If you are an EPC contractor, a solar installer moving into storage, or a C&I business adding a battery for peak shaving or blackout resilience, sizing a BESS comes down to two numbers: how many kilowatts (how fast the system delivers energy) and how many kilowatt-hours (how long it can do it). These are different quantities, and this is exactly where most buyers go wrong — they order "so many kWh" but the system can't carry the peak current, or they have enough power but not enough energy to cover the full peak duration. Below is the sizing method for container-scale projects (from 100–215 kWh upward), with worked examples for peak shaving and for backup.
Power (kW) vs energy (kWh): why you size both
This is the core distinction competitors rarely explain. Power (kW/MW) is speed: how much the system can deliver or absorb at a given instant, capped by the PCS (inverter). Energy (kWh/MWh) is the reservoir: how much it can deliver in total before it is empty, set by the battery modules. A short, sharp peak needs a lot of kW and little kWh; a long, gentle peak is the reverse.
The link between them is the C-rate — the ratio of power to energy. By definition: Power (kW) = Energy (kWh) × C-rate. A 500 kWh system at 1C delivers 500 kW for one hour; at 0.5C it delivers 250 kW for two hours. So you cannot order "a 500 kWh BESS" without a C-rate — without it, nobody knows what peak the system can shave.
You always calculate two numbers: power (kW) for the height of the peak, and energy (kWh) for its duration. One without the other is half a system.
The method: six steps
- Step 1 — define the goal. Peak shaving (cutting the demand charge) is kW-led; arbitrage / load shifting (TOU) is kWh-led; backup / blackout resilience is driven by the critical load and desired runtime; solar self-consumption is driven by surplus generation. The goal sets the C-rate and the kW-to-kWh ratio.
- Step 2 — pull the load profile. Get 12 months of hourly or 15-minute interval meter data. Identify the maximum demand peaks (kW), their duration and frequency, plus the tariff structure (a demand charge on power, separate from the energy charge).
- Step 3 — size power to shave the peak. Required power = how much of the peak you must cut. Example: reducing 1,000 kW to 750 kW means 250 kW of discharge. That is the floor for both battery and PCS.
- Step 4 — size energy for the peak duration. Usable energy (kWh) = power (kW) × peak duration (h). 250 kW for 45 minutes = 187.5 kWh usable.
- Step 5 — apply DoD, round-trip efficiency and end-of-life oversizing. Usable energy is not nameplate energy. Factor in depth of discharge (DoD), round-trip efficiency and degradation to end of life (detail below) to go from usable to installed capacity.
- Step 6 — match the PCS and C-rate. The PCS (inverter) rating must be ≥ energy (kWh) × operating C-rate. An undersized PCS becomes the bottleneck — the battery can deliver more than the inverter will pass.
From usable to installed capacity: DoD, efficiency, ageing
Nominal (nameplate) capacity is not what you actually get out. To convert a usable requirement into installed capacity:
- Round-trip efficiency (RTE). Modern LFP systems reach 92–95% at the battery level (DC–DC) and 88–92% at the system level (AC–AC) once the PCS and auxiliary loads are counted. So for every usable kWh out, budget roughly 1.1 kWh of charge in.
- Depth of discharge (DoD) and operating SoC reserve. Real systems don't discharge to zero — they hold a 10–20% SoC buffer to protect cycle life. That directly raises the installed capacity you need.
- Degradation / end-of-life oversizing. Every BESS contract is written against end-of-life (EoL) capacity, not nameplate. An LFP cell at 80% DoD typically delivers 3,000–6,000 cycles to 80% capacity retention. So you either oversize by 10–20% at commissioning, or plan augmentation (adding modules) around years 7–12 to hold the contracted power and energy.
Rule of thumb: installed capacity ≈ usable ÷ (RTE × SoC-reserve factor × degradation factor). In the example above, 187.5 kWh usable becomes roughly 240–250 kWh installed.
Worked example 1 — peak shaving
A factory peaking at 1,000 kW that must be held to 750 kW, with the peak lasting about 45 minutes twice a day:
- Power: 1,000 − 750 = 250 kW (the floor for PCS and battery).
- Energy: 250 kW × 0.75 h = 187.5 kWh usable.
- Installed: 187.5 ÷ (0.90 × 0.85 × 0.98) ≈ 240–250 kWh.
- C-rate: 250 kW ÷ 240 kWh ≈ 1C — typical for peak shaving. Spec the PCS at ≥ 250 kW.
Result — a container system of roughly 250 kW / 250 kWh. It must also recharge within the off-peak window; that is a separate validation step.
Worked example 2 — backup during outages
A site with a 100 kW critical load that must ride through a 4-hour blackout:
- Power: 100 kW (the full simultaneous critical load).
- Energy: 100 kW × 4 h = 400 kWh usable.
- Installed: 400 ÷ (0.90 × 0.85) ≈ 520 kWh (backup usually allows a deeper DoD; degradation is still budgeted).
- C-rate: 100 kW ÷ 520 kWh ≈ 0.2C — low, typical for backup / autonomy.
Backup is an energy-heavy case: the same power, but far more kWh. In markets where grid reliability is the driver — from Ukraine to parts of MENA and Africa — the hybrid inverter must include a backup/UPS transfer function.
Pairing with solar, the PCS, and quality checks
If the BESS charges from solar, the charge C-rate and energy are sized against the surplus-generation profile — spec the storage and array together, detail in the guide to importing solar panels and on the solar solutions page. The PCS/inverter must match on voltage class, BMS protocol and firmware profile — see choosing inverters for your project.
And crucially: the whole calculation only holds on real usable capacity. Cheap cells often test below nameplate, and without certification the shipment stalls at customs. Verify cell grade and paperwork — covered in the guide to LiFePO4 cells from China and the overview of importing a BESS. We run the full cycle: factory search, cell-grade and certificate verification (UN 38.3, IEC 62619), production QC and dangerous-goods logistics — not "just shipping".
Our methodology tracks the IEC 62933 series for electrical energy storage systems, which defines rated energy, DoD and SoC together with performance-assessment methods — precisely the terms behind a correct sizing.
Size a BESS for your load
Send your load profile (interval data, or at least peaks and tariff) to contact@silkwaysourcing.com or WhatsApp +380 97 883 4765 — we'll size the kW and kWh for your goal and quote a system straight from the factory.
