Energy Unlimited by Victron

Assuming a CoP of 2.5, 2 kW of cooling power will require 2 / 2.5 = 0.8 kW of electric power, which would, in 10 hours time, draw 0.8 x 1000 x 10 / 24 = 333 Ah from a 24 V battery.

6.4. Electric winches, windlass and bow thruster

More and more common, even on smaller boats, these products will draw very high currents, but for a short period.

- An electric winch or windlass on a 15 m boat is in general powered by a 1 horsepower motor (1 HP = 0.736 kW) and will draw at nominal load 736 / 12 = 61 A from a 12 V battery (current draw can increase to several hundreds Amps if the winch is under a near stalling load!). If operated for 1 minute, the Ah consumption will be 61 / 60 = 1 Ah (see sect. 6.2). So energy consumption is not the issue, but it is very important to properly dimension the fuse, contactors, cabling, and batteries to withstand the high currents and eliminate the risk of fire due to overheating. - A bow thruster will often take even more power, for example 300 A from a 24 V battery if fitted with a 10 HP motor. Current draw will be 10 x 736 / 24 = 300 A. One minute of operation will result in 300 / 60 = 5 Ah taken from the battery. A washing cycle at 60 °C with a standard household washing machine takes 0.9 kWh of electric energy, or 900 / 24 = 38 Ah from a 24 V battery. At 40°C this reduces to 0.6 kWh or 600 / 24 = 25 Ah from a 24 V battery. The energy required for dishwashing is of the same order of magnitude. Most of the energy goes into heating the water (hence the large difference in energy consumption between a 60 °C cycle and a 40 °C cycle), and using hot fill (supplying the washing machine and dishwasher with water at the right temperature instead of cold water) would further reduce energy consumption to a few hundred Wh! A standard household dryer, though, takes 3 kWh, which means 3000 / 24 = 125 Ah from a 24 V battery. This is because preheated air is used to evaporate all the remaining moisture. And I do not know of any dryer heating the air with a hot water heat exchanger instead of an electric heater…

6.5. A battery powered washing machine and dishwasher?

A wash-dry cycle of a small washer-dryer as is often used on boats will take approx. 2.7 kWh.

6.6. Ever thought that electric cooking on battery power was feasible?

I didn’t, until I made the calculations and verified in practice.

And since that time I have a two-hob electric induction stove on my trimaran, powered by a 24 V 200 Ah house battery and a 2.5 kW Multi.

When compared to other electric stove, my preference goes to induction. With electric induction it is not the hob that is heated, but the bottom of the pan directly. The heating is therefore extremely fast and the hob does not become hotter than the bottom of the pan, which increases safety. For that reason electric induction is also 20 % more efficient than other electric stoves (this is not just theory, I have measured it). But now the theoretical background, which is very simple: As stated in section 6.3.3, the heat capacity of water is 1.16 Wh per °C. Bringing 1 litre of water of 20 °C to the boil would therefore take 1.16 x (100 – 20) = 93 Wh. In practice it takes more than 100 Wh, depending on the heat capacity of the pan and other losses, which can be reduced by starting with warm water from the boiler instead. So the figure to remember is 100 Wh per litre.