Solution without a database

no-db.rb

@@ -0,0 +1,67 @@
+require "date"
+
+class Tariff
+  attr_reader :kilowatt_hour_rate # Arbeitspreis
+  attr_reader :basic_rate # Grundpreis
+  attr_reader :month
+
+  def initialize(kilowatt_hour_rate:, basic_rate:, month:)
+    @kilowatt_hour_rate = kilowatt_hour_rate
+    @basic_rate = basic_rate
+    @month = month
+  end
+end
+
+class Reading
+  attr_reader :date
+  attr_reader :kilowatts
+
+  def initialize(date:, kilowatts:)
+    @date = date
+    @kilowatts = kilowatts
+  end
+end
+
+class MonthlyConsumption
+  def self.calculate_for(month:, tariffs:, readings:)
+    start_reading = readings.find { |reading| reading.date == Date.new(month.year, month.month, 1) }
+    end_reading = readings.find { |reading| reading.date == Date.new(month.next_month.year, month.next_month.month, 1) }
+    kilowatts = end_reading.kilowatts - start_reading.kilowatts
+
+    tariff = tariffs.take_while { |tariff| tariff.month < month }.last
+
+    price = tariff.basic_rate + kilowatts * tariff.kilowatt_hour_rate
+
+    "In #{month.strftime('%B %Y')} you consumed #{kilowatts} kilowatts for #{price / 100} Euro"
+  end
+end
+
+tariffs = [
+  Tariff.new(
+    kilowatt_hour_rate: 35,
+    basic_rate: 1200,
+    month: Date.new(2022, 1, 1)
+  ),
+  Tariff.new(
+    kilowatt_hour_rate: 37,
+    basic_rate: 1200,
+    month: Date.new(2022, 5, 1)
+  ),
+  Tariff.new(
+    kilowatt_hour_rate: 30,
+    basic_rate: 1200,
+    month: Date.new(2022, 7, 1)
+  )
+]
+
+current_reading = 3220
+
+readings = (Date.new(2022, 1, 1)...Date.new(2022, 12, 31)).map do |date|
+  current_reading += [3, 4, 6].sample
+  Reading.new(
+    date: date,
+    kilowatts: current_reading
+  )
+end
+
+puts MonthlyConsumption.calculate_for(month: Date.new(2022, 6), tariffs: tariffs, readings: readings)