For engineers and facility managers

Battery peak-shaving sizer

A battery that discharges during the demand peak lets the site draw fewer kVA from the grid. This sizer works out the inverter kW and battery kWh to hold demand at a target, and what that is worth.

Inputs

Pre-filled with an illustrative case. Results update as you type. Jump to results

Demand peak

Current maximum demand

kVA

Target maximum demand

kVA

Power factor at the peak

Time above the target

hours

Shape of the peak

Peak events a year

per year

250 is about every working day.

Tariff

kVA demand charges

R/kVA/month

Energy rate when discharging

c/kWh

Energy rate when charging

c/kWh

Battery

Installed cost

R/kWh

Per nameplate kWh, inverter included.

Extra inverter cost

R/kW

0 if included per kWh.

Battery performance

Usable depth of discharge

Round-trip efficiency

Capacity left at end of life

The battery is sized to still do the job then.

Inverter margin

Cycle life

cycles

Calendar life

years

Results

175 kW inverter and 354 kWh battery cut demand by 150 kVA, worth about R134k a year; payback 15.9 years.

Inverter power: 175kW; 159 kW to shave + 10%
Battery energy: 354kWh; Nameplate; 239 kWh delivered per event
Annual value: R134k; Cost R2.1m
Simple payback: 15.9years; Expected life 15 years

NoteThe battery supplies real power only, so cutting 150 kVA needs 159 kW, not the 143 kW a kVA × power factor shortcut suggests. Correcting the power factor first makes the battery smaller.

RiskThe battery does not pay for itself within its expected life on these numbers. Peak shaving alone rarely does; stacking other value (backup, solar shifting, TOU arbitrage) usually decides the case.

Where the value comes from

Rand a year.

Sizing

Peak shape factor 0.75: the share of a flat block of energy the peak contains.

Measure	Value
Real power at the peak	950 kW
Reactive power (unchanged)	312 kvar
Real power allowed at the target	791 kW
Battery power to shave	159 kW
Energy delivered per event (AC)	239 kWh
Energy drawn from the cells (DC)	255 kWh
Nameplate energy	354 kWh
Power-to-energy ratio (C-rate)	0.50 C
Equivalent full cycles a year	200
Expected life	15.0 years
Net value over life (undiscounted)	−R115,128

Add to a business case

What is this?

Save this result as one measure in a project kept in this browser, then combine it with results from other tools for payback, NPV, IRR and a cost curve.

Saving, year 1: R133,953/yrAssumed
Energy added: 8,153 kWh/yrAssumed
Demand reduced: 150 kVACalculated
Capex: R2,124,420Assumed
Life: 15 yearsAssumed

Every input above is still the illustrative example, so the result is tagged assumed. Enter your own numbers to make it yours.

Name in the project

Running cost changeAssumed

R/yr

Maintenance, monitoring and insurance. Default: 1% of capex a year.

Method

Real and reactive power at the peak: P₁ = S₁ × pf, Q = S₁ × √(1 − pf²)
Battery power to reach the target: P₂ = √(S₂² − Q²), P_batt = P₁ − P₂; Reactive power is unchanged, so the target must be above Q.
Energy per event: E_ac = P_batt × hours × shape factor (flat 1, ramped 0.75, spike 0.5)
Nameplate energy: E_dc = E_ac ÷ √RTE, nameplate = E_dc ÷ (DoD × end-of-life capacity)
Life: cycles = events × E_dc ÷ (nameplate × DoD), life = min(calendar life, cycle life ÷ cycles)
Annual value: (S₁ − S₂) × R/kVA × 12 + events × (E_ac × c_discharge − E_ac ÷ RTE × c_charge) ÷ 100
Cost and payback: capex = nameplate × R/kWh + inverter kW × R/kW, payback = capex ÷ annual value

Assumptions

Every month has a similar peak, and the battery shaves it every month: the demand saving is counted 12 times a year.
The battery is fully recharged between events, off-peak, and is sized so it still covers the event at the end-of-life capacity.
Discharge efficiency is the square root of round-trip efficiency (losses split evenly between charging and discharging).
No degradation of the value over time, no discounting, no replacement, maintenance or insurance costs; the payback is simple.
Default costs, efficiency (88%) and cycle life (6,000) are illustrative for a lithium iron phosphate system.

Sources

NREL Annual Technology Baseline: commercial battery storageCost structure (energy and power components) and durations of commercial lithium-ion systems.
NREL Annual Technology Baseline: utility-scale battery storageRound-trip efficiency and cycling assumptions.
Eskom: Notified maximum demand fact sheetWhy one missed peak matters: demand and capacity charges follow the highest reading.

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