diff --git a/app/models/reading.rb b/app/models/reading.rb
index cdf66e4..6b1ec9d 100644
--- a/app/models/reading.rb
+++ b/app/models/reading.rb
@@ -30,5 +30,37 @@ class Reading < ActiveRecord::Base
   def total_kwh_produced_low=(kwh)
     write_attribute(:total_kwh_produced_low,kwh.round(1))
   end
-
+ 
+  # calculate difference with another reading
+  # return a hash with differences (self - reading)
+  def diff(reading)
+    { :total_kwh_consumed_high => self.total_kwh_consumed_high - reading.total_kwh_consumed_high,
+      :total_kwh_consumed_low => self.total_kwh_consumed_low - reading.total_kwh_consumed_low,
+      :total_kwh_produced_high => self.total_kwh_produced_high - reading.total_kwh_produced_high,
+      :total_kwh_produced_low => self.total_kwh_produced_low - reading.total_kwh_produced_low,
+      :current_kw_consumed => self.current_kw_consumed - reading.current_kw_consumed,
+      :current_kw_produced => self.current_kw_produced - reading.current_kw_produced,
+      :total_m3_gas_consumed => self.total_m3_gas_consumed - reading.total_m3_gas_consumed }
+  end
+  
+  #
+  # Class methods
+  #
+  class << self
+    # return readings from beginning of 'from' until end of 'to'
+    def days(from, to)
+      Reading.where("created_at > :begin AND created_at < :end", { :begin => from.to_date.beginning_of_day, :end => to.to_date.end_of_day})  
+    end
+    
+    def day(date)
+      Reading.where("created_at > :begin AND created_at < :end", { :begin => date.to_date.beginning_of_day, :end => date.to_date.end_of_day})        
+    end
+    
+    def diff_on(date)
+      readings_on = day(date)
+      first = readings_on.first
+      last = readings_on.last
+      first.diff(last)
+    end
+  end
 end