diff --git a/app/models/generator.rb b/app/models/generator.rb
index 5883374..ee388e6 100644
--- a/app/models/generator.rb
+++ b/app/models/generator.rb
@@ -6,11 +6,32 @@ class Generator < TechnicalComponentInstance
   validates :generator_type, :presence => true
 
   def takes_bids?
-    city.state.game.regulation_type == :auction
+    state.game.regulation_type == :auction
   end
 
-  def fuel_since time
-    operated_hours = Float(Time.now - time) / 1.hour
-    generator_type.marginal_fuel(operating_level) * operated_hours
+  def marginal_cost
+    generator_type.marginal_cost(state.game, operating_level)
+  end
+
+  def marginal_fuel_cost
+    generator_type.marginal_fuel_cost(state.game, operating_level)
+  end
+
+  def cost_since time
+    fuel_cost_since(time) + (marginal_cost * operated_hours(time))
+  end
+
+  def fuel_cost_since time
+    marginal_fuel_cost * fuel_used_since(time)
+  end
+
+  def fuel_used_since time
+    generator_type.marginal_fuel(operating_level) * operated_hours(time)
+  end
+
+  def step time
+    cost = cost_since(time)
+    state.cash -= cost
+    state.save
   end
 end
diff --git a/app/models/technical_component_instance.rb b/app/models/technical_component_instance.rb
index 88e2865..8bb3b61 100644
--- a/app/models/technical_component_instance.rb
+++ b/app/models/technical_component_instance.rb
@@ -11,6 +11,10 @@ def state
     city.state
   end
 
+  def operated_hours time
+    Float(Time.now - time) / 1.hour
+  end
+
   def to_s
     "#{buildable} created #{created_at} operating at #{operating_level}%"
   end
diff --git a/spec/models/bid_spec.rb b/spec/models/bid_spec.rb
index 9df299d..156d190 100644
--- a/spec/models/bid_spec.rb
+++ b/spec/models/bid_spec.rb
@@ -8,7 +8,7 @@
 
   it "should validate that no bid exists for the generator for the last day" do
     generator = Factory :generator
-    generator.city.state.game.regulation_type = :auction
+    generator.state.game.regulation_type = :auction
     generator.save
     bid = Factory :bid, :generator => generator
     lambda { another_bid = Factory :bid, :generator => bid.generator
@@ -17,7 +17,7 @@
 
   it "should validate that the game has auction-style regulation" do
     generator = Factory :generator
-    generator.city.state.game.regulation_type = :ror
+    generator.state.game.regulation_type = :ror
     generator.save
     lambda { bid = Factory :bid, :generator => generator }.should raise_error
   end
diff --git a/spec/models/generator_spec.rb b/spec/models/generator_spec.rb
index bae655d..e7d9070 100644
--- a/spec/models/generator_spec.rb
+++ b/spec/models/generator_spec.rb
@@ -9,23 +9,32 @@
   it { should validate_presence_of :generator_type }
 
   context "an instance of Generator" do
-    it "should return its current revenue"
-    it "should return its availability"
-
     context "with non-renewable fuel" do
       before do
         @generator = Factory :generator
       end
 
+      it "should return its current revenue"
+      it "should return its availability"
+
+      context "when the simulation is stepped" do
+        it "should deduct the marginal cost for that time from the state" do
+          original_cash = @generator.state.cash
+          @generator.step 1.hour.ago
+          @generator.state.cash.should eq(original_cash -
+              @generator.cost_since(1.hour.ago))
+        end
+      end
+
       it "should know how much fuel it has used since a point in time" do
-        @generator.fuel_since(2.hour.ago).should be_close(
+        @generator.fuel_used_since(2.hour.ago).should be_close(
             @generator.operating_level *
               @generator.generator_type.fuel_efficiency * 2, 1)
       end
 
       it "should use more fuel over time" do
-        @generator.fuel_since(1.hour.ago).should be > (
-            @generator.fuel_since(10.minutes.ago))
+        @generator.fuel_used_since(1.hour.ago).should be > (
+            @generator.fuel_used_since(10.minutes.ago))
       end
     end
 
@@ -35,7 +44,7 @@
       end
 
       it "should not use any fuel" do
-        @generator.fuel_since(1.hour.ago).should eq 0
+        @generator.fuel_used_since(1.hour.ago).should eq 0
       end
     end
   end