How much electricity do you need?

To calculate this you need to go through:

• what draws electricity on your boat
• how much electricity each thing typically consumes
• which of these do you use while the engine isn't running, i.e. what is the "hotel load"

I approach this by assuming I use everything onboard as I would over the course of 24 hours while at anchor.

This gives me a theoretical maximum consumption.

To avoid having to alter my behaviour to conserve power, I use this maximum consumption to size my house battery bank.

In reality, my actual power consumption will be less - for example, on a hot summer day the heater won't be used at all. Conversely, on a cool day, the fridge and freezer won't have to work as hard.

I don't bother calculating these different scenarios in detail - sizing the batteries based on maximum consumption gives me a comfortable margin.

Hotel load on a Swift Trawler 37

Our boat has a gas galley (as opposed to electric) and no air conditioning.

Here is an example of estimated power consumption over 24 hours:

This sums up to:

• a minimum of about 90 Ah with the fridges running when not on the boat
• about 140 Ah if spending time onboard listening to music, watching TV etc, without using 220V appliances like dishwasher and coffeemaker
• around 200 Ah when using all amenities

Dimensioning the house battery bank

We went with lithium (LiFePO4) for the house battery bank.

We want full authonomy for about two days at anchor without needing to conserve power.

This means we need up to about 400 Ah to power everything onboard.

Assuming we never go below 10% charge in the batteries, that means we need a house battery bank of about 450 Ah in total.

That will give us two days of autonomy with all the bells and whistles for two days even if the sun doesn't shine one bit on our solar cells.

To extend this further, we can add additional batteries to the house bank and/or add a generator or fuel cell.