Abstract: Storage duration — how many hours the battery can discharge at full power — is the first sizing decision in any C&I battery project, and it drives 40–60% of the system cost. A 1-hour system shaves demand spikes; a 2-hour system balances peak shaving with short backup; a 4-hour system shifts large loads and covers extended outages. Match a 1-hour, 2-hour, or 4-hour system to your site's load profile, tariff structure, and outage risk; the worked example below compares annual value.
What "duration" means in practice
A BESS is rated in power (MW) and energy (MWh). Duration is simply energy ÷ power. A 1 MWh battery paired with a 500 kW PCS gives 2 hours of discharge at full power — and up to 3–4 hours at reduced output.
The number matters because it decides:
- How much demand you can shave per event
- How long you can carry critical loads in an outage
- How many arbitrage cycles you can complete per day
- The battery's cycle life over the contract period
The three standard durations
1-hour systems are power-focused. They are designed to clip short peaks — compressor starts, welding bays, EV fast-charging surges. It discharges for 30–60 minutes, recharges from solar or grid, and is ready for the next spike. Typical size: 100 kW / 100–120 kWh.
2-hour systems are the standard commercial configuration. They handle both demand charge reduction and 1–2 hour backup for critical loads. In most EU and US commercial tariffs, 2 hours captures 80–90% of the demand-charge saving available from storage. Typical size: 500 kW / 1 MWh.
4-hour systems are energy-focused. They shift solar generation to evening peaks, cover longer outages, and support large loads such as EV fleets or data centers. They cost more per kWh of battery but unlock arbitrage and resilience value that shorter systems cannot. Typical size: 1 MW / 4 MWh.
How the load profile decides duration
Pull one year of 15-minute interval data from your utility. Look at three numbers:
1. Peak duration. How long does the site's peak demand last? If the peak is a 20-minute spike at 2 PM daily, a 1-hour battery shaves it. If the site runs near maximum for 3 hours every evening, you need 4 hours.
2. Demand charge structure. Some tariffs charge the single highest 15-minute average in the month. Others charge the average of the top 2–4 intervals. A 1-hour system erases one interval; a 2–4 hour system erases several — worth more in multi-interval tariffs.
3. Outage exposure. Check the site's outage history and the cost of one hour of downtime. A food processing plant losing a freezer room has different backup needs than an office block.
Worked example: 500 kW site, three durations
Assumptions: 500 kW average peak demand, €40/kW/month demand charge, 3 peaks per day, 20-year project life.
| 1-hour | 2-hour | 4-hour | |
|---|---|---|---|
| System | 500 kW / 500 kWh | 500 kW / 1 MWh | 500 kW / 2 MWh |
| Demand shaved per event | 500 kWh | 500 kWh | 500 kWh (limited by demand) |
| Backup coverage | 0.5–1 h | 1–2 h | 2–4 h |
| Demand charge saving / yr | €180,000 | €180,000 | €180,000 |
| Arbitrage value / yr (3 cycles) | ~€30,000 | ~€60,000 | ~€70,000 |
| Typical installed cost | ~€180k | ~€280k | ~€480k |
| Simple payback | ~4.2 yr | ~3.5 yr | ~5.3 yr |
The 2-hour system wins on payback here because it captures the same demand-charge value as the 4-hour unit while costing less. The 4-hour unit pays for itself only if the site needs the extra backup or arbitrage value.
When 1 hour is enough
Choose 1-hour when:
- The tariff charges the single peak interval
- Peaks are short and predictable (EV chargers, welders, cold stores)
- The site already has a generator for outages
- Budget is the binding constraint
When 2 hours is the safe default
Choose 2-hour when:
- The site has mixed loads and you want margin
- Outage backup for 1–2 hours is part of the business case
- You are unsure about future load growth
For most commercial buildings, manufacturing plants, and cold storage facilities, 2 hours is the recommendation we give first.
When 4 hours is justified
Choose 4-hour when:
- The site shifts large evening loads (data centers, EV fleets)
- Outages of 4+ hours are a real risk (hurricane zones, rural grids)
- Solar penetration is high and you want to shift afternoon generation to night
- The tariff has a wide day-night spread that rewards multiple-hour arbitrage
The cost trap to avoid
Battery cost scales with MWh, not MW. Doubling duration from 1 to 2 hours roughly doubles the battery cost but adds only 20–30% more demand-charge value in most tariffs. Longer duration does not automatically increase savings; model the tariff first and choose the duration with the highest payback, not the longest runtime.
Also check the PCS rating. A 500 kW PCS with a 2 MWh battery gives 4 hours, but if the site's peak shaving only needs 500 kW for 1 hour, you are paying for unused energy capacity. Size battery energy capacity to the measured load and tariff application, not a generic product spec.
What to send to your supplier
To get a correct duration recommendation, provide:
- One year of 15-minute load data
- The utility tariff sheet with demand charge structure
- Outage history and cost of downtime
- Any planned load additions (EV chargers, new machinery)
- Whether the system must provide backup or only arbitrage
A supplier can model duration against the tariff and recommend a specific system only after receiving the load data, tariff sheet, and outage history. Without those, any duration quote is a guess.
Ruibit sizes Dawnice commercial cabinets — from 100 kW / 215 kWh up to 1 MW / 2 MWh containerized systems — from the site's load data, not from a fixed product range. Send us your interval data and tariff sheet, and we return a duration comparison with payback per option before you commit.
FAQs
Q: Is a longer-duration BESS always better? No. Duration adds battery cost that must be justified by tariff structure, backup needs, or arbitrage spread. In many tariffs, 2 hours delivers most of the demand-charge saving at lower cost than 4 hours.
Q: What is the most common duration for commercial BESS? 2 hours at rated power. It balances peak shaving, backup, and cost for most commercial sites.
Q: Can a 1-hour battery provide backup for a long outage? It can carry critical loads for 30–60 minutes. For longer outages, pair it with a generator or choose a 2–4 hour system.
Q: How does duration affect battery cycle life? More hours at full power means deeper cycles per event. A 4-hour system cycling fully twice daily reaches its cycle-life limit sooner than a 1-hour system cycling shallowly. Match the cycle depth to the warranty period.
Q: Do I need the same duration if the system also does arbitrage? Not necessarily. Arbitrage value depends on the tariff spread and how many full cycles per day you can complete. Model the spread before adding duration for arbitrage alone.
