pge-ev-rate-schedule-peak-periods-residential-charging-reduce-costs-kw-engineering-energy-audit-consultants

How to Save Money when EV Charging

You shop for the best gas prices, why not your electricity rates? If you’re considering buying an electric vehicle (EV), or are already an owner, it’s important to know what you’re paying to charge your EV so you can minimize your costs. With changing electricity rate schedules and different pubic electric charging station vendors, charging costs will vary greatly based on when are where you charge.

Battery Math

There are many kinds of EVs on the road now with varying sizes of batteries. Let’s dive into how to calculate your charging costs, so you can determine the most cost-effective way to charge.

Battery Size

EV manufactures express battery size (capacity) in kWh, which stands for ‘kilowatt hour’ (a measurement of electricity that’s equivalent to the amount of energy expended in one hour by one kilowatt of power). Plug-in hybrid and electric-only car batteries range in size from 5 – 100 kWh.

Charging Time

Charging times depends on your batter size, the type of charger used and your battery’s charge controller. Residences may use Level 1 or Level 2 charging. Level 1 uses a standard wall outlet, which is 110-volts. Level 2 requires professional installation and uses a 240-volt dedicated circuit, similar to ones used in large appliances like electric dryers.

Charging time is about cut in half when using a Level 2 charger depending on the size of the circuit and battery capacity. Most commercial chargers are Level 2 but some are Level 3 (such as the Tesla Supercharger).

The other factor for charging time is the charge controller on your EV’s battery. It regulates the rate at which you can draw power. Tesla battery charge controllers have much higher ratings so they can draw more power, allowing for quick charges at the Supercharger.

How Does Voltage Impact Charging Time at Home?

The amount of time it takes to charge your EV battery at home can be estimated using the following equation.

ev electric vehicle battery charging lower costs kw engineering energy audit consultants

In this equation, volts is the voltage of your circuit, in homes either 120 or 240, and amps is the amperage rating on your charging cable. Even though your circuit may be rated for 20 amps, you’ll only be able to use the maximum rated by your charging cable.

Let’s look at an example. I have a Ford C-Max Energi with a 7.6 kWh battery capacity which charges on a 120V circuit using a cable with a 10A rating.

ev electric vehicle battery charging lower costs kw engineering energy audit consultants

Using this equation, if the battery was fully discharged, it would take 6.33 hours to recharge.

However, if I had a Tesla Model S with a 75 kWh battery needing a full charge, I would need a 240V circuit and a 40A wall charger to charge in 7.81 hours. But, it would use 13x the amount of power (kWh) than the Ford C-Max. (The 75 kWh battery was recently discontinued but for using here for calculation simplicity.)

So, how does the charging time impact my wallet?

Electricity Rates and Home EV Charging

With TOU rates, your charging cost will vary by when and how long it takes to charge your EV. Many customers are already on time of use (TOU) rates which charge more for using electricity during partial peak and peak periods which can extend to 11 PM. Some utilities offer EV TOU rates, which can be more affordable for larger EV battery charges.

If your battery takes longer than 8 hours to charge, you’re like to be charging during a peak or partial peak period at a more expensive rate. Or if you plug in during peak hours, it will also be more expensive. For this reason, many EVs offer the ability to set charging schedules. So, it’s important to know the size of your battery, your charging time, and your rate schedule, since how much your charge costs will vary accordingly.

TOU and EV Charging

Typically, utilities’ EV rate schedules benefit owners of larger capacity EV batteries, offering significantly lower rates during off peak hours. Let’s use PG&E’s EV-A rate schedule for the summer as an example. The current peak schedule is as follows and shown in the graph below. For weekdays it’s:

  • off peak 11pm – 7am
  • part peak 7am – 2 pm and 9pm – 11pm
  • peak 2pm – 9pm

Weekend off peak is 7pm – 3pm and peak is 3pm – 7pm.

pge-ev-rate-schedule-peak-periods-residential-charging-reduce-costs-kw-engineering-energy-audit-consultants

Source: PG&E

Next let’s look at the cost to charge a fully discharged battery using our previous Ford and Tesla examples.

pge-ev-full-charge-rate-schedule-peak-periods-residential-charging-reduce-costs-kw-engineering-energy-audit-consultants

You can see the big difference in cost between charging during the different peak periods. Over time, these differences in charging costs can add up.

The 7.6 kWh battery (Ford C-Max Energi) is officially rated to have a 19-mile range. With my commute pattern, I charge the battery fully, daily. If I charged during only peak hours, it’d cost almost $575 more a year, as shown in the table below.

pge-ev-Ford-annual-cost-difference-rate-schedule-peak-periods-residential-charging-reduce-costs-kw-engineering-energy-audit-consultants

Let’s compare this to the Tesla.

pge-ev-tesla-annual-cost-difference-rate-schedule-peak-periods-residential-charging-reduce-costs-kw-engineering-energy-audit-consultants

The 75 kWh Tesla battery is rated to have about 250-mile range. Assuming you drove 250 miles/week, you’d charge weekly. Over the course of the year you’d spend over $750 more if you charged only during peak hours, which you could only do if you had a dedicated 240V circuit with a 40A charger. If you didn’t have that, your charging time would be significantly longer, resulting in charging during part peak or peak hours. This would result in a higher cost, which would reduce the savings of charging purely off-peak.

Which Is Cheaper to Drive Per Mile, an EV or Gas Car?

To put charging costs in perspective, let’s look at how much it costs per mile to drive a Prius with an 11.3 gallon tank and a range of 452 miles assuming a gas cost of $3.50.

pge-ev-prius-cost-difference-rate-schedule-peak-periods-residential-charging-reduce-costs-kw-engineering-energy-audit-consultants

You can see driving a Prius is cheaper than an EV when charging at peak prices, but compared to off-peak prices, it the cost per mile is more than 3 times an EV.

Non-TOU Rate Schedules

PG&E still offers a tiered rate plan. Tiered rates divide your monthly usage in different tiers which carry different rates. Your location has an assigned territory, each of which has a different seasonal baseline use. If you use more than 100% of your territory’s baseline usage, you pay a higher rate on the extra electricity usage. Overall, tiered rates are significantly higher (between $0.22281 and $0.49124 for customers on the current E-1 schedule) than the off peak EV TOU rates.

However, if your household uses less electricity than average, but you have a peak usage needs like AC, and you have a smaller EV battery, a tiered rate plan could be more cost-effective. PG&E offers a rate comparison calculator when viewing your account online to explore your options. You can change your rate once a year.

Charging Costs at Public EV Charging Stations

In our area, I’ve seen various EV charging vendors install stations in public parking lots. For some vendors, property managers set the charge and cost model for customers, so they all vary in terms of cost and price structure, even among the same charging network. From free to rates that charge per kWh to those that charge per hour, prices vary, so it’s worth investigating what options are available in your area.

The best deal in my area (Concord, California) are the free Volta chargers. They are free to the public and Volta offers free installation to property owners, touting higher customer sales and longer stays on your property as incentive. It’s hard to beat that.

ChargePoint is also very popular network where I live. Their fee structures vary from $/kWh to $/hr to free for a number of hours and then $/hr after. A mall near our house charges $0.20/kWh which is better than the lowest tiered rate.

As a comparison, the local movie theater parking lot, also ChargePoint, charges $1.75/hr. Charging by the hour discourages EV parking spot hogs. Since chargers are typically in limited supply, it’s common courtesy to move your vehicle after it’s done charging and paying for occupying that spot is a good incentive. ChargePoint encourages you to move your EV by notifying you on your phone, via their app, when your car is done charging.

Calculating Energy Charge per Hour

To calculate your energy cost ($/kWh) with an hourly rate, you can use the following formula, but it can vary depending on whether you are the only EV charging on a shared station.

Let’s look at our movie theater parking lot example. To calculate, you need to know the chargers power capacity, which you can find in the app or on the website.

ev 2 electric vehicle battery charging lower costs kw engineering energy audit consultants

In our example, the charger power capacity is 6.6 kW and parking fee is $1.75/hr. Using the equation above, the energy cost is $0.265/kWh. However, this assumes you are the only EV charging at this charger.

Optimizing Charging Times

Typically charging stations have a pair of charging cables to charge two EVs. If you and another EV are charging at the same time, you’ll only receive half of the charger’s power if you’re both charging.

In the previous example, instead of receiving 6.6 kW of power in your hour of parking, you’ll only receive 3.3 kW. This doubles your charging cost, making this rate very pricey in terms of typically electricity rates.

ev 4 electric vehicle battery charging lower costs kw engineering energy audit consultants (3)

In general, to increase your charging speed, always park at an unoccupied station (and hope no one parks next to you 😊)

Be Aware of Pricey Chargers

Some of the most expensive rates in our area are Blink chargers. Their rates are currently as high as $0.69/kWh. That’s more than triple a tiered household electricity rate. It’s no wonder I never see anyone using them.

Temperature and HVAC Impacts on EV Mileage

Additional factors of your actual mileage per charge are ambient temperature conditions and the use of HVAC. Earlier this year AAA studied five different EV models and the impact on driving range in varying temperatures and use of heating and cooling equipment. Results showed a reduction in mileage of up to 12% with cold temperature conditions, and up to 41% reductions as when using HVAC in those conditions. Warmer conditions also reduced driving range. This means you’ll need to be charging more frequently to go the same distance.

Ready to Find the Best EV Charging Deal?

As battery technology and charging infrastructure improve along with pushes for decarbonization, it will become easier and cheaper to charge your EV. Though there’s some additional nuances to how much charge your battery needs and your mileage, you now know enough to get a good estimate of how much it costs to charge your EV and what factors drive this cost. If you’re a business owner and looking to investigate the impact of charging on your business, contact us anytime.

 

Like this post? Share it with our network on LinkedIn.

No Comments

Sorry, the comment form is closed at this time.

How can we help you save money and lower your carbon footprint?