Can rooftop solar charge the EV that is away from home all day?

The roof is sunny at noon. The car is parked at the office. That timing problem is easy to miss in a quotation promising “free EV charging from solar”.

A Nepal household can use rooftop solar and own an EV, but the energy has to reach the car when it is connected. Direct daytime charging, exchange with the grid and battery storage are different arrangements, with different costs and approvals. Work out which one the proposal actually provides.

Convert driving into an energy requirement

For an example car travelling 30 km daily at 16 kWh per 100 km, the battery needs 4.8 kWh. At an assumed 90% charging efficiency, the supply delivers about 5.3 kWh. That is the starting energy requirement, before considering the rest of the house.

A 3 kWp rooftop array does not produce 3 kW continuously. Its output depends on sunlight, orientation, shading, temperature and system losses. If an illustrative day yields four equivalent full-output hours and 80% overall performance, generation is 3 × 4 × 0.8, or 9.6 kWh. A cloudy day can be very different.

Those figures are a sizing example, not a production estimate for a Kathmandu roof. Obtain a site survey and seasonal generation calculation. A neighbouring building's shadow may matter more than a larger panel nameplate.

The household uses some of that energy too. If the house consumes 6 kWh during solar hours, only 3.6 kWh remains in the simplified example. A proposal cannot allocate the same generated units to the house and the EV twice.

Direct use needs the car at home

If the car is parked during the day, charging can be coordinated with solar output. The charger and vehicle still have minimum operating requirements and limits. A system that continually stops and restarts under passing clouds needs competent design rather than an improvised extension lead.

Daytime charging may suit someone working from home, a second household car or a vehicle returning between shifts. It may be less convenient for the commuter whose car leaves before useful generation begins and returns after it ends.

A weekend can provide some direct use, but do not value every weekday unit as direct EV charging if the car is absent. Record the actual parking pattern before sizing around it.

Grid exchange needs confirmed terms

A grid-connected system may export electricity while the car is away and draw electricity later. Whether exported units reduce the bill, at what value and under which metering arrangement depends on the approved connection and current rules.

AEPC publishes Nepal grid-connected solar planning guidance and implementation resources. Use those materials to prepare the project, then confirm the applicable arrangement with NEA. A planning document or a past export rate is not approval for your property.

Ask the installer for the connection process, metering proposal, export treatment and expected charges in writing. Check eligibility separately for any subsidy or finance programme. A commercial rooftop call with a minimum project size should not be advertised as an automatic grant for every household.

Storage adds another battery and another bill

A stationary battery can shift some solar generation into the evening. It also has purchase costs, conversion losses, an operating range and eventual replacement requirements. Compare the value of shifted energy with those costs rather than calling the stored electricity free.

Imagine an assumed 6 kWh battery whose planned usable window is 80%. That offers 4.8 kWh before conversion losses, less than the example EV's 5.3 kWh supply requirement. It may also be expected to keep lights, a refrigerator or a pump running. Size it for the combined job.

A home backup and a car charger can demand different power. A small inverter suitable for lights may not support the EV charging load. A battery with enough stored energy may still have too little output power for the intended equipment.

An ordinary grid-tied array is not automatic outage backup

Grid-connected systems require appropriate protection, including arrangements that prevent energising a disconnected grid. Do not assume rooftop panels will keep the charger running during a power cut. Backup operation needs equipment and switching designed for that purpose.

AEPC's technical resources for grid-connected systems are a useful starting point for the designer. Installation, protection and connection should be handled by qualified professionals. This article is not a wiring guide.

Ask what happens during an outage, which loads remain powered and how the system is isolated. Do not connect an EV's vehicle-to-load outlet to the house through an improvised socket arrangement.

Ask for two financial comparisons

Request a solar-only household proposal and a proposal that includes the actual EV charging arrangement. Show installed costs, maintenance, seasonal generation, self-consumption, export assumptions and storage separately.

If the car is absent most sunny hours, the useful purchase may still be household solar without a large storage battery. If it sits at home, managed daytime charging may use more of the generation directly. The roof and car can work well together, but the parking schedule has to appear in the calculation.

Sources and scope

• AEPC project planning.
• AEPC technical resources.
• NEA consumer information.
• The energy examples use stated assumptions; they are not a rooftop yield estimate.