inverter, meter work and paperwork are already covered, though this depends on whether your inverter has headroom. Check that too.

The saving is the units you no longer buy at your BESCOM slab rate, compared with what you used to spend on petrol. How much of that you capture depends on your tariff slab, as tariff changes and payback shows. We have not put a rupee figure on this because tariffs, EV prices and driving habits vary too much to quote one honestly. Build the sum with your own bill and your own kilometres.

Mistakes to avoid.

• Sizing the system before you know whether the EV is coming. If it is likely within a year or two, mention it now, so the inverter and roof space leave room.
• Assuming the subsidy scales with the EV. Check what PM Surya Ghar subsidy covers for your capacity, and do not size up just to chase it.
• Installing the charger first and the wiring check later.
• Counting on solar to charge a car that is never home in daylight, without understanding how credits will offset night import.

The short version.

Measure the EV's real units, convert them to extra kW using your roof's own estimate, confirm how net metering will treat your charging pattern, and check your load and wiring before buying the charger. Then compare quotes with and without the EV.