How Weather Changes EV Charging Demand

RMI’s GridUp EV load forecasting tool now includes seasonal changes, helping utilities and regulators make prudent investments in grid infrastructure.

As spring weather arrives, drivers in electric vehicles (EVs) may notice that their cars are going farther between charges. They aren’t imagining things — like all vehicles, EVs operate more efficiently in temperate weather. To help grid planners and regulators better account for these seasonal effects, RMI is releasing a set of new scenarios in our GridUp EV load forecasting tool to showcase how changes in temperature can affect EV charging demand throughout the year.

What factors affect EV efficiency?

A vehicle’s efficiency boils down to two main factors: the friction it needs to overcome to keep moving forward, and the energy it uses to keep the passenger comfortable through air conditioning or heating (often called auxiliary energy use).

Friction reduces vehicle efficiency in three main ways: air resistance, rolling resistance (tires on the road), and drivetrain losses. In the case of air resistance and drivetrain losses, EVs are often more efficient than internal combustion engines, thanks to design decisions that reduce resistance and far fewer moving parts.

All vehicles, including EVs, use more auxiliary energy when the ambient temperature is either colder or hotter than what is comfortable (such as 68°F/20°C). In these conditions drivers use climate control systems to heat or cool the cabin. This is energy intensive, especially on very hot or cold days. While some electric vehicles use heat pumps to improve climate control efficiency, warming a vehicle requires more energy when the ambient temperature drops regardless of the technology used.

EVs also have a third, smaller factor that can impact their charging speed. Low temperatures can affect battery performance for most common battery chemistries, so some vehicles are designed to heat the battery pack to keep it within an optimal temperature range. This impact is typically only noticeable during high-speed charging in very cold weather, when the vehicle needs to warm the battery more to receive the higher power of a fast charge.

Why does EV efficiency matter?

Vehicle efficiency dictates overall energy demand, regardless of the fuel source. The efficiency of the EV fleet — including variations due to temperature — has important implications for the electric grid: all else equal, a less efficient EV fleet will require building more charging and grid infrastructure to meet the greater demand. While there are tools to mitigate the impacts of EV charging on the grid, making good, data-driven investments relies on decision makers at utilities and regulatory agencies being able to anticipate the scale and location of EV charging demand. This led us to develop our EV load forecasting tool GridUp.

This latest update to GridUp, which incorporates EV efficiency variations throughout the year, helps stakeholders such as utility distribution system planners and public utility commission regulatory staff gain more confidence in their ability to make the prudent infrastructure investments needed to serve EV load.

Additionally, while EVs have zero tailpipe emissions, their efficiency still influences upstream power sector emissions. As three-quarters of US electricity still comes from nonrenewable sources, lower EV efficiency means burning additional fossil fuels and therefore greater carbon and local air pollutant emissions (although EVs still have a much smaller carbon footprint than gasoline vehicles).

In numbers: the seasonal temperature effects on EV charging energy demand

To demonstrate how seasonal temperature changes can affect the energy needed to charge EVs, let’s take a closer look at the results from the new, temperature-dependent GridUp scenarios for two cities with very different climates: Phoenix and Minneapolis. We modeled the energy use of millions of trips individually, incorporating trip speed and seasonal snapshots of ambient temperature based on hourly weather data to determine how changes in operating friction and climate control use impacted the amount of energy used by a vehicle.

Phoenix

In Phoenix, the average daily temperature is 76°F (24°C) year-round. On days like that, GridUp’s forecast for unmanaged EV charging in 2035 shows peak power demand reach 2,525 MW. However, on a hot day in July, the temperature climbs to a sweltering average temperature of 96°F (35°C). Then the peak power for charging rises 14% to 2,871 MW. In other words, hot days result in significantly increased energy and power demand from these vehicles, primarily from air conditioning usage. (Gasoline usage is also higher on these days, as drivers of all car types use more energy to cool down.)

Minneapolis

In Minneapolis, the average daily temperature is 47°F (8°C) year-round. During these days, GridUp’s forecast for unmanaged EV charging in 2035 shows peak power demand to be 387 MW. However, the temperature can plummet to a frigid 16°F (-9°C) on average in January. The peak power for charging then rises dramatically to 540 MW, 39% above the median day. Cold weather also brings additional energy and power demand for cars as drivers try to stay comfortable and their vehicles must overcome an increase in air resistance.

Seasonal temperature swings change the shape and size of loads that utilities and regulators must plan for, not just the range of an individual EV. For grid planners, relying on EV load forecasts that only consider average conditions can systematically underestimate energy needs for charging, especially during extreme weather, which already stresses the grid. Understanding seasonal variation in vehicle efficiency is a prerequisite for making prudent, least-cost investments that keep costs down and improve reliability.

Consider an example from Minneapolis: In 2035, EV charging is excpected to draw 5,496 MWh for an average temperature day compared to 7,433 MWh in our cold scenario, and this difference of 1,947 MWh occurs day after day. At the end of a cold month (e.g., January), energy demand from charging exceeds an average scenario by 60,357 MWh. Perhaps more importantly, if even a fraction of this demand lines up with evening hours when winter peaks often occur — almost a certainty given typical unmanaged charging patterns — feeders, transformers, and generation capacity that are adequate in an average scenario may become constrained. Planners should treat seasonal EV efficiency as a key part of preparing for peak conditions in different parts of the year.

Whether the weather is hot or cold, utilities and regulators have a set of options to mitigate the impact of EV charging demand; the key will be to plan ahead using good data and planning tools, and take advantage of such opportunities. For example:

• Utilities can and should lean into managed charging and other load management strategies to shift charging to off-peak hours, reducing the need for infrastructure to be built solely for extreme-weather conditions.
• Utilities, regulators, consumer advocates, and other stakeholders can support these cost-effective options by earnestly incorporating EV load flexibility into planning exercises, prioritizing development of programs to harness this flexibility, and ultimately making participation easy for customers.

The new seasonal scenarios in the GridUp tool are designed to make this actionable: they can be used to stress-test infrastructure planning against extreme conditions, model how much incremental load shows up on extreme days, and, importantly, explore what amount of flexibility can be obtained from EV charging to keep upgrades focused where they are truly needed.

RMI’s GridUp Tool

GridUp forecasts when and where energy and power demand will materialize from vehicle electrification. The tool is uniquely detailed and flexible, allowing users to gain greater insight into how driving behavior will create and shape charging demand.

Explore the Tool

RMI would like to thank FedEx for their generous support of this work.