Energy Unlimited by Victron

6.Electric equipment and energy consumption

6.1. Introduction

Now that we know, more or less, how to charge batteries, it is time to discuss the consumers, which will discharge the batteries. In order to better understand the impact on energy consumption of the different consumers on board, it is advisable to think in 3 categories:

- Continuous consumers , which could for example, be the standby power taken by the VHF or the SSB, the refrigerator and the freezer.

- Long duration consumers (navigation lights, autopilot, cabin lighting, water maker, air conditioning) that need power from between one hour to several hours a day.

- Short duration consumers (pumps, electric winches, bow thruster, microwave, washing machine, dishwasher, electric stove) that need power for between a few seconds up to, say, one hour per day.

In my experience everybody, myself included, tends to underestimate the daily energy consumption of continuous and long duration consumers and to overestimate energy consumption of short duration consumers.

6.2. Power and energy

Especially when the source of electricity is a battery, it is important to differentiate between power and energy.

Power is instantaneous, it is energy per second, and is measured in Watts (W) or Kilowatts (1 kW = 1000 W).

Energy is power multiplied by time. A battery stores energy, not power. Low power but consumed over a long period can result in a lot of energy consumed and drain a battery. Power is measured in Watt-hours (Watts x hours, or Wh) or Kilowatt-hours (1 kWh = 1000 Wh).

Energy is also the product of battery capacity (Ampere-hours) and voltage: Wh = Ah x V and kWh = Ah x V x 1000.

So a power of 2 kW during 1 hour is 2 kW x 1 hour = 2 kWh of electric energy, and will drain 2 kWh / 12 V = 2000 Wh / 12 V = 167 Ah from a 12 V battery.

2 kW during 1 second (i. e. 1 / 3600 of an hour) amounts to (2000 / 3600) / 12 = 0.046 Ah. Next to nothing!

2 kW during 1 minute (i. e. 1 / 60 of an hour) amounts to (2000 / 60) / 12 = 2,7 Ah. A notebook battery would do this (if it could deliver very high currents)!

2 kW during 10 hours will drain 2000 x 10 / 12 = 1667 Ah. A huge battery!

As a preparation for the chapters to come, some examples of power and energy consumption of household appliances and other equipment are discussed in the next sections.