Solar EV Charging Integration: 8-Step Design Guide

Last updated: November 2026

EV charging and solar often land on the same project. The EV load is large, and it does not follow the sun. A good design handles both in the right order.

This guide has 8 steps, in the order you should do them. It is for installers and electrical engineers who quote home and business sites. The numbers in the examples are example inputs. Replace them with your own site data.

The 8 steps:

1. How much energy will the EVs use?
2. How big should the solar array be?
3. Do you need a battery?
4. Should you choose DC or AC coupling?
5. How do you size the circuits and meet code?
6. Where does a solar carport fit?
7. How does smart charging work?
8. What does the utility need?

Step 1: How Much Energy Will the EVs Use?

Start with the charger type and how long each car stays plugged in. Dwell time (how long a car sits at the charger) matters more than the car’s top charging speed.

The US Department of Energy’s Alternative Fuels Data Center (AFDC) describes three charger types.

Source: AFDC, Electric Vehicle Charging Stations.

AFDC also says that many drivers can meet daily needs by charging overnight with Level 1, if a power outlet on a dedicated branch circuit is near the parking spot. See AFDC, charging at home. So Level 1 can be enough for a short daily trip. Ask the driver for their real daily miles first.

If you plan DC fast charging, check the site’s electrical service early. AFDC says these units typically need a three-phase AC input.

How to find the daily energy for one car

Use this formula: daily kWh = daily miles × kWh per mile.

Get kWh per mile from the car’s rating. AFDC gives an example of 27 kWh per 100 miles, which is 0.27 kWh per mile. Charging has losses. The fueleconomy.gov EV page says battery charging efficiency is often 84% to 93%, based on published studies. If your rating is measured at the battery and not at the wall, add the charging loss.

How to find the load for many ports

The diversity factor is the share of installed charger power that runs at the same time. Not every car plugs in at once.

Here is an example. A site has 10 Level 2 chargers at 11.5 kW each. Installed power is 10 × 11.5 = 115 kW. Assume a diversity factor of 0.7. Expected demand is 115 × 0.7 = 80.5 kW. The 0.7 is an assumption. Replace it with interval data from a similar site, or from your own site’s utility records. Ask the utility for at least 12 months of interval data, if your site has it.

Use the diversity factor to size the array and the battery. Do not use it to size circuits. Each circuit must carry its own charger (see Step 5).

EV load adds to the building load. It does not replace it. Check that the extra demand does not push the site past its service capacity.

Step 2: How Big Should the Solar Array Be?

Use this formula: array kW = daily kWh ÷ peak sun hours ÷ system factor.

• Peak sun hours show how much sun your site gets in a day. Get them for your site from PVWatts or your design software.
• System factor covers losses from heat, dirt, wiring and the inverter. This guide uses 0.80 as a cautious example. PVWatts lists an estimated system loss of 14% by default, and the user can change it. Model your real site before you quote.

The table below shows an example for one car. It uses 0.27 kWh per mile (AFDC’s example), a system factor of 0.80 and 400 W panels. The sun-hour values are examples, not site data.

How to read one row: 40 miles × 0.27 = 10.8 kWh. Then 10.8 ÷ 5.0 ÷ 0.80 = 2.70 kW. Then 2.70 ÷ 0.4 = 6.75, which rounds up to 7 panels.

For a business site, run the same formula with the demand from Step 1. For the 10-port example, work out the daily kWh from how many hours the chargers actually run. Size the building’s own load in a separate calculation, then add the two array sizes. Good solar design software lets you model each load alone before you combine them, and the generation and financial tool shows what the combined system returns.

Before you finalize the layout, check shade on the array with shadow analysis.

Step 3: Do You Need a Battery?

Not always. A grid-connected site can charge EVs from solar when the sun is up and from the grid at other times.

A battery makes sense when your utility bill has demand charges. AFDC says electricity charges are typically based on the highest 15-minute average use in a billing period. It also says DC fast charging is more likely to trigger demand charges than Level 1 and Level 2 charging. See AFDC, operation and maintenance.

So one session at a high-power charger can set the peak for the whole bill. A battery can cover that peak. Here is how to check:

1. Get the tariff and read the demand charge in dollars per kW.
2. Find the site’s highest 15-minute peak with and without the chargers.
3. Ask for a written battery quote.
4. Compare the yearly demand charge saved with the battery cost.

Your battery needs may also come from backup power or from time-of-use prices. Add those to the same comparison. For more on this topic, see the solar storage design guide and adding battery storage services.

Step 4: Should You Choose DC or AC Coupling?

Coupling means where the battery connects. DC coupling joins the battery to the panels before the inverter. AC coupling joins it after the solar inverter.

The US National Laboratory of the Rockies (NLR, formerly NREL) notes that DC coupling can raise round-trip efficiency when the battery charges from the coupled solar. AC coupling charges less efficiently from solar because of more AC and DC conversions. See the NLR page. Its figures are for a utility-scale model.

AC coupling is often easier when solar is already installed. Enphase, for example, says its IQ Battery 10C can be added to existing third-party solar while “leaving the existing inverter and panels in place” (Enphase).

For a full comparison, read the DC vs AC coupling guide and the engineering version.

Step 5: How Do You Size the Circuits and Meet Code?

AFDC says the National Electrical Code (NEC) treats EV charging as a continuous load, and that Article 625 holds most of the rules for charging equipment. See AFDC, charging at home.

AFDC gives an example. Most home Level 2 chargers run at up to 30 amps and deliver 7.2 kW. They need a dedicated 40-amp circuit to meet NEC Article 625. See AFDC, Electric Vehicle Charging Stations.

Follow these steps:

1. Read the charger’s installation manual for the circuit and breaker rating.
2. Check which NEC edition your local authority (the AHJ) has adopted.
3. Size each circuit for its own charger. Do not use the diversity factor here.
4. Ask the AHJ about permits and forms before you submit.
5. Ask whether the AHJ lets an energy management system limit charger power to reduce the circuit or service size.

Hire a qualified electrician for the panel and service check. AFDC notes that some homes may not have enough electrical capacity for Level 2 charging.

Step 6: Where Does a Solar Carport Fit?

A carport puts panels over parking spaces and gives the chargers a place to sit. It suits sites with large parking areas and not much roof.

Compare the carport with a roof system on cost, shade, structure and permits. Ask your AHJ about structural and permit rules for the carport. For the layout, spacing and panel choices, read the solar carport design guide.

Step 7: How Does Smart Charging Work?

Smart charging lets software control when and how fast each charger runs. It can follow solar output and keep the site under a power limit.

Chargers talk to the management system through a protocol. OCPP (Open Charge Point Protocol) is an open standard. The Open Charge Alliance says OCPP 1.6 was released in 2015, OCPP 2.0.1 in 2020 and OCPP 2.1 in 2025. It says OCPP 2.1 adds a functional block for bidirectional charging and one for distributed energy resource (DER) control, and improves smart charging. See the Open Charge Alliance.

Before you buy chargers, ask each maker which OCPP versions they support. Ask which functions work in the version you plan to use.

Step 8: What Does the Utility Need?

A solar system that connects to the grid needs the utility’s approval. The standard for connecting distributed energy resources to the grid is IEEE 1547-2018, titled “IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces” (IEEE). Your utility sets its own forms and rules.

Follow these steps:

1. Ask the utility for its interconnection application.
2. List every source: solar, battery and any charger that can send power back out.
3. Wait for the interconnection agreement and the permission to operate before you switch on.

If a vehicle can send power to the grid, ask the utility whether that needs its own application.

What About Bidirectional Charging?

Bidirectional charging lets a car send power back out. It can power a home (V2H), power a device (V2L) or send power to the grid (V2G).

It needs three things: a vehicle that supports it, a charger that supports it and a management link that supports it. Read the vehicle maker’s own documents for what each model can do. Model support changes often, so this guide does not list models.

For design detail, read the vehicle-to-home solar design guide and the bidirectional EV charger selection guide.

What Costs Should You Quote?

Get a dated written quote for each line. Do not use a price from memory.

Conclusion

Do the steps in order. A wrong EV load estimate carries into every later step.

• Get the real daily miles and the charger type from the customer.
• Size the array, then check whether demand charges justify a battery.
• Size every circuit for its own charger and confirm the code with the AHJ.
• File the utility application early.

For help modeling the array and the loads together, book a SurgePV demo and bring one of your own sites.

Frequently Asked Questions

How many solar panels do I need for an EV charger?

Divide the daily EV energy use by the peak sun hours at your site, then by a loss factor such as 0.80. Example: 10.8 kWh per day (40 miles at 27 kWh per 100 miles), 5.0 peak sun hours and 0.80 gives 2.7 kW of array. With 400 W panels that is 7 panels. The inputs are example values, so replace them with your own.

Can I charge an EV directly from solar panels without a battery?

Yes, if the system is grid-connected. The charger uses solar power when it is available and grid power when it is not. The vehicle must be plugged in while the sun is up to use solar directly. A battery lets you store solar power for evening charging.

What is the NEC requirement for EV charger circuit sizing?

The US Department of Energy’s Alternative Fuels Data Center says the NEC treats EV charging as a continuous load, and that Article 625 holds most of the rules for charging equipment. It gives one example: a 7.2 kW (30 amp) Level 2 charger needs a dedicated 40 amp circuit. Check the NEC edition your local authority has adopted and the charger’s installation manual.

How does a diversity factor affect a multi-port commercial EV charging design?

The diversity factor is the share of installed charger power in use at the same time. If a 10-port site has 11.5 kW chargers, installed power is 115 kW. At an assumed factor of 0.7, the expected demand is 80.5 kW. Use this to size the array and battery. Do not use it to size each circuit, because each circuit must carry its own charger.

When does adding battery storage to a solar-EV site make financial sense?

Check your utility bill for demand charges. The Alternative Fuels Data Center says electricity charges are typically based on the highest 15-minute average use in a billing period, and that DC fast charging is more likely to trigger demand charges than Level 1 and Level 2 charging. A battery can lower that peak. Compare the savings with a written battery quote.

What is OCPP 2.1 and why does it matter for solar integration?

OCPP (Open Charge Point Protocol) is the open standard that lets chargers talk to a management system. The Open Charge Alliance says OCPP 2.1 was released in 2025. It adds a functional block for bidirectional charging and one for distributed energy resource (DER) control, and it improves smart charging.

What does bidirectional charging need?

Bidirectional charging lets a vehicle send power back out. It needs a vehicle that supports it, a bidirectional charger and a charger-to-management link that supports it. OCPP 2.1 adds support for this. Ask your utility if sending power to the grid needs an interconnection application. Read the vehicle maker’s own documents for what each model supports.