Robson » Little Red Book » Electricity Bill

Chapter 4 - Question 6

Computing an electricity bill involves calculating the number of units consumed and allocating them to a variety of tariffs.

Tariffs are based on two components - a standing charge and a price per unit. There may be more than one price range - perhaps the first 50 units are charged at a higher rate than subsequent units.

Given that the initial tariffs offered are:

Tariff	Costs	Price
A	Standing charge	10.00
	First 80 units	.105
	Subsequent units	.03
B	Standing charge	3.00
	First 250 units	0.10
	Subsequent units	.075
C	Standing charge	NIL
	All units	0.125

Customers must choose the tariff they wish to be charged under. Design a program that accepts the number of units consumed and reports the most economic tariff.

Solution 1

C++.

#include <stdio.h>
#include <stdlib.h>

using namespace std;

// iUnits = The total amount of units consumed
// fStart = The standing charge
// fSwitch = The amount that electricity switches to lower rate
// fStandardRate = The high rate that applies to the first units
// fLowRate = The low rate that applies to subsequent units
float ic_CalculateCost(int iUnits, float fStart,
float fSwitch, float fStandardRate, float fLowRate=0)
{
// stores the total
float fCost = fStart;

// work out the price for this tariff
// see if they used enough units to use both brackets
if (fSwitch && iUnits > fSwitch)
{
// the first few units are at standard rate
fCost += fSwitch * fStandardRate;
// remaining units at low rate
fCost += (iUnits - fSwitch) * fLowRate;
}
else
// all units at standard rate
fCost += iUnits * fStandardRate;

// return the total price
return fCost;
}

int main(int argc, char *argv[])
{
// convert the units to a number and store
int iUnits = atoi(argv[1]);
// this stores the prices for each company
float fPrices[2];

// work out the price for tariff a
fPrices[0] = ic_CalculateCost(iUnits, 10, 80, 0.105, 0.03);
// work out the price for tariff b
fPrices[1] = ic_CalculateCost(iUnits, 3, 250, 0.1, 0.075);
// work out the price for tariff c
fPrices[2] = ic_CalculateCost(iUnits, 0, 0, 0.125);

// show all the total prices
printf("Tariff A: %c%.2f\nTariff B: %c%.2f\nTariff C: %c%.2f",
156, fPrices[0],
156, fPrices[1],
156, fPrices[2]);
}

Solution 2

Excel formulas.

Tariff A:

=IF(A1 > 80, 10 + (80 * 0.105) + ((A1 - 80) * 0.03), 10 + (A1 * 0.105))

Tariff B:

=IF(A1 > 250, 3 + (250 * 0.1) + ((A1 - 250) * 0.075), 3 + (A1 * 0.1))

Tariff C:

=A1*0.125

The formulas above were then used to construct the following chart:

Excel chart showing the cost of different amounts of units

Tariff C is the best choice for less than 120 units. Tariff B is the best choice for 120 to 190 units. Tariff A is the best choice for more than 190 units.

Log

Auguest 7, 2005 - Added solution 1 and 2.