Managed Electric Vehicle Charging Benefits Tested in Solar Synchronization Pilot

March 11, 2020 | | Education

Managed electric vehicle charging can have benefits for two different groups—the demand side (the electric vehicle owner, the building owner, the power customer) and the supply side (the whole electric grid system). This blog, published originally by the Great Plains Institute (GPI facilitates Drive Electric Minnesota), looks at the benefits of managed electric vehicle charging and how it works in practice, using 2019 examples from the Greenway Building (GPI’s headquarters) where charging levels are synchronized with on-site solar production.

Key takeaways on the benefits of managed electric vehicle charging:

  • Without managed charging, even electric vehicle (EV) chargers paired with solar can create significant demand from the grid.
  • Managed charging can ensure EVs charge mostly from energy generated on-site.
  • Synchronizing EV charging with times of solar energy production can reduce energy bills and grid stress.

This is the third post in a series sharing GPI’s results and insights from the pilot project at our headquarters to synchronize on-site solar energy production with EV charging using a managed charging system. Here’s what to expect:

Part 1. Results from our analysis of how seasonal and daily variation in rooftop solar generation levels impacts their ability to provide electricity year-round, using EV charging as an end-use test case.

Part 2. Intro to how we’re deploying a managed EV charging system to synchronize rooftop solar production with the varying energy demand set by EV charging levels.

Part 3. Describes the benefits of managed charging and details how it has worked in practice, using specific charging scenarios to illustrate the data we’ve analyzed.

Part 4. Reviews the actual impacts of managed charging on EV drivers at our office building in 2019.

Why Managed Electric Vehicle Charging?

For a building owner, managed charging provides the potential to cut down on power demanded from the grid (and therefore, demand charges) by using energy generated on-site. Even if the solar and the EV chargers are behind the same meter, EVs don’t typically charge at the same time that a solar array is generating energy (see figure 1). This means without managing when an EV is charging, the EV would likely be charging from the grid, and the building owner would have to pay for that power demand.

Figure 1: Mismatch between solar array output and EV charging 

Source: National Renewable Energy Laboratory analysis (2019) for the GPI-led Minnesota Solar Energy Innovation Network project showing the modeled charging patterns for six workplace chargers and the solar production curve for a January week. The timing mismatch is consistent across days but can be matched with a shift of only a couple of hours.

If the building owner could shift the time in which EVs charge so that they are charging directly from a solar array, they could reduce the demand from the grid, smoothing energy consumption to help avoid peak demand charges.

(Want to know about peak demand charges? Check out our white paper Overcoming Barriers to Expanding Fast Charging Infrastructure in the Midcontinent Region).

To help illustrate how our solar synchronized managed charging works, we’ve outlined several different types of days and charging scenarios below.

  1. Chargers ramping up power as the sun rises

As solar generation gradually increases in the morning as the sun rises, the chargers plugged into vehicles gradually ramp up their power level proportionally until they reach the vehicle maximum charging level. Below, you can see the charger plug into the car, immediately start charging at the level of available solar, and then take three “steps” to reach the charger maximum level. Once the vehicle is fully charged, it gradually ramps the power level back down to zero. You might also notice two small “bumps” in the charger power. Each type of EV will have a slightly different end of charge procedure, which creates these individual charging “signatures” for each car.

Figure 2. EV Charging ramps up with solar production

Source: Great Plains Institute.

  1. Charging on a day without sufficient solar

On days with little to no sun, vehicle can still charge at a minimum power level. On this foggy day in March, a vehicle plugs in at 10 am and immediately starts charging at the minimum power level. Because of the lack of sun, it never rises above the minimum charging level and takes longer to fully charge, ramping back down after it completes charging at 5pm.

Figure 3. EV charging at a minimum on low solar day

Source: Great Plains Institute.

  1. Chargers reacting to intermittent solar

On days with intermittent sunlight, such as a partly overcast day where clusters of clouds regularly pass overhead, the chargers will rapidly ramp power up and down to match the level of solar available. Here you can see the total charging load “nested” in the same pattern as the solar generation.

Figure 4. EV Charging adjusts dynamically to intermittent solar array generation

Source: Great Plains Institute.

From a broader, grid-level perspective, managed charging can help reduce during times of high energy demand. Each day, the grid experiences what is called peak demand (also known as peak load)—typically around midday when the most electricity is being demanded from the grid. In order to meet that demand, the grid relies on expensive and inefficient energy generation facilities to provide extra electricity during the peak. The physical infrastructure to accommodate this maximum amount of energy—substations, transmission lines, distribution feeders and transformers, etc. also needs to be built, representing significant investment and resources.

EV charging (and in some cases, even create new demand peaks). Managed charging can help mitigate that demand by keeping charger demand at a minimum unless there is available solar energy to offset it. By maximizing the amount of EV load that is met with on-site solar production, the net energy needed at the building relatively low compared to a scenario where EVs are only charged up using their maximum power rate, likely exceeding the amount of solar energy generated on site and requiring energy to be drawn from the grid.

The ability to synchronize with a highly variable resource means managed charging can adapt to a wide variety of scenarios and locations. We’ll continue to explore how managed charging works in a variety of seasons and solar patterns in our fourth post in this series.

Want to learn more about our electric vehicle+solar synchronization project? As GPI has tracked the interaction of electric chargers with the solar array, we’ve had a chance to make some observations and generate some key takeaways. Stay tuned for part four of this series to learn more!

About the Great Plains Institute (GPI): A nonpartisan, nonprofit organization, the GPI is transforming the energy system to benefit the economy and environment. Working across the US, we combine a unique consensus-building approach, expert knowledge, research and analysis, and local action to find and implement lasting solutions. Learn more at