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Jun 01, 2023

Matt Traverso

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Electric vehicles (EV) are designed to be as comparable to their traditional gas-powered counterparts as possible. However, in terms of energy usage and efficiencies, most of the standards break down or are challenging to interpret.

This article is part of an ongoing series where I examine EV behavior. Here, after meticulous data collection, I will quantify the capabilities of charging, efficiency, and range in EV mode of my Plug-in Hybrid Vehicle (PHEV) and compare those numbers to the advertised fueleconomy.gov/Monroney sticker values.

The Toyota Rav4 Prime (R4P) PHEV is a hybrid vehicle equipped with a large battery that allows it to drive roughly 40 miles (65 km) on electricity alone (EV mode). When the battery is low, the PHEV operates identical to a Hybrid Vehicle (HV mode) with a gas engine and generator. The PHEV seems to be the ideal vehicle type for my single-vehicle home, powering all short commutes on cheap, clean electricity while allowing for long-range road trips on high-efficiency, fast-filling gas.

In this article, I share the real-world data on how much I charge my PHEV and how far I’m able to drive on that charge. To collect the data, I plug my charger through a wattmeter (~$20 on amazon) to measure the power it takes to charge the battery.

Before getting into range and efficiency, I wanted to spotlight wall charging. I can measure how quickly it charges and how much energy it takes to fully charge so I can estimate the cost for a "tank" of electricity.

Reminder: it's easy to confuse power (measured in kilowatts, kW) and energy (measured in kilowatt hours, kWh). For our purposes, energy is the amount of charge stored on a battery while power is how quickly it is getting used (when driving) or filling (when plugged in).

The R4P accepts either Level 1 (L1) or Level 2 (L2) chargers. Both L1 and L2 use the same port on the car. L1 uses a standard wall outlet while L2 requires a high voltage circuit (like a clothes dryers or ovens) and charges about 5x faster (6.6 vs. 1.4 kW). All of my data is from L1.

The vehicle usually pulls 1340–1350 W. During the final 30–45 min, the power draw dipped to 850 W. The total charge time was 11 hrs and 51 min, just a little faster than the official Toyota spec of 12 hrs. At the average US cost of electricity, this fill-up cost $2.59 but electricity costs vary by region.

I performed a few other tests on the charging cable and the battery to measure their resilience.

A few other observations about charging:

All and all, I am pleased with charger performance. Very little energy is wasted and the charge time matches specifications. A typical daily commute can be supported by regular overnight charging.

Electricity is usually measured and paid for by the kWh, so any numerics listing the fuel economy of an EV would ideally be labeled in mi/kWh. Instead, the Monroney sticker standard in the US (example, Figure 3) includes two other measures of EV efficiency that we then need to convert to kWh.

The first value is Miles per Gallon electric (MPGe). As I wrote previously, MPGe uses a translation constant assuming 33.705 kWh = 1 gallon of gas. Therefore, a score of 94 MPGe is equivalent to 2.789 mi/kWh.

The second value is the amount of kWh it takes to drive 100 mi. 36 kWh translates to 2.778 mi/kWh. I will split the difference between these calculated values in kWh of 2.778 and 2.789 and use 2.78 mi/kWh (4.47 km/kWh) as the published fuel economy benchmark.

These results are for the R4P but the calculations can be applied to any EV and PHEV. For example, I used fueleconomy.gov to determine that the Tesla Model Y has about 3.6 mi/kWh and the Nissan Leaf about 3.3 mi/kWh.

The electric range on EVs and PHEVs is included on the Monroney sticker (Figure 4) but tricky to independently verify. Range can be calculated by multiplying the mi/kWh value by the battery capacity. However, the total battery is not completely accessible to EV mode. To visualize this, I have pieced together the approximate partition sizes based on available information (Figure 5).

The R4P has a 18.1 kWh battery. The amount of total capacity is often given as a percent and referred to as the State of Charge (SoC). The Monroney sticker in Figure 4 indicates a 42 mi (68 km) range. At 2.78 mi/kWh, this indicates that 15.1 kWh of the 18.1 kWh battery (83%) is available for EV charging.

I fully charged the R4P on five different days. Interestingly, every measurement was above 15.1 kWh. 42 mi (68 km) may be a conservative range estimate to account for variation in conditions and vehicles. At 2.78 mi/kWh, we would expect the R4P to drive over 44 mi (71 km) based on the largest charge value.

Regenerative braking and coasting is the likely reason that no two charge amounts match. Any regenerative events in HV mode performed immediately prior to charging will cause the EV-accessible part of the battery to start filling. I included a partition to show that regen encroaches on the EV-accessible battery.

The battery needs a partition for HV functions. This partition must be roughly matched to the total battery capacity of the Rav4 Hybrid trims (1.6 kWh). The vehicle, not the operator, controls the amount of charge on this partition and it's charged by the gas engine, not an outlet.

After the EV and HV mode allocations, there is still an unaccounted for portion of the battery that's roughly 1–1.5 kWh, which I have labeled as the "buffer" in Figure 5. Consensus on the Rav4 forums seems to indicate that the buffer may be included to improve battery longevity. Owners should feel safe charging the battery to 100% of the EV-accessible portion.

As long as my vehicle meets or exceeds the drive-efficiency benchmark of 2.78 mi/kWh, it will also exceed the range benchmark of 42 mi (68 km).

Now that we’ve covered electric driving efficiency metrics and battery capacity, we can measure my vehicle's performance and compare it to the official fueleconomy.gov numbers. Using the odometer and a wattmeter, I was able to calculate the mi/kWh for my EV-only trips.

The efficiency improved throughout the first month, indicating a burn-in period. I am not the first person to observe this on a R4P. I was in a bit of a panic during the first few weeks of ownership, myself. Hopefully this work will help to characterize it and prepare new owners better.

Excluding the burn-in period, I charged 101.76 kWh total and was able to drive 282.3 miles, averaging 2.774 mi/kWh (4.46 km/kWh), within error of the official 2.78 mi/kWh stats and exceeding the driving range of 42 mi in every charging trial. My old ICE listed 26 MPG on fueleconomy.gov but typically got 23 MPG, so my Toyota Rav4 Prime PHEV matches EV mode mileage and range expectations.

The high variability between different trips (Figure 8) is likely a reflection of different driving conditions and styles. I am working on a follow-up to understand this better. I’m considering factors like hills, average speed, and exterior temperature.

One final note on efficiency: The dashboard display shows an average mi/kWh that would seem easier to use than calculating the values yourself. It's wrong. I think I know why. I will write about my findings soon.

I have written about cost and emissions per mile of EV vs ICE and Hybrid before. In summary, the average costs in the U.S. as of April 2023 was $0.165/kWh and $3.83/gal of gas (BLS data). At those averages, it costs a little under $0.06/mi to drive the R4P in EV mode vs. $0.10/mi in HV mode and $0.127/mile for ICE trims.

This is a transitional time for EVs. The current efficiency standards of MPGe and kWh/100 mi may be helpful to demonstrate that EVs are more affordable and cleaner to drive than similar ICEs, but we should begin getting more accustomed to mi/kWh as a unit and using a wattmeter.

This work shows that EV mode efficiency on the R4P matches the fueleconomy.gov specifications. As I collect more information, I plan to publish more articles examining HV mode efficiency, EV performance under different conditions, and swapping between modes while driving.