Daily guide · FR · 2026-08-31
What a home battery earned in France 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 week's clearest signal is how much the cheap window mattered. On 24 August the midday trough averaged 7,96 EUR/MWh and the evening peak 183,50 EUR/MWh, producing a cycle value of 1,57 EUR. By contrast, on 25 and 26 August the cheap window itself sat above 116 EUR/MWh, and daily earnings fell to 0,71 and 0,60 EUR respectively. When solar does not push midday prices down, arbitrage margins compress quickly.
The strongest days came when charging was effectively free. On 29 August the cheap window averaged -0,09 EUR/MWh and on 30 August -1,00 EUR/MWh, delivering 1,22 and 1,75 EUR per cycle. The full week returned 7,66 EUR from a 10 kWh battery at 90 percent round-trip efficiency, on an average spread of 128,04 EUR/MWh, which annualises to 399 EUR.
The concrete takeaway: the charge window was consistent, starting at 13:00 on most days, while discharge timing varied from 06:00 to 22:00. Fix your charging around the midday trough, but stay flexible on discharge, because that is where the week's value gap between days actually opened up.
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 FR this week.
€7.66
captured this week — one cycle per day, wholesale component, after losses
128.04
€/MWh average cheapest-2h → priciest-2h spread this week
≈€399
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 |
|---|---|---|---|---|---|
| 2026-08-24 | 13:00–15:00 | 7.96 | 20:00–22:00 | 183.5 | €1.57 |
| 2026-08-25 | 13:00–15:00 | 116.07 | 19:00–21:00 | 207.9 | €0.71 |
| 2026-08-26 | 14:00–16:00 | 121.02 | 19:00–21:00 | 200.68 | €0.6 |
| 2026-08-27 | 13:00–15:00 | 115.24 | 19:00–21:00 | 202.48 | €0.67 |
| 2026-08-28 | 13:00–15:00 | 46.29 | 06:00–08:00 | 177.8 | €1.14 |
| 2026-08-29 | 15:00–17:00 | -0.09 | 22:00–24:00 | 135.72 | €1.22 |
| 2026-08-30 | 13:00–15:00 | -1 | 20:00–22:00 | 193.72 | €1.75 |
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": "FR", "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-fr-2026-08-31".
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