10.5. The extra’s
From the extra’s mentioned in section 9.5 of the previous chapter, the electronic navigation system, SSB and radar have been included in the list of section 10.2. In addition to the other extra’s mentioned in 9.5 there are a few more to consider, now that we are looking at bigger boats.
10.5.1. Hot water kettle
Very convenient for boiling water. The heat capacity of 1 litre of water is 1.16 Wh per °C, see sect. 6.6. So bringing 1 litre of water to the boil will take approx. 100 Wh, i.e. 4.2 Ah from a 24 V battery.
10.5.2. Electric stove
Gas on board is hazardous and lugging gas bottles about is no pleasant task. With 2 electric hobs of 2 kW each the peak power required will be 4 kW (i.e. almost 200 A at 24 V), 4 hobs will require a maximum of 6 to 8 kW. Cooking a meal for 4 people takes approx. 1.2 kWh, i.e. 50 Ah (see sect. 6.6).
10.5.3. Small washing machine
This has been touched on in sect. 6.5. The energy consumption for a wash-dry cycle is about 2.7 kWh.
Most of the electricity needed is for heating the water and for the drying cycle. Hot fill (= supplying the washing machine with hot instead of cold water) and no drying cycle would reduce energy consumption to about 0.5 kWh.
10.5.4. Small dishwasher
Consumption approx. 1 kWh Hot fill would reduce energy consumption to less than 0.5 kWh.
We will now calculate the total consumption per 24-hour period if all this luxury would be taken on board, assuming a crew of 4 and a tropical climate, so that instead of the heater, the air conditioning is used. In our example the air conditioning is switched on for 12 hours per day and the compressor runs with an average duty cycle of 50 %. Similarly a high-pressure pump type water maker is used instead of the much more energy efficient DC water-hydraulic type. We also assume that the washing machine and dishwasher are operated without hot fill.
Consumers
Consumption
Time / 24-hours
Consumption / 24-hour period
Watt 1500 2000 6000
Amp
Hours
kWh Ah (24 V)
Microwave oven
0.25
0.4 0.6 1.2
16 25 50 (9) 60 56 42 10
Kettle, 6 litres per day Electric cooker, 4 persons
Heater
3
6 x 0.5 = 3 12 x 0.5 = 6
Air conditioning, cooling capacity 4 kW
1400
8.4 1.4
350
Water maker, 200 litres per day
Small washing machine, once every 2 days
2000 2000
0.5 x 2.7
Small dishwasher, daily
1.0
Miscellaneous
Consumption per 24-hour period
25
609
The table shows that air conditioning is an especially power hungry consumer, although we have assumed only 12 hours of operation per day. With 8.4 kWh of energy needed per day, which would translate into 350 Ah per day if running (with an inverter) from the battery, air conditioning requires more energy than all other equipment together! A 1500 Ah battery would be needed to achieve a generator free period of 20 hours. Although such installations do exist, the more common solution is to run a generator whenever the airco is on, and to cope with the noise, maintenance and fuel consumption.
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