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Compulsive Number Crunching

In the old days it was easy to track personal expenses. Gasoline for the car was recorded in Moneydance as an transportation:auto:fuel expense. The electric bill was recorded as an household:utilities:electricity expense. And any improvements to the house were tracked in an account for adjusting the basis for some future time when the house might be sold.

Enter the Prius Prime

But things got a little confusing when we purchased our 2017 Prius Prime and switched to an EV Time of Use (EV-TOU) rate plan with our electrical utility.

The most advantageous time to charge a PHEV or battery Electric Vehicle (BEV or EV) is during “super off peak” hours. And for our utility those hours vary based on the day of the week and the month of the year.

In theory the Prius Prime can be programmed to charge on a schedule. In practice our utility’s schedule has cannot be programmed into the Prius Prime. And since the EV range of the Prime is so low, you want to charge it as often as possible. For example, between shopping outings as long as it is between 10 AM and 2 PM on weekdays in April, but not in May. The fix for that was to use a Z-Wave controlled outlet with Home Assistant setting up the schedule.

Unfortunately, there was no good way to measure the power delivered to the Prius Prime. So when asked “how much does it cost to charge?” I had no definitive answer.

On the one hand, the car claimed about 4 mi/kWh and I could guess the EV miles. That gives a total kWh value which, based on the super off peak tariff, we could say those miles cost a certain amount.

On the other hand, by going to a TOU rate plan and making some modest lifestyle changes like using delayed start on the dishwasher, our electrical bill actually went down even though we were using a bit more electricity. From that point of view the EV miles on the Prius Prime were free.

In any case, there were no hard measurements to put into the finance tracking program. And that situation lasted until last year.

Preparing for a EV

In researching EVs I became aware that the federal tax credit for residential install of a Electric Vehicle Service Equipment (EVSE) device (a.k.a. charger) was going to expire on December 31, 2021. So we went ahead and installed a ChargePoint Home Flex which is far more than needed for a Prius Prime but is perfect for the new Ioniq 5 that replaced it. One feature of the Home Flex is it knows your electric company’s tariffs so it knows the cheapest time to charge. And it records the kWh and cost of each charging session and can give summaries for each month, etc.

The ChargePoint Home Flex is not smart enough to handle all the details of the utility’s convoluted TOU schedule. But the Ioniq 5 has enough range that we don’t need to charge it every day much less in the middle of the day between errands.

Now the old accounting for fuel with real numbers became possible again. At the end of the month I could look at the ChargePoint app for the home charging cost for the month. Then in Moneydance I add a transfer transaction, decreasing the household:utilities:electricity expense and increasing transportation:auto:fuel expense for cost of the power delivered to the car. Compulsive number crunching “sanity” restored!

Restored, at least, until you factor in electrical costs based on solar.

Rooftop Solar Photovoltaic

In an effort to reduce our carbon footprint we installed a grid tied rooftop solar photovoltaic system in 2021. This is basically an improvement to the house so, under my normal accounting, the after tax credit cost of the system was added to the basis of the house.

With “Net Energy Metering” (NEM) the over production we have during the sunny part of the day usually covers our day time use and our super off peak car charging. So if I make a monthly transfer transaction from household:utilities:electricity to transportation:auto:fuel based on the utility company’s TOU tariff and out ChargePoint Flex measured usage then our accounting software indicates that our household electrical costs are negative which isn’t true.

Cost of Electricity

So what is our “true cost” of electricity now? And how much of our electricity cost should be charged to the car as fuel?

After going down a rabbit hole trying to model time of use charges, fixed utility charges, non-recoverable use based fees, how much solar production we had based on average time of day over the month, etc. I finally decided to cut my losses and stick with things that are easy to measure and record. And I decided to simplify the costing model as much as possible.

Cost of Solar Electricity

The electricity we are getting from our roof top system is not free: We paid a pretty big amount for it. One way of looking at it is that we paid for 25 years of electricity up front (the production guarantee and warrantee life). With linear depreciation, the monthly cost of our solar electricity is simply the cost of the system divided by the life of the system. This over states the costs as it is very likely that the system will continue to produce power for years after the warrantee expires.

The output of the system varies due to weather and month of the year. We now have a full year of production data to examine. By this accounting method our monthly solar electricity cost ranged between $0.04/kWh and $0.10/kWh with a year long average of $0.06/kWh.

Cost Of Utility Power

You can get lost in the weeds figuring out the utility’s statement which seems to be intentionally designed to be confusing. Then trying to allocate costs between various uses based on the time of use is not easy as we don’t track exactly when we run appliances, etc.

So take the simple way out and look at the total (usually negative) energy delivered to the house and the monthly charge on the statement.

And, yes, there are charges when we have a net monthly export to the utility. And it is irksome to pay the electric company to take our power.

Monthly Cost of Electricty

The simple model is now:

  • The monthly kWh consumed is the utility kWh plus our solar kWh. Most months the utility kWh is negative (we produce more than we consume). But the sum of utility and solar is always positive and is a believable number.
  • The monthly cost of the electricity is the fixed amount of our solar depreciation plus whatever the utility bills us.

In our case the yearly average cost was $0.12/kWh. Yes, the utility power costs a lot more than our solar power. I guess one way to justify this is to think of the utility as a large battery that we are paying to access.

Allocating Costs

Rather than try to allocate TOU costs between household use and EV charging with the confusing issue that we are selling power to the utility during some hours at one billing rate and buying it back at other hours with different billing rates, it seems reasonable to simply split the dollar cost based on the power consumed.

We have the total consumption (solar plus utility). And we have a measure of the amount used to charge the car. So the household use is simply total consumption minus the car charging.

Now things are reasonably simple to measure and record. And accurate enough for my level of compulsive record keeping. Each month:

  • Record the kWh used to charge the car.
  • Record the kWh produced by the solar system.
  • Record the kWh supplied by or to the utility company.
  • Record the cost charged by the utility company.
    Plop it all into a pretty simple spreadsheet and get the dollar amount that should be allocated to the house and to the car.