the fine points of analyzing PG&E E9A

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astrorob

Well-known member
Joined
Apr 21, 2011
Messages
192
so a couple of months ago, i installed a TED5000. i was not able to capture the whole house, just the sub-panel. so i miss the garage lights and outlets, and also the Blink. but that turns out to be good for modeling different leaf charging scenarios.

i wrote a perl script to take the TED5000 output (hourly) and compute the E1A and E9A costs for my electrical use. using the blink's statistics i hacked the leaf consumption back in and added in a fudge factor to pick up the usage that's not caught by the TED.

the result is a little counter-intuitive. first of all, with my current consumption (around 800kwh/month) the E1 scenario is cheaper than any E9 scenario i can come up with. furthermore charging the leaf at peak times seems to be marginally cheaper than charging at off-peak times. on average i'm using about 4kwh/day on the leaf. E9A does not become cheaper than E1 until i start using about 7kwh per day on the leaf.

i'm thinking that either the script is buggy, or my continuous load of 300-400kw just drives the E9 cost into the stratosphere.

so the question is - is it legitimate to compute the tiered usage on a day-by-day basis? in other words, the script starts at midnight with daily usage 0 and adds up the usage for each hour. using the daily baseline quantity and the time of day/day of week, i figure out the current rate and add in the cost for the current hour. i think this is why charging the leaf overnight gives a more expensive result - by 3 or 4am i'm already in the middle/higher tiers, and then the rest of the day costs more than it ordinarily would.

or is this wrong, in the sense that any unused lower tiers for a given day must be "banked" to be squared up at the end of the month? in other words if for 30 days the baseline quantity is (say) 300kwh and i (somehow) use 300kwh on one night during off-peak time, is that full 300kwh charged at the lowest tier? my script would allocate the lion's share of that usage to the highest tier, as it's way, way beyond the daily baseline amount of 10kwh.

i think initially i convinced myself that going day by day was equivalent to computing it monthly, but writing the above makes me think that was wrong. reduction to the absurd is often helpful :)

anyway, anyone have any insights on this? thanks.
 
astrorob said:
so the question is - is it legitimate to compute the tiered usage on a day-by-day basis?
No, you can't do it on a day by day basis, you need to use monthly totals.

Here's my understanding of how to compute tiered TOU bills:

From the usage data, determine monthly totals for each TOU category. Your rate in a given tier will be the blended rate of the different TOU rates in that tier, in proportion to your TOU totals for the month. E.g. if your usage totals are 20:100:80 for the three TOU categories, then your rate at each tier will be 10% TOU rate A + 50% TOU rate B + 40% TOU rate C.

Now that you have the blended tiered rates, you computer your total bill as the sum of (tier X rate * usage within tier X).

Cheers, Wayne
 
wwhitney said:
astrorob said:
so the question is - is it legitimate to compute the tiered usage on a day-by-day basis?
No, you can't do it on a day by day basis, you need to use monthly totals.

Here's my understanding of how to compute tiered TOU bills:

From the usage data, determine monthly totals for each TOU category. Your rate in a given tier will be the blended rate of the different TOU rates in that tier, in proportion to your TOU totals for the month. E.g. if your usage totals are 20:100:80 for the three TOU categories, then your rate at each tier will be 10% TOU rate A + 50% TOU rate B + 40% TOU rate C.

Now that you have the blended tiered rates, you computer your total bill as the sum of (tier X rate * usage within tier X).

Cheers, Wayne

okay, thanks for the help. i guess i got wrapped around the axle because they give a baseline quantity per day, but of course that's just to figure the monthly baseline qty since meter reads might not be exactly 1 month apart.

i'll hack on the script and try to fix it. it sounds like my E1 computation might be broken as well given what you describe above. although my script gave the right dollar amount when compared to my real bill.
 
astrorob said:
wwhitney said:
astrorob said:
so the question is - is it legitimate to compute the tiered usage on a day-by-day basis?
No, you can't do it on a day by day basis, you need to use monthly totals.

Here's my understanding of how to compute tiered TOU bills:

From the usage data, determine monthly totals for each TOU category. Your rate in a given tier will be the blended rate of the different TOU rates in that tier, in proportion to your TOU totals for the month. E.g. if your usage totals are 20:100:80 for the three TOU categories, then your rate at each tier will be 10% TOU rate A + 50% TOU rate B + 40% TOU rate C.

Now that you have the blended tiered rates, you computer your total bill as the sum of (tier X rate * usage within tier X).

Cheers, Wayne

okay, thanks for the help. i guess i got wrapped around the axle because they give a baseline quantity per day, but of course that's just to figure the monthly baseline qty since meter reads might not be exactly 1 month apart.

i'll hack on the script and try to fix it. it sounds like my E1 computation might be broken as well given what you describe above. although my script gave the right dollar amount when compared to my real bill.

You can compute the amounts on a day by day basis IF you know the billing period end date. The number of days in you billing period times the baseline allotment for your zone determines the trip points for the tier levels. Two things I have noticed, PG&E reduced the baseline by about 10% on June 20th and if you have a Net metering device it is stuck on the older daylight savings time that started later in the year and ended in October. Now that I finally have E9A, my PG&E readings have vanished so I have to rely on TED like system (Current Cost) to estimate the use.
 
Also, don't forget to subtract off the E9a baseline credit.

One example of a real E9a bill with some commentary is http://www.mychevroletvolt.com/first-chevy-volt-electricity-bill-pge.
 
Nekota said:
You can compute the amounts on a day by day basis IF you know the billing period end date.
No, you can't. Consider this simple (albeit extreme) example:

On one day in the month I use 150% of my daily baseline.
On all the remaining days I use 95% of my daily baseline.

For a 30 day month that would work out to (1.5 * b + 29 * 0.95 * b)/30 = 0.968 * b where b = baseline.

Calculated on a monthly basis my entire usage would be at tier 1. Calculated on a daily basis I would have some tier 2 and some tier 3 usage.

Ray
 
Nekota said:
You can compute the amounts on a day by day basis IF you know the billing period end date. The number of days in you billing period times the baseline allotment for your zone determines the trip points for the tier levels. Two things I have noticed, PG&E reduced the baseline by about 10% on June 20th and if you have a Net metering device it is stuck on the older daylight savings time that started later in the year and ended in October. Now that I finally have E9A, my PG&E readings have vanished so I have to rely on TED like system (Current Cost) to estimate the use.

okay, i think i understand what you mean. in my example above with 300kwh used on the first day, you would just assign all of that to tier 0. on the next day you'd be in the next tier, until you reach 130% of that monthly baseline, and so on. intuitively that makes sense to me. i hacked the script to use the monthly baseline amount compared against the cumulative usage when computing the tier and the results look better (but still indicate that i should stay on e1a if i don't shift my non-leaf heavy loads to off-peak times)

the lowered baselines are apparently part of a rate rejiggering they did to cut super-heavy users a break. they did this by adding a new tier to the CARE rates and lowering everyone's baseline quantity, while lowering the per-kwh rates in the highest tiers.
 
planet4ever said:
Nekota said:
You can compute the amounts on a day by day basis IF you know the billing period end date.
No, you can't. Consider this simple (albeit extreme) example:

On one day in the month I use 150% of my daily baseline.
On all the remaining days I use 95% of my daily baseline.

For a 30 day month that would work out to (1.5 * b + 29 * 0.95 * b)/30 = 0.968 * b where b = baseline.

Calculated on a monthly basis my entire usage would be at tier 1. Calculated on a daily basis I would have some tier 2 and some tier 3 usage.

Ray


that was my example in the first post. however, on that first day, the fact that you used 150% of your daily baseline does not matter according to Nekota because you are comparing it against the monthly baseline quantity when you compute the tier. the 150% usage all gets billed at tier 1 in the daily model with monthly baseline.

if i look at how the TED does it (at least for non-TOU) it seems to be doing the same thing - for the first part of the month the reported tier is 1, and as your monthly usage goes up the "instantaneous" tier increases.
 
JeffN said:
Also, don't forget to subtract off the E9a baseline credit.

One example of a real E9a bill with some commentary is http://www.mychevroletvolt.com/first-chevy-volt-electricity-bill-pge.

this i have definitely forgotten to do. i'll have to look at your link to see if it explains how to do it.
 
planet4ever is right. You cannot use daily amounts to determine the tier; it's all monthly. Other factors that might affect the accuracy of your computations are how you change your electric usage because you are on a tiered TOU rate. For example, I now run the dishwasher after 9pm or before 2pm much of the time, instead of always right after dinner, to get out of peak period. You may not expect to change your habits, but you don't know when writing your script how that might vary.
 
I calculate my electric use on a day by day basis by applying the hourly readings. If you know how many kwhr you can use for the month and you make hourly readings you can calculate the cost for each hour and add up the amounts and know exactly how much you are paying for electricity at that moment (hour) of time. I will use a real example.

For PG&E individual metered baseline quantity, my zone is X code B (non electric heating) summer = 11.0kwhr/day and winter = 11.7kwhr/day. In July there were 31 billing days so my baseline quantity is 341 kwhr. Using an hour by hour measurement for 31 days requires 744 hourly readings times the tier rate for each of those readings. You use tier one rates using the time of day rate (offpeak, partpeak, and peak) until the total power consumed is greater than 341 kwhr. I used up all my baseline kwhr in 13 days when I crossed into tier2 (130% of baseline = 443.3) and then fairly quickly (~4 days) into tier3 above 443kwhr. I did not cross into tier4 at 682kwhr since my total use for the 31 days was 675kwhr. Finally subtract the baseline credit for 341 kwhr @ 0.01729 ($5.89) and you will have your bill. I have a smartmeter and there is no meter charge for a smartmeter. All the charges for Generation, Transmission, Distribution ... ect. are rolled up into the TOU rates but you do have to add the energy commission tax and utility users tax to obtain the total electric charges.
 
We've also varied our lifestyle for the summer tiers--dishwasher runs after midnight and laundry done only before 5PM on weekends (luckily it's only 2 of us). Still, with the two hot spells we had in the 6/16-7/15 period, we got killed on E-9A. Our August 15th bill will be very interesting as the A/C hasn't run much at all during the period. Last summer, we used 27 kWh/day during the "hot" months. This year, we are using ~14 kWh/day for charging the LEAF, so we exceed the baseline of 366/month pretty quickly.
 
thanks for all the responses. i was on vacation so i did not have the chance to reply. i did make the fixes to the script to take into account the baseline quantity over the range being analyzed and the results now make more sense.

bottom line is that the script shows that e1 is still the best place for me to be, unless i want to try to shift the heavy loads to off-peak times. however, after 40 years of using electricity just whenever i want to, i think that's going to be a hard sell... especially for the rest of the family.

solar seems to be way too expensive for me given the design of my roof. i think i might just have a 2nd meter installed for the leaf and go onto e9b, but i need to do a breakeven analysis there first.

what is the purpose of the baseline credit? it seems like it's a way to lower the baseline per-kwh charge in proportion to the amount of electricity you use, up to the baseline amount. i wonder what the origin of that is? could this stuff be any more obfuscated?
 
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