BESS Sizing Calculator: Battery Capacity and PCS Design

The equations below are derived here from conservation of energy and constant-power sizing. They are a screening model; this tool does not execute SimSES or reproduce HTW Berlin's annual simulation.

1. Describe the service at the AC boundary. For self-consumption, use measured energy to shift and maximum simultaneous battery output. For peak shaving, $P_s=P_{site}-P_{cap}$ and $E_s=P_s t k$, where $k=1$ for a rectangle and $k=0.5$ for a triangular excess-demand event of the same total width. Backup energy is $E_s=P_{avg}t$. Multiply service energy and maximum AC output by $1+m$ to obtain design energy and discharge power. The peak-event area method follows Fisher et al. (2018), source s1 p.2, Eqs. 1–2.

2. Keep converter, battery-terminal and stored-energy boundaries distinct. Enter PCS and internal battery efficiencies separately in both directions:

Stored energy withdrawn is $E_{dc}=E_{design}/\eta_d$, and AC recharge input is $E_{charge}=E_{design}/\eta_{rt}$. Required constant AC charging power is $P_c=E_{charge}/t_c$. Battery-terminal charging power is $P_{term,c}=P_c\eta_{PCS,c}$; stored-energy gain is only $\eta_{b,c}P_{term,c}$. Terminal discharge power is $P_{term,d}=P_{design}/\eta_{PCS,d}$; stored-energy withdrawal rate is $P_{term,d}/\eta_{b,d}$. Internal battery losses must never be subtracted before checking terminal charging C-rate.

3. Size nameplate energy at beginning of life. Let $u=SOC_{high}-SOC_{low}$, $r$ be retained capacity and $f_T$ the temperature factor. The energy constraint is $E_E=E_{dc}/(u r f_T)$. Under the constant-nominal-voltage approximation, use terminal power for C-rate:

Actual charging and discharge C-rate proxies are $P_{term,c}/(E_{nom}r f_T)$ and $P_{term,d}/(E_{nom}r f_T)$. The usable SOC window does not multiply these terminal-power constraints. This follows the terminal DC power/capacity definition in SAM's Battery Storage help, sections “Computed Properties” and “Power Converters” (source s5). HTW Berlin's Weniger/Tjaden/Quaschning manuscript, s3 p.4, supplies the separate usable-capacity and loss context.

4. Rate the PCS and verify recharge. For a symmetric bidirectional PCS, $P_{PCS}=\max(P_{design},P_c)$ and $S_{PCS}=P_{PCS}/PF$. Design-life AC energy is $E_{nom}u r f_T\eta_d$. The battery's maximum AC charging input is $C E_{nom}r f_T/\eta_{PCS,c}$; internal battery charging efficiency does not enter this terminal-current limit. The fastest refill uses the smallest of this limit, site AC headroom and PCS rating, and assumes that power is available throughout charging. Check PV surplus energy separately. Select actual equipment above the continuous minima and verify both directional ratings.

The sizing chart holds peak discharge power fixed while varying service energy and refill duty. Its minimum envelope is shown last as separated sample markers over the continuous constraint lines; the full envelope vector remains available in the calculation steps. The capacity-fade chart is an energy sensitivity for a fixed installed battery, not an aging forecast or a guarantee of power feasibility at every retention value.

• One service and one design event are sized at a time. Duties are not added or stacked. Battery starts at the entered upper SOC limit; energy below the lower limit is reserved and unavailable.
• Peak-shaving inputs describe net site demand after concurrent generation. A triangular event starts and ends at the target import limit. Use a rectangle for a conservative envelope when the shape is unknown. Peaks separated by insufficient recharge time must be grouped or simulated explicitly.
• Backup has no replenishment during the outage. PV self-consumption is a representative-day energy balance; the constant recharge window plus minimum full-power discharge time must fit within a 24-hour day. Adequate total surplus and power headroom do not establish an annual self-sufficiency rate or guarantee their timing.
• Constant PCS and internal battery efficiencies, approximately constant nominal battery voltage, and constant recharge power are assumed. The four efficiency inputs have separate boundaries; do not enter an already combined efficiency as a component efficiency. Auxiliary energy must be included in the service energy/load or the loss assumptions. Idle standby consumption outside the event is not modeled.
• Retention and temperature factors independently reduce available total capacity. Terminal charging and discharge power are compared with the permitted C-rate times this remaining capacity. This assumes limits scale with remaining capacity and uses terminal power/nominal energy as a constant-voltage proxy for current/capacity; supplier voltage, current, SOC, temperature and age limits govern actual equipment.
• The 10% allowance, 80% retained capacity, 10–90% SOC window, 0.5C limit, 97% PCS efficiencies and 98% internal battery efficiencies are editable illustrative design choices, not universal manufacturer guarantees. The temperature default of 1 assumes rated conditions. Enter product-specific warranted values.
• PCS ratings are continuous, symmetric and available at the design operating conditions. The entered minimum power factor is used for apparent-power screening in both directions. Surge and reactive dispatch require separate checks.

Use for initial energy and continuous-power specification of residential or commercial stationary battery systems. The arithmetic is chemistry-neutral; constant efficiency and constant-voltage assumptions are most suitable for preliminary lithium-ion screening. Lead-acid reference examples validate the energy balance only: actual lead-acid capacity requires discharge-rate, temperature and aging data.

The design-life input is a specified capacity retention, not an aging law or a forecast of years or cycles. SimSES (2022, DOI 10.1016/j.est.2021.103743) provides a more detailed framework for efficiency, component behavior and aging; its citation does not imply numerical equivalence to this tool. Use time-series simulation for annual PV performance, sequences of peak events, variable charge headroom, forecast errors, tariffs or competing services. HTW Berlin's German residential findings do not transfer automatically to homes in the Netherlands, Poland or another location.

The outputs are minimum continuous ratings before selecting commercially available equipment. A warning about insufficient charging power or PV energy means the entered refill duty is not feasible even if the battery is large enough. Confirm part-load efficiency, temperature-dependent power, overload capability, phase imbalance, islanding/transfer behavior, DC voltage/current limits, protection and grid requirements with the selected equipment.

No tariff, subsidy, market-price or regulatory design rule is implemented. Installation work belongs to a qualified person; verify applicable local requirements and check against the standard in force. This tool does not establish installation compliance.