Demand charges for AV fleet charging, explained with the math

How does a utility measure demand?

The interval length changes what a short burst costs. Austin Energy bills the 15-minute interval of greatest use.³ FPL bills the 30-minute period of greatest use.⁴ SRP in Phoenix uses the highest 30-minute demand.⁹ A 10-minute spike moves a 15-minute average more than a 30-minute one, so a yard that briefly runs every charger at once pays more on a 15-minute meter.

The Department of Energy's Alternative Fuels Data Center defines it for fleets as a fee on your greatest power draw during peak periods, on top of the rate you pay for the energy itself.¹² NREL found demand charges often make up 30% to 70% of a commercial electric bill.¹

What does one peak cost a robotaxi depot in a month?

The 461 kW is not a made-up worst case. It is 24 Level 2 chargers at 19.2 kW all starting when the cars come back, the yard from our overnight charging schedule: 40 robotaxis, 80 kWh each, 3,200 kWh a night. Nothing about the energy changes when you cap the yard at 300 kW. The same 3,200 kWh simply takes 10.7 hours instead of 7, which fits in a 12-hour overnight window like PG&E's 9 p.m. to 9 a.m. off-peak.⁶

At low use the charge gets worse per kWh, not better. NREL cites an EVgo fast-charging station whose demand charges reached $1.96 per kWh in summer 2016, over 90% of its monthly bill, because it set a high peak and sold little energy.² A depot in its first months, with a few cars and full-size chargers, can look like that station.

Why can the same peak cost $5,855 or $364 on FPL?

A robotaxi yard whose 461 kW peak falls at 11 p.m. pays 461 × $12.70, or $5,855, on GSD-1, and 461 × $0.79, or $364, on GSDT-1. Move that same peak to 7 a.m. on a January weekday, inside FPL's winter on-peak hours of 6 to 10 a.m., and GSDT-1 costs $5,850.⁴ The rate rewards a schedule that is finished before the morning peak, and punishes one that isn't.

FPL also has an EV version of GSD-1 with a demand limiter: billed demand can never exceed the month's kWh divided by 75 hours. For our yard that cap is 1,280 kW, far above its peak, so it does nothing. It is built for public chargers that sell little energy, and the tariff requires the charging to be open to the public.⁴

What is a demand ratchet, and why does one summer peak last a year?

APS bills on-peak demand on that rate at $18.941 per kW for the first 100 kW and $12.654 per kW after that, on a 15-minute interval, with weekday on-peak hours of 3 to 8 p.m.⁵ A yard that lets 461 kW of charging run into one summer weekday afternoon owes $6,462 that month. Through the ratchet, it keeps paying on at least 369 kW of that peak every month for the next year, even if it never charges on-peak again.

Ratchets aren't in every tariff, and most EV rates don't have one. Search your tariff for "ratchet", "80%" or "highest demand in the preceding" before you assume a bad month only costs a month.

How does a depot keep its demand charge down?

• Set a site cap. Managed charging spreads the night's energy across the window instead of front-loading it. The Department of Energy's Federal Energy Management Program lists avoiding higher demand charges as a reason to manage fleet charging.¹¹
• Stagger the starts. Cars that return at 9 p.m. and leave at 6 a.m. don't all need to start at 9:00.
• Know your interval. On a 15-minute meter, even a short burst of every charger counts.³
• Finish before on-peak. On time-related demand rates like FPL GSDT-1 and APS E-32 TOU L, the cost depends on when the peak happens, not just how high it is.⁴⁵
• Consider storage only for peaky loads. NREL found batteries alone are more effective at cutting costs for peaky or low-use loads;¹⁰ a yard that already charges flat all night gets less from one.
• Read the EV riders, and their end dates. NV Energy's EV demand reduction for DC fast charging is down to 30% and ends April 1, 2029.⁸ SCE's $0 demand charge on its EV rates lasts until the CPUC decides otherwise.⁷

We priced one 461 kW peak on five tariffs

Method: we took the per-kW demand or subscription prices from each tariff in the sources, applied them to a 461 kW peak and to a 300 kW cap, and assumed the peak falls overnight, outside on-peak hours. We counted demand only: energy, fixed customer charges, taxes and riders are left out, so these figures show what the peak alone costs. PG&E's BEV-2-S figure is the subscription a yard would choose to cover that peak, with no overage.⁶

The lesson isn't that one utility is cheap. It is that the rate design decides whether a peak matters at all. On Austin Energy and FPL GSD-1, the height of the peak is everything. On FPL GSDT-1 and APS, the timing is. On PG&E, it is the subscription you choose in advance and the overage if you exceed it: a yard subscribed for 300 kW that hits 461 kW pays $615 in overage on top.⁶

When demand charges aren't your problem

• Your load is flat. A yard that charges at a steady 300 kW around the clock has a high load factor, and the demand charge is a small share per kWh. Work on the energy price and your time-of-use window instead.
• Your rate has no demand charge right now. On SCE's TOU-EV-8 the facilities demand charge is $0 per kW for now.⁷ Budget for it coming back.
• You charge at someone else's station. Then the station's demand charge is its owner's cost, and your concern is the price per kWh you agreed.

That last case is how XoomPark works with station owners: robotaxis charge in the hours a station's stalls sit unused, never block its regular customers and never push the station past its monthly peak. When the station fills, the robotaxi leaves.

Questions depot energy managers ask