diff --git a/bpfin/financials/monthly_saving.py b/bpfin/financials/monthly_saving.py
new file mode 100644
index 0000000000000000000000000000000000000000..0f2ee41c66a42cac81d23c06eb05e0a673dc1638
--- /dev/null
+++ b/bpfin/financials/monthly_saving.py
@@ -0,0 +1,26 @@
+import datetime
+from bpfin.utilbills.bill_lib import cal_last_day
+
+
+def monthly_saving(commissioning_date, proforma_date, prior_saving_list, post_saving_list):
+    """
+    Calculate monthly saving on usage, or on charge, considering the commissioning date.
+    Args:
+        commissioning_date (date): the date that construction work finished, saving starts at NEXT month
+        proforma_date (list): list of dates, months of pro-forma time period
+        prior_saving_list (list): list of float, usage or charge without (prior to) savings
+        post_saving_list (list): list of float, usage or charge with (post to) savings
+    Return:
+        list: list of float, monthly saving values on usage or charge
+    """
+    saving_start_date = datetime.date(
+        commissioning_date.year,
+        commissioning_date.month,
+        cal_last_day(commissioning_date.year, commissioning_date.month))
+    saving_dict = {}
+    for date, prior, post in zip(proforma_date, prior_saving_list, post_saving_list):
+        if date > saving_start_date:
+            saving_dict[date] = prior - post
+        else:
+            saving_dict[date] = 0
+    return list(saving_dict.values())
diff --git a/bpfin/tests/test_financials/test_monthly_saving.py b/bpfin/tests/test_financials/test_monthly_saving.py
new file mode 100644
index 0000000000000000000000000000000000000000..c9ad36722fcf762d13a8f334a9b9d5c6fd580a2c
--- /dev/null
+++ b/bpfin/tests/test_financials/test_monthly_saving.py
@@ -0,0 +1,25 @@
+import datetime
+from bpfin.financials.monthly_saving import monthly_saving
+
+
+def test_monthly_saving():
+    input_proforma_date = [
+        datetime.date(2017, 1, 31),
+        datetime.date(2017, 2, 28),
+        datetime.date(2017, 3, 31),
+        datetime.date(2017, 4, 30),
+        datetime.date(2017, 5, 31),
+        datetime.date(2017, 6, 30),
+        datetime.date(2017, 7, 31),
+        datetime.date(2017, 8, 31),
+        datetime.date(2017, 9, 30),
+        datetime.date(2017, 10, 31),
+        datetime.date(2017, 11, 30),
+        datetime.date(2017, 12, 31), ]
+    input_commissioning_date = datetime.date(2017, 3, 14)
+    input_prior_list = [100] * 12
+    post_post_list = [100, 90, 80, 70, 60, 50, 40, 30, 40, 50, 60, 70]
+
+    out_put_list = [0, 0, 0, 30, 40, 50, 60, 70, 60, 50, 40, 30]
+    result_list = monthly_saving(input_commissioning_date, input_proforma_date, input_prior_list, post_post_list)
+    assert out_put_list == result_list
diff --git a/bpfin/utilbills/data_generation.py b/bpfin/utilbills/data_generation.py
index d7219d8e2c0fc2fe1f8801489536159c4b77ca76..3f8da74081bbe57af83b8257a26e6c9ad78c52d5 100644
--- a/bpfin/utilbills/data_generation.py
+++ b/bpfin/utilbills/data_generation.py
@@ -1,9 +1,16 @@
+# import datetime
+# import copy
 # from bpfin.utilbills.bill_month_normalize_rough import bill_month_normalize_rough
 # from bpfin.utilbills.bill_prior_proj_rough import bill_prior_proj_rough
+# from bpfin.financials.monthly_saving import monthly_saving
+# from bpfin.lib import other as lib
 # from bpfin.utilbills import bill_lib as bl
+# from bpfin.financials import financial_lib as fl
 # from bpfin.tests.testdata import sample_data as db
 
 
+
+
 # raw_bill = db.raw_bill
 # norm_bill = bill_month_normalize_rough(raw_bill)
 # prior_bill_electricity = bill_prior_proj_rough(norm_bill, raw_bill, db.analysis_date, db.inflation_coeff_dict)
@@ -11,3 +18,29 @@
 
 # print(annual_bill_electricity)
 
+
+# prior_energy_bill = db.bill_overview_organized
+# raw_income_input = db.raw_income_input
+
+# IS_prior = fl.Income_Statement_Table(raw_income_input, prior_energy_bill)
+# IS_prior.project(-2.0, db.analysis_date, prior_energy_bill)
+
+# post_energy_bill = copy.deepcopy(db.bill_overview_organized)
+# saving_list = monthly_saving(datetime.date(2018, 1, 1), db.proforma_date_to, db.prior_proj_rough_charge, db.post_proj_rough_charge)
+# # print(saving_list)
+# post_saving_e_bill = lib.sublist(
+#     db.prior_proj_rough_charge,
+#     saving_list
+#     )
+# # print(post_saving_e_bill)
+
+# post_energy_bill['electricity'] = bl.annualizing_projection(
+#     db.post_bill_rough['date_to'], post_saving_e_bill)
+
+# IS_post = fl.Income_Statement_Table(raw_income_input, post_energy_bill)
+# IS_post.project(-2.0, db.analysis_date, post_energy_bill)
+
+# prior_noi = (IS_prior.get_noi_dict().values())
+# post_noi = (IS_post.get_noi_dict().values())
+
+# print(lib.sublist(post_noi, prior_noi))