Daily guide · SI · 2026-07-26
What a home battery earned in Slovenia this week, hour by hour
All numbers computed from the week's published auction results at publication and frozen — this article is an honest snapshot, not a page that rewrites itself. Reproduce via the API.
The standout feature this week is the timing of cheap power. On most days the trough sat in the early afternoon, starting around 12:00 to 14:00, driven by midday solar, while peaks landed in the evening between 18:00 and 20:00. The exception was 25 July, when the cheap window opened at 12:00 with an average of -7,55 EUR/MWh and the peak arrived at 02:00. Negative prices mean a battery is paid to charge, which is why that day tied for the best cycle value at 1,64 EUR.
For a reader with a 10 kWh battery at 90 percent round-trip efficiency, the week returned 9,37 EUR across seven cycles, with an average spread of 152,14 EUR/MWh. Daily earnings ranged from 1,09 EUR on 21 and 22 July to 1,64 EUR on both 24 and 25 July. The spread was wide throughout, but tighter cheap windows and higher charging costs on 20 and 21 July capped returns.
The concrete takeaway: charge in the early-afternoon solar dip and discharge into the evening peak. Held consistently, this pattern points to roughly 489 EUR per year.
A home battery makes money on one thing: the gap between the day's cheapest and priciest hours. Charge low, discharge across the evening peak instead of buying it, repeat daily. The size of that gap is set every afternoon at the day-ahead auction — so it's measurable, not hypothetical. Here is what one full daily cycle of a 10 kWh battery (at 90% round-trip efficiency — losses included, unlike most brochures) would have captured in SI this week.
€9.37
captured this week — one cycle per day, wholesale component, after losses
152.14
€/MWh average cheapest-2h → priciest-2h spread this week
≈€489
a year at this week's spreads — the honest run-rate, not the best-day fantasy
Day by day: the two windows that mattered
| Day | Charge (2h) | at €/MWh | Discharge (2h) | at €/MWh | Cycle value |
|---|---|---|---|---|---|
| 2026-07-19 | 10:00–12:00 | 0.3 | 18:00–20:00 | 167.56 | €1.51 |
| 2026-07-20 | 10:00–12:00 | 53.61 | 19:00–21:00 | 184.55 | €1.12 |
| 2026-07-21 | 14:00–16:00 | 69.56 | 18:00–20:00 | 198.63 | €1.09 |
| 2026-07-22 | 14:00–16:00 | 55.96 | 19:00–21:00 | 182.79 | €1.09 |
| 2026-07-23 | 12:00–14:00 | 42.77 | 20:00–22:00 | 189.4 | €1.28 |
| 2026-07-24 | 13:00–15:00 | 14.73 | 20:00–22:00 | 198.16 | €1.64 |
| 2026-07-25 | 12:00–14:00 | -7.55 | 02:00–04:00 | 173.3 | €1.64 |
Local time, hourly auction means. Days where charging was negative-priced mean you were paid to fill the battery.
Do it automatically
The windows move every day — automation is the whole game. One API call returns a cost-optimal schedule your inverter or energy manager can follow, and there's a full battery simulator to backtest your exact capacity, power and cycle limits against realized prices before you trust it:
curl -X POST -H "Authorization: Bearer YOUR_KEY" \
-H "Content-Type: application/json" \
-d '{"zone": "SI", "capacity_kwh": 10, "max_power_kw": 5,
"round_trip_efficiency": 0.9, "max_cycles_per_day": 1}' \
https://voltcast.com/api/v1/optimize/batteryHome Assistant users get the schedule as a sensor via the integration. And because the value depends entirely on tomorrow's spread, the P10–P90 forecast — scored publicly every day — tells you whether tomorrow is worth a deep cycle at all.
Method & citation. Prices are hourly means of published day-ahead auction results (native 15-minute periods averaged; ENTSO-E/SMARD, attributed). Wholesale-price component only — grid fees and taxes come on top and vary by supplier. Cite as "Voltcast Research, voltcast.com/guides/battery-arbitrage-si-2026-07-26".
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