Hello,

Due to the Format that you are using, if for whatever reason the resulting number is zero, you will get an empty string placed into the TextBox, which I suspect might be part of the problem you are having.

I would suggest that you look into using Double.TryParse to grab your input values, this way you can be sure that what you are getting from the user is valid data. As an example, take a look at this:

Code:
Partial Public Class _Default
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    End Sub

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
        'CURRENT FIXTURE part 1
        Dim a As Double
        Dim b As Double
        Dim c As Double
        Dim d As Double

        If Not Double.TryParse(TextBox1.Text, a) Then
            Throw New ArgumentException("Unable to validate the Light Wattage Input")
        End If

        If Not Double.TryParse(TextBox2.Text, b) Then
            Throw New ArgumentException("Unable to validate the QTY Input")
        End If

        If Not Double.TryParse(TextBox3.Text, c) Then
            Throw New ArgumentException("Unable to validate the Operation Time Input")
        End If

        Dim annualKw As Double = a * b * c * 365

        TextBox4.Text = Format(annualKw, "#,###,")

        If Not Double.TryParse(TextBox5.Text, d) Then
            Throw New ArgumentException("Unable to validate the charge per kw Input")
        End If

        'CURRENT FIXTURE part 2
        'Multiply the annual Kw usage * kw charged
        Dim annualCost As Integer = annualKw * d

        TextBox6.Text = Format(annualCost, "#,###,")
    End Sub

    Protected Sub Button2_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button2.Click
        MsgBox("yay")
    End Sub
End Class
In this sample I am simply throwing an exception when something goes wrong, but you would most likely want to handle this a bit more gracefully.

Hope that helps!

Gary