Creating a running total in VB 2010

I have a home utility auditing program that I am working on. I am very new to VB and am just learning the intricacies of the language. Basically I have everything working as I want except one feature. The program uses text boxes to accept user entry and then creates a value in a text box based on those entries (basically a user enters the cost per hour of electricity, selects an appliance, enters the amount of hours used, and the cost of using the appliance is presented in a text box. What I now need to have happen is for the program to recognize when the data in the result text box has changed, then add all of the numbers up to give a total for all appliances entered. This will allow the user to keep a running total of costs for however many appliances they want to enter. I think I need a do loop to handle this but I am unsure of the syntax. So for the following code the values that need to be added together will be from the opCostTextBox and the total should be displayed will be given in the TextBoxTotalCost. This is the first VB program I have written, so I apologize in advance for any obvious answers I am missing.

Public Class Form1
    'declaring variables to store program values
    Dim costEntry As Single
    Dim applianceSelect As String
    Dim powerRequired As Single
    Dim hoursUsed As Single
    Dim operCost As Single
    Dim strGallons As Single
    Dim strCostPerGallon As Single
    Dim washerAddCost As Single
    Dim strListBoxEntry As String
    


    'this section for subroutine to run once calculate button is pressed
    Private Sub calcButton_Click(sender As System.Object, e As System.EventArgs) Handles calcButton.Click
        'converting variables into single data type
        costEntry = CSng(costEntryTextBox.Text)
        applianceSelect = applianceComboBox.Text
        powerRequired = CSng(powerTextBox.Text)
        hoursUsed = CSng(hoursTextBox.Text)


        'math to calculate the solution
        If applianceSelect = "Washer" Then
            operCost = (costEntry * hoursUsed * powerRequired) + (strGallons * strCostPerGallon)
            operCost = CSng(FormatCurrency(operCost, 2))
            MessageBox.Show("$ " + CStr(washerAddCost) + "  has been added to the cost of operating the washer to account for water useage")
        Else

            operCost = costEntry * hoursUsed * powerRequired
            operCost = CSng(FormatCurrency(operCost, 2))
        End If
        'formatting results for display
        opCostTextBox.Text = FormatCurrency(operCost, 2)

        'causes data to be dispayed in listbox
        strListBoxEntry = ((applianceComboBox.Text) + ",  Cost per Hour:" + Str(costEntry) + ",  Power Req'd:" + Str(powerRequired) + ",  Hours Used:" + Str(hoursUsed) + ", Cost of Operation= $" + Str(operCost))
        ListBox1.Items.Add(strListBoxEntry)
        
    End Sub

    'section for handling the combobox and assigning the multiplication values to determine the power requirements for each appliance
    Private Sub applianceComboBox_SelectedIndexChanged(sender As System.Object, e As System.EventArgs) Handles applianceComboBox.SelectedIndexChanged
        If applianceComboBox.Text = "Toaster" Then
            powerTextBox.Text = CStr(1.146)
        ElseIf applianceComboBox.Text = "Coffee Pot" Then
            powerTextBox.Text = CStr(1.2)
        ElseIf applianceComboBox.Text = "Can Opener" Then
            powerTextBox.Text = CStr(0.1)
        ElseIf applianceComboBox.Text = "Microwave Oven" Then
            powerTextBox.Text = CStr(1.45)
        ElseIf applianceComboBox.Text = "Electric Range" Then
            powerTextBox.Text = CStr(12.2)
        ElseIf applianceComboBox.Text = "Air Conditioner" Then
            powerTextBox.Text = CStr(1.9)
        ElseIf applianceComboBox.Text = "Washer" Then
            powerTextBox.Text = CStr(0.512)
            strGallons = CSng(CDbl(InputBox("Enter the number of gallons of water used by the washer per hour.")))
            strCostPerGallon = CSng(CDbl(InputBox("Enter the cost per gallon of water used by the washer.")))
            washerAddCost = strGallons * strCostPerGallon
        ElseIf applianceComboBox.Text = "Dryer" Then
            powerTextBox.Text = CStr(2.79)
        ElseIf applianceComboBox.Text = "Television" Then
            powerTextBox.Text = CStr(0.147)
        ElseIf applianceComboBox.Text = "Lamp" Then
            powerTextBox.Text = CStr(0.05)
        End If
    End Sub

    Private Sub costEntryTextBox_KeyPress(sender As Object, e As System.Windows.Forms.KeyPressEventArgs) Handles costEntryTextBox.KeyPress
        'validation controls to ensure numeric data only is entered in cost entry text box
        If Asc(e.KeyChar) < 46 Or Asc(e.KeyChar) > 57 Then
            e.Handled = True
            'displays box telling the user to enter only numbers
            MessageBox.Show("Please enter only numbers")

        End If
    End Sub

    'This section handles the reset button functionality to clear all textboxes
    Private Sub resetButton_Click(sender As System.Object, e As System.EventArgs) Handles resetButton.Click
        costEntryTextBox.Text = ""
        applianceComboBox.SelectedIndex = -1
        powerTextBox.Text = ""
        hoursTextBox.Text = ""
        opCostTextBox.Text = ""
    End Sub

    Private Sub powerTextBox_KeyPress(sender As Object, e As System.Windows.Forms.KeyPressEventArgs) Handles powerTextBox.KeyPress
        'validation controls to ensure numeric data only is entered in cost entry text box
        If Asc(e.KeyChar) < 46 Or Asc(e.KeyChar) > 57 Then
            e.Handled = True
            'displays box telling the user to enter only numbers
            MessageBox.Show("Please enter only numbers")

        End If
    End Sub

    Private Sub hoursTextBox_KeyPress(sender As Object, e As System.Windows.Forms.KeyPressEventArgs) Handles hoursTextBox.KeyPress
        'validation controls to ensure numeric data only is entered in cost entry text box
        If Asc(e.KeyChar) < 46 Or Asc(e.KeyChar) > 57 Then
            e.Handled = True
            'displays box telling the user to enter only numbers
            MessageBox.Show("Please enter only numbers")

        End If
    End Sub

    Private Sub TotalCostTextBox_TextChanged(sender As System.Object, e As System.EventArgs) Handles TotalCostTextBox.TextChanged

    End Sub
End Class