What does it cost to run a heat pump?

Introduction

Alternative energy is a relatively new and fascinating field in South Africa, when comparing to Europe and first world countries. It is common experience with any new technology, for instance with solar and heat pumps, that it is difficult to make a solid financial decision when investing in such technologies. To give an example, Carte Blanche, (South African television program) had a program, this year of 2010, comparing solar and heat pumps and tried to make an educated conclusion on which would be the better technology and a better investment purely from a cost point of view. Regardless of the outcome of the program and which hailed the superior technology, there is another point that must be stressed.

Why is it, that five years later, after the introduction of solar and heat pumps into this country, there are programs, like Carte Blanche, still raising questions about the efficiency and payback of these technologies? This observation must mean that customers are purchasing alternative technologies based only on the fact that, “Alternative technology will save energy”, and that, “it’s good for the environment”.

The specific questions that should be raised are; “How much energy will it save? How much will it save for a family of two, three or four? How much will it save if I have water saving shower heads installed? These are some realistic questions posed by homeowners but without quoting statistics and random percentages, what exactly, “IN RANDS AND CENTS” will I save every month if I make the investment?

Note to the reader: There are definitely many ways of calculating the energy usage of any alternative energy system on the market. Even solar can be calculated using historical data, weather patterns, and other energy calculations. Heat pump efficiency can be calculated by calculating coefficient of performance, insulation coefficients and many more fancy tools available to the average engineer or technical person but at the end of the day, there is nothing more certain than an energy meter like the one in your electricity board, and a water flow meter to give you final, accurate results with no speculations or assumptions.

Getting down to business

The following article is going to measure the water and energy used on a domestic 300L heat pump. The heat pump in question is a German machine consisting of a tank and heat pump in one unit. It has higher efficiency than split units. (500 watt to heat 300L) See figure 1. The experiment was conducted in the following manner:

Experimental procedure

A cold water flow meter was connected to the heat pump, “cold inlet”. This will in turn measure the hot water leaving the heat pump only. The heat pump works like a geyser; the hot water leaving the top of the heat pump is replaced with cold water entering the bottom. The flow meter was purchased from a local plumbing supplier and is the same meter supplied to the municipality for measuring your total water supply to your home.
The energy meter, purchased from a local electrical supplier, measures the energy used by the heat pump. No other electrical appliances were connected to this unit. It was exclusively allocated to measuring the heat pump energy consumption.
The test was carried out from the 22 March 2007 till 31st of October 2007 and a final reading was taken August of this year to determine a daily average.
An average monthly result for the water and electricity consumed: (Remember that this heat pump services a family of four with two bathrooms, each having one shower).

 If the monthly electricity cost is displayed in a graph format at the current electric unit price, the following result can be seen in Graph 1:

 Graph 1: Monthly cost of running the heat pump

Notice that the monthly hot water electricity bill for a family of four is R86.64. This is a staggeringly low amount! Remember that this amount is charged at 1.01 Cents per electric unit for 2010. If your unit price is higher or lower, use the electricity consumption in Table 1 and multiply it with your unit price.
 

Electricity bill average over three years

The final calculation will consist of average electricity consumption over a three year period from 2007 till 2010. For the sake of this report the electrical calculations were left out for simplicity. If you require the calculations please download the full report, “How much electricity is the heat pump going to save? – full report”

 The average electricity consumption over the past three years is plotted in Graph 2:

 Graph 2 The electricity consumption of the heat pump over the past three years with an average of 82 electric units.

So, for a family of four, showering once a day every day, the electricity consumption is 82 units per month. To determine what it would cost in your area, multiply the 82 units with your unit cost price to see the final amount in Rands and cents. For example, in George in the Western Cape, the unit cost price from July 2010 is R1,01/kWh or R1,01/electric unit. Therefore: R1,01 x 82 kWh = R 82.82 for a family of four for one month.

The results are staggering. It is no wonder that heat pumps are a viable proposition for the homeowner.

For interest sake, the heat pump electrical consumption can be reduced even further by connecting a glycol solar panel to the heat pump described in the article “the results are in”