What values should be used for battery and converter parameters when sizing an off-grid system, and what does a complete result look like from input to financial indicator?
Default parameter set
The values below are the platform defaults for integrated AC-DC battery storage in off-grid configurations. They are starting points; the rationale column states what each is based on and when it should be moved.
|
Parameter
|
Default
|
|
Calendar life
|
15 years
|
|
Cycle life
|
8,000 cycles
|
|
O&M cost
|
3 USD/kWh/year
|
|
C-rate options
|
0.5 / 0.25
|
|
Charge efficiency
|
95%
|
|
Discharge efficiency
|
95%
|
|
Initial SOC
|
95%
|
|
Minimum SOC
|
5%
|
|
Maximum SOC
|
95%
|
|
State of health
|
94%
|
|
PCS capacity margin
|
20%
|
|
Maximum PV/battery ratio
|
2
|
Worked example
A single off-grid configuration, from optimisation output through to financial indicators.
Resulting configuration
|
Component
|
Model
|
Units
|
Total
|
|
PV modules
|
650 W
|
16313
|
10,603 kW
|
|
Inverters
|
330 kW
|
24
|
7,920 kW
|
|
Battery storage
|
2.5 MWh, 0.5C
|
12
|
15 MW / 30 MWh
|
The configuration is resolved to equipment counts and model selection rather than to system-level capacity, which is what allows the CAPEX figure to be built from a bill of materials rather than from a per-kWh assumption.
The PV-to-inverter ratio of approximately 1.34 and the four-hour storage duration are outputs of the optimisation under the stated constraints, not preset design choices.
Financial result
|
Indicator
|
Before tax
|
After tax
|
|
NPV
|
22,963,133 USD
|
15,048,385 USD
|
|
IRR
|
45.25%
|
33.15%
|
|
Equity IRR
|
45.25%
|
33.15%
|
|
Payback period
|
2.1 years
|
2.8 years
|
CAPEX 9,137,380 USD. LCOE 0.0910 USD/kWh.
Reading the result
An after-tax IRR above 30% with a payback under three years reflects a specific situation: displacement of diesel generation at a site where fuel cost is high and supply is unreliable. Returns of this magnitude are a property of the baseline being replaced rather than of the technology.
How to verify
The parameters above are testable against the equipment datasheets of any supplier. The example is reproducible given the input conditions: same resource data, same load profile, same constraints should return the same configuration, since the method is deterministic.
The optimisation method and its modelling assumptions are set out in EcoSizing: Method and Scope. Full parameter documentation is in White Paper on Intelligent Microgrid Planning and Sizing.
