This is my data:

``````Date        Electricity Electricity2 Electricity3 Water Oil level (cm)
2011-06-30  8508        16041        7521         794   /
2011-07-10  8568        16170        7601         800   /
2011-08-01  8733        16473        7739         808   /
2011-09-03  9020        17007        7986         822   /
2011-10-16  9292        17532        8240         839   91
2011-10-25  9350        17635        8285         842   89
``````

These are counters for electricity, water and oil level at certain date. Is there any way to get an approximation how much electricity, water or oil do I spend per month?

I can get a pretty nice scatter plot graph, but that's not quite what I am searching for...

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On the example of electricity, if those numbers represent the state at some point in time (how much you've spend so far), then by intepolating (you already know how to get trendlines which is Excel's name for regression equations) what your state would be at for example 30.1. and 1.1. and subtracting one from the other, you would've gotten your consumption of electricity for that month (january).

Or have I completely misunderstood your question?

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You understood it correctly. But I don't have values for first and last day in the month for example. I can probably get them with the help of formula on the graph (for example for electricity1 the formula is y=7,3142x-289355), but how can I get this formula and use it? – sventevit Nov 2 '11 at 18:28
@slobodan - Here. I've worked you up an example. You can download it from mediafire.com/?6wso8zqxbmf4mao ... see if that's what you were aiming at. – Rook Nov 2 '11 at 18:44
Wow, exactly what I needed, thanks :) – sventevit Nov 2 '11 at 18:53

You can use the built-in SLOPE function to estimate your average monthly usage:

The formula in B10 (which is copied and pasted into C10:F10) is

`=SLOPE(B2:B7,\$A2:\$A7)`

You'll notice that these are the same slopes calculated by the chart trendline formulas, but they are in the worksheet, where they are useful to this exercise.

This formula provides the daily usage. To get the monthly usage in B11, assuming 365.25 days per year and 12 months per year, I use this formula:

`=B10*365.25/12`

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