EN 
TH 
How Much Solar Should You Add for an EV? Start with Your Driving Distance and Charging Schedule
When an EV joins the household, charging becomes part of your home's everyday electricity use. It is often the point when homeowners start considering solar, whether they are planning their first installation or already have panels on the roof.
The next question is often: “How many more kilowatts of solar do I need to charge my car?”
The answer starts with how far you drive, how much energy your car uses, and when it is parked at home to charge. Short local trips and daily journeys between provinces can create very different energy needs, even for the same vehicle.
For example, driving an average of 50 kilometres a day would require approximately 2.2 kWp of solar panel capacity to match the average charging energy, using the assumptions in this article. This is an initial energy estimate. The capacity you should install also depends on household electricity use, charging times and the expected output of your particular roof.
First, Understand the Three Units: kW, kWh and kWp
The units used for cars, chargers and solar panels describe different things.
| Unit | Meaning | Example |
|---|---|---|
| kW, or kilowatt | Electrical power at a given moment | A charger rated at 7 kW |
| kWh, or kilowatt-hour | An amount of energy; one kWh is one electricity billing unit in Thailand | A charging session using 10 kWh consumes 10 units |
| kWp, or kilowatt-peak | The combined rated power of solar panels under standard test conditions | A solar array rated at 5 kWp |
A 7 kW charger does not automatically require a 7 kWp solar array. Solar capacity is sized around the energy you want to generate. A charger's rating describes its power delivery capability, while actual charging power also depends on the car, settings and electrical supply.
If your aim is to charge entirely from solar at a particular moment, the design also needs to check whether the solar power remaining after household use can meet the charging demand.
1. Convert Your Driving Distance into Charging Energy
Start with your average daily or monthly driving distance and your car's energy consumption, often expressed in kWh per 100 kilometres.
Consider this illustrative example:
- Average driving distance: 50 kilometres per day
- Average energy drawn from the vehicle battery: 16 kWh per 100 kilometres
- Charging efficiency from the household electrical supply to the vehicle battery: 90%
- All energy used for driving in this example is replenished at home
Daily driving energy = 50 × 16 ÷ 100 = 8 kWh
Allowing for charging losses, energy drawn from the household supply = 8 ÷ 0.90 ≈ 8.9 kWh per day, or approximately 267 kWh over 30 days.
The consumption rate and charging efficiency above are assumptions used to demonstrate the calculation. Actual figures vary with the vehicle, driving patterns and charging conditions.
If you already have energy records from your charger meter, use them to inform the estimate. They reflect your home charging directly. Check whether the reading measures energy entering the charger or reaching the vehicle battery. If charging losses are already included, do not add them again.
If you regularly charge away from home, include only the share you intend to charge at home so the system reflects your actual use.
2. Estimate the Solar Capacity That Matches Your Charging Energy
Once you know the energy required, the next step is to estimate how much electricity each 1 kWp of panels would produce on your roof.
Output depends on location, orientation, tilt, shading and system losses. Your design team should assess the actual site before recommending a capacity.
For this example, assume that each 1 kWp of panels produces an average net AC output of 4 kWh per day. This is an illustrative assumption, not a guaranteed yield for roofs in Southern Thailand or for every season.
quivalent solar capacity (kWp) = average daily home charging energy (kWh) ÷ average daily yield per kWp
Using the earlier example: 8.9 ÷ 4 ≈ 2.2 kWp
The table below uses the same assumptions throughout: vehicle consumption of 16 kWh/100 km, charging efficiency of 90%, and net solar yield of 4 kWh/kWp/day.
| Average driving distance | Average home charging energy | Solar capacity equivalent to the EV's energy needs |
|---|---|---|
| 30 km/day | 5.3 kWh/day | Approximately 1.3 kWp |
| 50 km/day | 8.9 kWh/day | Approximately 2.2 kWp |
| 80 km/day | 14.2 kWh/day | Approximately 3.6 kWp |
| 100 km/day | 17.8 kWh/day | Approximately 4.4 kWp |
This table compares average energy quantities. It does not mean solar will fully power every charging session. It also excludes other household electricity use and any additional losses from storing the energy in a home battery first.
If your roof's average yield is lower than assumed, the equivalent panel capacity will be higher. Even where average generation is sufficient, rainy or heavily overcast days may produce too little energy for that day's charging.
3. When Your Car Is at Home Affects How You Can Use Solar
Imagine two households that each drive 50 kilometres a day. In one, the car is parked at home during daylight hours. In the other, it returns after sunset. Their energy needs may be similar, but they will use solar differently.
A car parked at home during the day can use electricity as the panels generate it.
recommends charging EVs during the day or when surplus solar is available.
Household demand must be accounted for first. If solar is producing 4 kW at a given moment and the home is using 1.5 kW, approximately 2.5 kW remains for the car before considering any other loads. Charging at 7 kW would require another source to make up the shortfall in a grid-connected system, according to its design.
A system that adjusts charging power to match surplus solar may help manage this. Compatibility between the charger, vehicle, meter and controls must be checked, along with the minimum charging power supported.
A car that returns home at night cannot use freshly generated solar while charging. Using daytime solar energy at night requires suitable energy storage.
Homeowners should therefore compare options around their actual routines: charging from the grid at night, charging during daylight on days the car is home, or adding a home battery. Ask the design team to assess energy needs, losses and costs for each option before deciding.
4. If You Already Have Solar, Should You Add the Capacity in the Table?
Check the surplus from your existing system first. Some homes may already have spare solar energy during the hours when the car can charge.
Suppose the car needs an average of 8.9 kWh per day, and the existing system provides an average of 3 kWh per day of surplus energy that can actually be used during charging hours. The remaining energy requirement would be approximately 5.9 kWh per day.
Using the same assumed yield of 4 kWh/kWp/day, the additional equivalent panel capacity would be approximately 1.5 kWp. An actual expansion still requires checking whether the existing inverter can accommodate more panels, along with roof space and other system constraints.
Use generation and consumption data broken down by time of day. If the existing system does not provide enough information about surplus energy, further monitoring may be needed. Total daily generation alone does not show how much energy is available when the car is charging.
For a new installation, assess household and EV demand together from the start, separating daytime and nighttime energy needs.
5. Plan the Charger and Electrical System for Simultaneous Use
Alongside panel capacity, consider how much EV charging the home's electrical system can support while other appliances are running, such as air conditioners, electric hobs and water heaters.
The design team should check the electrical service and meter rating, single-phase or three-phase supply, distribution board, charger circuit and protective devices for the chosen vehicle and charger. They should also assess whether load management is needed to control the home's total power demand.
When expanding solar, check the existing inverter's ratings and capabilities, connection arrangements and applicable requirements for modifying the system. Additional panels and the charging point should be planned together.
Prepare These Five Details for an Estimate Suited to Your Home
1. Vehicle model and average driving distance. Use monthly figures if daily journeys vary.
2. Home charging energy. Include meter or charger records where available, plus the share of charging done away from home.
3. Parking and charging times. Cover both working days and days off.
4. Household electricity use. Bring electricity bills and load profiles if available, along with plans for more appliances or another EV.
5. Existing system details. Include panel capacity, inverter model, battery information and remaining roof space.
These details give a clearer basis for deciding how much solar to add: the energy required, the power needed by devices running together, and the hours when solar can be put to use.
ConNEXT Living plans Solar / Energy systems around the way your household lives, from site surveys and system design through to installation and after-sales support. If you are preparing your home for an EV, bring your driving distance, charging schedule and existing solar details to our team to explore an approach suited to your home.