Deep dive · In the raw · optional

The battery's day plan, as the controller wrote it

About 12 min · 3 questions ·

Thursday
The situation

Before the first car, the energy management system of the site writes its plan for the day: for every hour, how much the chargers will need, how much the grid may supply, what the battery does and where its state of charge will stand.

Stop 05 showed the day as it happened. This is the plan it started from, in the table the controller keeps, with the rule it applied in each hour.

The questionWhich rule decided each hour, and where did price, reserve and the grid limit pull in different directions?

Guess firstThe connection is 200 kW and the battery 400 kWh. Why does the plan refill at 2 PM at 7.8 ct when the night will offer 7.2 ct?RevealHide

Because the evening peak comes first. The reserve rule outranks the price rule: a battery that waits for the cheapest hour would face the 5 PM rush half empty.

What you are looking at

An energy management system plans a day in hours: a demand forecast from past days and traffic, the fixed grid limit, the battery's energy and power, and the prices of the day, which the day-ahead market sets the day before, in quarter hours. The four rules of stop 05 become one line per hour. The battery's own , which the plan relies on for its usable capacity, is what EU law requires the to hold.

Select the lines to see which rule won in that hour.

Energy management system, plan written Thursday 5:00 AMThe day, hour by hour

Demand and grid in kilowatt-hours per hour. Battery: energy into the battery from the grid (positive) or out of it to the cars (negative). Cells: what that changes in the stored energy, with 8% lost each way. State of charge at the end of the hour.

  2. Line 1The day's parameters

    The controller plans the day at 5 AM from the demand forecast, the grid limit and tomorrow's prices. Five numbers frame it: the connection, the battery's energy and power, the window it is used in, and the losses between grid, cells and cars. The window and the 8% on the way out are the assumptions of stop 03; the plan assumes the same 8% on the way in.

  3. HOUR DEMAND GRID BATTERY CELLS SOC PRICE RULE
  4. kWh kWh kWh kWh % ct/kWh
  6. 07:00 60 60 0 0 95 8.1 grid covers demand
  8. 09:00 110 110 0 0 95 9.0
  9. 10:00 120 120 0 0 95 8.2
  10. 11:00 190 190 0 0 95 7.9
  12. 13:00 250 200 -50 -54 54 7.3 peak continues
  14. 15:00 170 200 +30 +28 73 8.6 refill
  15. 16:00 230 200 -30 -33 65 10.2 early evening
  19. 20:00 90 200 +110 +101 55 11.8 refill at full grid
  21. 22:00 10 10 0 0 55 9.1 wait
ems-plan-2026-10-08.txt: the whole file, to copy
Thursday 8 October 2026. Site: two chargers of 300 kW, grid 200 kW, battery 400 kWh and 400 kW. Window 10% to 95%, 8% lost each way, start of day 90%.
HOUR   DEMAND  GRID  BATTERY  CELLS   SOC   PRICE   RULE
       kWh     kWh  kWh      kWh     %     ct/kWh
06:00      5    27     +22     +20     95     7.4   refill to the top
07:00     60    60       0       0     95     8.1   grid covers demand
08:00    130   130       0       0     95     9.6   grid covers; peaks within the hour from the battery
09:00    110   110       0       0     95     9.0   
10:00    120   120       0       0     95     8.2   
11:00    190   190       0       0     95     7.9   
12:00    300   200    -100    -109     68     7.1   midday peak: discharge the difference
13:00    250   200     -50     -54     54     7.3   peak continues
14:00    150   200     +50     +46     66     7.8   refill with spare capacity
15:00    170   200     +30     +28     73     8.6   refill
16:00    230   200     -30     -33     65    10.2   early evening
17:00    310   200    -110    -120     35    13.9   evening peak: the busiest hour
18:00    260   200     -60     -65     18    15.6   peak continues; reserve holds
19:00    150   200     +50     +46     30    14.1   refill, price still high
20:00     90   200    +110    +101     55    11.8   refill at full grid
21:00     40    40       0       0     55    10.4   wait: price falls after 11 PM
22:00     10    10       0       0     55     9.1   wait
23:00      0   173    +173    +159     95     7.6   cheap: refill to the top
00:00      0     0       0       0     95     7.2   at the top; flexibility offered until 5 AM, floor 60%

A composed plan with the site of this path. Prices are day-ahead values per hour, known the day before; the flexibility offer at night is the market access RED III asks Member States to open to small systems.

What the plan adds up toOne day in four numbers
Through the battery350kWh

delivered to cars over the day

Lowest state18%

after the evening peak, floor 10%

Lost on the way65kWh

in and out, of 435 kWh drawn for refilling

Refilled in the cheap hours195kWh

at 6 AM and after 11 PM, 7.4 and 7.6 ct; 240 kWh at 7.8 to 14.1 ct

Computed from the plan, with 8% lost each way. Less than half of the refilling happened in the cheap hours: the reserve for the peaks came first.

The four rules, in the order they win

  • Never below the floor. 10% stays in the battery whatever happens; the plan's lowest point is 18%.
  • Cover the peak. Whenever the chargers need more than the connection, the battery supplies the difference; this is why the site exists (stop 01).
  • Keep the reserve. Refill before the next expected peak, even at a high price. Hours 14, 15, 19 and 20 are this rule.
  • Then follow the price. With the reserve safe, wait for the cheap hours: 21 and 22 wait, 23 refills.

The actual day of stop 05 ran lower, to its 10% floor at 6 PM, because more cars came than forecast. A plan is a forecast; the rules are what hold when it is wrong.

What the plan is worth

Without the battery, the site would have slowed cars in five of the eighteen hours, and in the peak minutes of others. With it, the draw from the grid never rose above the 200 kW of the connection: the battery covered the peaks and shifted energy into quieter hours, at the cost of about 65 kWh lost in conversion. A capped draw and a low peak are also what the grid operator and the reward (Charging infrastructure, deep dive). The cheap refill at night and the flexibility offer are the second use of the battery, after the cars.

Check your understanding

3 questions. Answer all of them to complete this stop. Each answer explains itself.

01Question 1 of 3

At 2 PM the plan refills 50 kWh at 7.8 ct although 11 PM will cost 7.6 ct. Why?

02Question 2 of 3

What does the 10% floor do in the plan?

03Question 3 of 3

Why does hour 08 show a battery value of zero although single minutes reached 260 kW?

Further in other paths

  1. Energy and grid integration · Deep diveOne night of prices and one grid signalThe night of stops 04 to 06 as the wallbox's controller saw it: quarter-hour prices from the day-ahead market, the retail price built on them, the grid operator's signal at 7:30 PM, and the charging schedule the controller chose.
  2. Load management · Stop 07How is a load-managed site planned?From the energy the cars need to the power the site reserves: the steps of planning a site with load management, computed for one car park.
  1. OKTE (nominated electricity market operator): 15-minute MTU in SDAC was implemented, 1 October 2025. https://okte.sk/en/information/news/2025-10-01-15-minute-mtu-in-sdac-was-implemented. Checked 11 Oct 2026.
  2. EUR-Lex: Regulation (EU) 2023/1542 on batteries, Articles 3, 12, 14 and 77 and Annex V. https://eur-lex.europa.eu/eli/reg/2023/1542/oj. Checked 09 Oct 2026. © European Union, reused under Commission Decision 2011/833/EU.
  3. OKTE (nominated electricity market operator): 15-minute MTU in SDAC was implemented, 1 October 2025. https://okte.sk/en/information/news/2025-10-01-15-minute-mtu-in-sdac-was-implemented. Checked 11 Oct 2026.
  4. EUR-Lex: Directive (EU) 2023/2413 (RED III), Article 20a inserted into Directive (EU) 2018/2001. https://eur-lex.europa.eu/eli/dir/2023/2413/oj. Checked 09 Oct 2026. © European Union, reused under Commission Decision 2011/833/EU.
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