softwareengineerprogrammer · softwareengineerprogrammer · Jan 16, 2026 · Jan 16, 2026 · Jan 16, 2026 · Jan 16, 2026
diff --git a/.bumpversion.cfg b/.bumpversion.cfg
@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 3.10.25
+current_version = 3.11.1
 commit = True
 tag = True
 

diff --git a/.cookiecutterrc b/.cookiecutterrc
@@ -54,7 +54,7 @@ default_context:
     sphinx_doctest: "no"
     sphinx_theme: "sphinx-py3doc-enhanced-theme"
     test_matrix_separate_coverage: "no"
-    version: 3.10.25
+    version: 3.11.1
     version_manager: "bump2version"
     website: "https://github.com/NREL"
     year_from: "2023"

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
@@ -4,6 +4,11 @@ Changelog
 
 GEOPHIRES v3 (2023-2025)
 ------------------------
+3.11
+^^^^
+
+3.11: `SAM Economic Models Project Payback Period fix <https://github.com/softwareengineerprogrammer/GEOPHIRES/pull/123>`__ | `release <https://github.com/NREL/GEOPHIRES-X/releases/tag/v3.11.1>`__
+
 
 3.10
 ^^^^

diff --git a/README.rst b/README.rst
@@ -58,9 +58,9 @@ Free software: `MIT license <LICENSE>`__
     :alt: Supported implementations
     :target: https://pypi.org/project/geophires-x
 
-.. |commits-since| image:: https://img.shields.io/github/commits-since/softwareengineerprogrammer/GEOPHIRES-X/v3.10.25.svg
+.. |commits-since| image:: https://img.shields.io/github/commits-since/softwareengineerprogrammer/GEOPHIRES-X/v3.11.1.svg
     :alt: Commits since latest release
-    :target: https://github.com/softwareengineerprogrammer/GEOPHIRES-X/compare/v3.10.25...main
+    :target: https://github.com/softwareengineerprogrammer/GEOPHIRES-X/compare/v3.11.1...main
 
 .. |docs| image:: https://readthedocs.org/projects/GEOPHIRES-X/badge/?style=flat
     :target: https://nrel.github.io/GEOPHIRES-X

diff --git a/docs/_images/fervo_project_cape-5-sensitivity-analysis-irr.svg b/docs/_images/fervo_project_cape-5-sensitivity-analysis-irr.svg
diff --git a/docs/_images/fervo_project_cape-5-sensitivity-analysis-lcoe.svg b/docs/_images/fervo_project_cape-5-sensitivity-analysis-lcoe.svg
diff --git a/docs/_images/fervo_project_cape-5-sensitivity-analysis-project_npv.svg b/docs/_images/fervo_project_cape-5-sensitivity-analysis-project_npv.svg
diff --git a/docs/conf.py b/docs/conf.py
@@ -18,7 +18,7 @@
 year = '2025'
 author = 'NREL'
 copyright = f'{year}, {author}'
-version = release = '3.10.25'
+version = release = '3.11.1'
 
 pygments_style = 'trac'
 templates_path = ['./templates']

diff --git a/setup.py b/setup.py
@@ -13,7 +13,7 @@ def read(*names, **kwargs):
 
 setup(
     name='geophires-x',
-    version='3.10.25',
+    version='3.11.1',
     license='MIT',
     description='GEOPHIRES is a free and open-source geothermal techno-economic simulator.',
     long_description='{}\n{}'.format(

diff --git a/src/geophires_x/EconomicsSam.py b/src/geophires_x/EconomicsSam.py
@@ -521,21 +521,43 @@ def _calculate_project_vir(cash_flow: list[list[Any]], model: Model) -> float:
 
 def _calculate_project_payback_period(cash_flow: list[list[Any]], model) -> float | None:
     """
-    See payback period output parameter's tooltip text for details relevant to this implementation.
-    """
+    Calculates the Simple Payback Period (SPB).
+    SPB is the time required for the cumulative non-discounted after-tax net cash flow to turn positive.
 
+    The calculation assumes annual cash flows. The returned value represents the number of years
+    from the start of the provided cash flow list until the investment is recovered.
+    """
     try:
+        # Get flattened annual after-tax cash flow
         after_tax_cash_flow = _after_tax_net_cash_flow_all_years(cash_flow, _pre_revenue_years_count(model))
-        cumm_cash_flow = np.zeros(len(after_tax_cash_flow))
-        cumm_cash_flow[0] = after_tax_cash_flow[0]
-        for year in range(1, len(after_tax_cash_flow)):
-            cumm_cash_flow[year] = cumm_cash_flow[year - 1] + after_tax_cash_flow[year]
-            if cumm_cash_flow[year] >= 0:
-                year_before_full_recovery = year - 1
-                payback_period = (
-                    year_before_full_recovery
-                    + abs(cumm_cash_flow[year_before_full_recovery]) / after_tax_cash_flow[year]
-                )
+
+        cumulative_cash_flow = np.zeros(len(after_tax_cash_flow))
+        cumulative_cash_flow[0] = after_tax_cash_flow[0]
+
+        # Handle edge case where the first year is already positive
+        if cumulative_cash_flow[0] >= 0:
+            # If the project is profitable immediately (rare for SPB), return 0 or fraction.
+            # For standard SPB logic where Index 0 is an investment year, this is an edge case.
+            pass
+
+        for year_index in range(1, len(after_tax_cash_flow)):
+            cumulative_cash_flow[year_index] = cumulative_cash_flow[year_index - 1] + after_tax_cash_flow[year_index]
+
+            if cumulative_cash_flow[year_index] >= 0:
+                # Payback occurred in this year (year_index).
+                # We need to calculate how far into this year the break-even point occurred.
+
+                previous_year_index = year_index - 1
+                unrecovered_cost_at_start_of_year = abs(cumulative_cash_flow[previous_year_index])
+                cash_flow_in_current_year = after_tax_cash_flow[year_index]
+
+                # Fraction of the current year required to recover the remaining cost
+                fraction_of_year = unrecovered_cost_at_start_of_year / cash_flow_in_current_year
+
+                # Total years elapsed = Full years prior to this one + fraction of this one.
+                # If we are at year_index, the number of full years passed is equal to year_index.
+                # Example: If year_index is 5 (6th year), 5 full years (Indices 0..4) have passed.
+                payback_period = year_index + fraction_of_year
 
                 return float(payback_period)
 

diff --git a/src/geophires_x/__init__.py b/src/geophires_x/__init__.py
@@ -1 +1 @@
-__version__ = '3.10.25'
+__version__ = '3.11.1'
diff --git a/tests/examples/Fervo_Project_Cape-4.out b/tests/examples/Fervo_Project_Cape-4.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.25
- Simulation Date: 2026-01-13
- Simulation Time:  11:08
- Calculation Time:      1.855 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:41
+ Calculation Time:      1.775 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -34,7 +34,7 @@ Simulation Metadata
       After-tax IRR:                                        27.55 %
       Project VIR=PI=PIR:                                    1.45
       Project MOIC:                                          4.20
-      Project Payback Period:                                       2.33 yr
+      Project Payback Period:                                       3.33 yr
       Estimated Jobs Created:                                 1300
 
                           ***ENGINEERING PARAMETERS***

diff --git a/tests/examples/Fervo_Project_Cape-5.out b/tests/examples/Fervo_Project_Cape-5.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.25
- Simulation Date: 2026-01-14
- Simulation Time:  11:38
- Calculation Time:      1.828 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:42
+ Calculation Time:      1.791 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -38,7 +38,7 @@ Simulation Metadata
       After-tax IRR:                                        24.46 %
       Project VIR=PI=PIR:                                    1.45
       Project MOIC:                                          5.05
-      Project Payback Period:                                       4.93 yr
+      Project Payback Period:                                       5.93 yr
       Estimated Jobs Created:                                 1269
 
                           ***ENGINEERING PARAMETERS***

diff --git a/tests/examples/Fervo_Project_Cape-6.out b/tests/examples/Fervo_Project_Cape-6.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.25
- Simulation Date: 2026-01-15
- Simulation Time:  11:34
- Calculation Time:      1.834 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:42
+ Calculation Time:      1.811 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -38,7 +38,7 @@ Simulation Metadata
       After-tax IRR:                                        35.45 %
       Project VIR=PI=PIR:                                    1.69
       Project MOIC:                                          5.26
-      Project Payback Period:                                       2.88 yr
+      Project Payback Period:                                       3.88 yr
       Estimated Jobs Created:                                 272
 
                           ***ENGINEERING PARAMETERS***

diff --git a/tests/examples/Fervo_Project_Cape-6.txt b/tests/examples/Fervo_Project_Cape-6.txt
@@ -73,7 +73,7 @@ Reservoir Volume Option, 1, -- FRAC_NUM_SEP: Reservoir volume calculated with fr
 Number of Fractures per Stimulated Well, 150, -- The model assumes an Extreme Limited Entry stimulation design (Fervo Energy, 2023) utilizing 12 stages with 15 clusters per stage (derived from Singh et al., 2025) and 81–85% stimulation success rate per 2024b ATB Moderate Scenario (NREL, 2025).
 Fracture Separation, 9.8255, -- Based on 30 foot cluster spacing (Singh et al., 2025) marginally uprated to align with long-term thermal decline behavior trend towards wider fracture spacing (Fercho et al., 2025).
 
-Fracture Shape, 4, -- Bench design and fracture geometry Singh et al., 2025 are given in rectangular dimensions.
+Fracture Shape, 4, -- Bench design and fracture geometry in Singh et al., 2025 are given in rectangular dimensions.
 Fracture Width, 305, -- Matches intra-bench well spacing of 500 ft (corresponding to fracture length of 1000 ft) (Singh. et al., 2025)
 Fracture Height, 95, -- Actual fracture geometry is irregular and heterogeneous; this height complies with the minimum height required by the implemented bench design (200 ft; 60.96 meters) and yields an effective fracture surface area consistent with simulation results in Singh. et al., 2025.
 

diff --git a/tests/examples/example_SAM-single-owner-PPA-2.out b/tests/examples/example_SAM-single-owner-PPA-2.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.22
- Simulation Date: 2025-12-15
- Simulation Time:  09:15
- Calculation Time:      1.013 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:42
+ Calculation Time:      1.086 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -34,7 +34,7 @@ Simulation Metadata
       After-tax IRR:                                        59.73 %
       Project VIR=PI=PIR:                                    4.58
       Project MOIC:                                          9.59
-      Project Payback Period:                                       1.13 yr
+      Project Payback Period:                                       2.13 yr
       Estimated Jobs Created:                                 976
 
                           ***ENGINEERING PARAMETERS***

diff --git a/tests/examples/example_SAM-single-owner-PPA-3.out b/tests/examples/example_SAM-single-owner-PPA-3.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.22
- Simulation Date: 2025-12-15
- Simulation Time:  09:15
- Calculation Time:      1.189 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:41
+ Calculation Time:      1.221 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -34,7 +34,7 @@ Simulation Metadata
       After-tax IRR:                                        30.00 %
       Project VIR=PI=PIR:                                    2.27
       Project MOIC:                                          4.61
-      Project Payback Period:                                       2.94 yr
+      Project Payback Period:                                       3.94 yr
       Estimated Jobs Created:                                 125
 
                           ***ENGINEERING PARAMETERS***

diff --git a/tests/examples/example_SAM-single-owner-PPA-4.out b/tests/examples/example_SAM-single-owner-PPA-4.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.22
- Simulation Date: 2025-12-15
- Simulation Time:  09:15
- Calculation Time:      1.205 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:42
+ Calculation Time:      1.231 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -34,7 +34,7 @@ Simulation Metadata
       After-tax IRR:                                        22.13 %
       Project VIR=PI=PIR:                                    1.76
       Project MOIC:                                          3.38
-      Project Payback Period:                                       4.29 yr
+      Project Payback Period:                                       5.29 yr
       Estimated Jobs Created:                                 125
 
                           ***ENGINEERING PARAMETERS***
@@ -434,7 +434,6 @@ Interest earned on reserves ($)                  	0            	0          	0
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 
 
-
                                 ***EXTENDED ECONOMICS***
 
       Royalty Holder NPV:                                   50.59 MUSD

diff --git a/tests/examples/example_SAM-single-owner-PPA-5.out b/tests/examples/example_SAM-single-owner-PPA-5.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.24
- Simulation Date: 2025-12-24
- Simulation Time:  15:35
- Calculation Time:      1.840 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:42
+ Calculation Time:      1.865 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -34,7 +34,7 @@ Simulation Metadata
       After-tax IRR:                                        16.54 %
       Project VIR=PI=PIR:                                    1.58
       Project MOIC:                                          5.72
-      Project Payback Period:                                       8.92 yr
+      Project Payback Period:                                       9.92 yr
       Estimated Jobs Created:                                 250
 
                           ***ENGINEERING PARAMETERS***

diff --git a/tests/examples/example_SAM-single-owner-PPA.out b/tests/examples/example_SAM-single-owner-PPA.out
@@ -4,10 +4,10 @@
 
 Simulation Metadata
 ----------------------
- GEOPHIRES Version: 3.10.22
- Simulation Date: 2025-12-15
- Simulation Time:  09:15
- Calculation Time:      1.196 sec
+ GEOPHIRES Version: 3.11.0
+ Simulation Date: 2026-01-16
+ Simulation Time:  11:41
+ Calculation Time:      1.222 sec
 
                            ***SUMMARY OF RESULTS***
 
@@ -34,7 +34,7 @@ Simulation Metadata
       After-tax IRR:                                        24.35 %
       Project VIR=PI=PIR:                                    1.93
       Project MOIC:                                          3.85
-      Project Payback Period:                                       3.91 yr
+      Project Payback Period:                                       4.91 yr
       Estimated Jobs Created:                                 125
 
                           ***ENGINEERING PARAMETERS***
-Original file line number
+Diff line change
@@ Expand Up / @@ -4,6 +4,11 @@ Changelog @@
     GEOPHIRES v3 (2023-2025)
     ------------------------
+.11
+    ^^^^
+.11: `SAM Economic Models Project Payback Period fix <https://github.com/softwareengineerprogrammer/GEOPHIRES/pull/123>`__ | `release <https://github.com/NREL/GEOPHIRES-X/releases/tag/v3.11.1>`__
 .10
     ^^^^
@@ Expand Down @@