If your broadband supplier is Zen Internet, you may have wondered which of their broadband packages will be the best value for your level of monthly usage. This page compares Zen's three tariffs, but you can use the same method to compare tariffs from other providers.

Very simply, if your monthly download usage is low, the Zen Lite package will be cheapest for you. If your monthly download usage is high, the Zen 8000 Pro package will work out cheapest. Somewhere between high and low, then the Zen 8000 Active package will be your cheapest option. But where are the lines drawn between low, medium and high monthly usage?

The price that you'll pay each month is determined by several things:

- the fixed monthly fee for the broadband package you're using;
- the usage limit that comes with that package;
- how many gigabytes you use in that month, and whether you go over the limit;
- the price per extra gigabyte of usage over the monthly limit.

All these numbers make it tricky to know which package will cost you the least for a given amount of download usage in a month. But with a bit of tinkering in OpenOffice 2.1 Calc we get the following graph:

This is a nice way of showing how the cost of each package varies as monthly usage goes up. The graph shows that if you reach less than six gigabytes of broadband download usage each month, then the Zen 8000 Lite package will work out cheaper than the other two packages. And if you use more than about twenty-six gigabytes in a month, the Zen 8000 Pro package will work out the cheapest of the three. If your monthly usage falls between about six and twenty-six gigabytes in a month then the Zen 8000 Active package is your cheapest option. [Based on prices in February 2007.]

If you want to know the exact boundary values, a bit of equation solving will offer the answer. For the point at which the Lite and Active packages become equal in price, we need to solve the following, where g is monthly usage in gigabytes:

1.49(g - 2) + 17.99 = 24.99 (where 2 < g < 20).

So 1.49g - 2.98 + 17.99 = 24.99

So g = (24.99 - 17.99 + 2.98) / 1.49 = 6.70

Which means that at 6.7 gigabytes in a month, the Active package costs the same as the Lite package. And, looking at the graph, after 6.7 gigabytes in a month, the Active package is cheaper than the Lite package.

For the point at which the Active and Pro packages become equal in price:

1.49(g - 20) + 24.99 = 34.99 (where 20 < g < 50).

So 1.49g - 29.8 + 24.99 = 34.99

So g = (34.99 - 24.99 + 29.8) / 1.49 = 26.71

Which means that at 26.71 gigabytes in a month, the Pro package becomes the same price as the Active package. And, looking at the graph, after 26.71 gigabytes in a month, the Pro package becomes the cheapest package of the three. [Based on prices in February 2007.]

In case the prices have changed for Zen's ADSL packages, or in case you want to modify the spreadsheet to compare packages from another company, here is the spreadsheet I created to draw the above graph:

Zen ADSL package comparison spreadsheet.

Note that it's in OpenOffice 2.1 format.