After spending way too much time figuring out how to size the power system for our boat, I thought it would be good to create a simple guide to estimating your needs. It is an excel file you can download/modify with your own sailing or RV needs. I ran through this math with our teardrop trailer, and now with the more complicated sailboat. This whole file is set up for a 12-volt system, you may need to make some modifications to this file for different voltages.
Start at the top of the document and fill out the yellow boxes. The file The results will show up at the bottom.
Think of your system as the combination of three different calculations:
- Energy generation from solar, wind turbine, generator, and/or alternator
- Energy storage to/from the battery bank
- Energy draw to the system demands
Each system needs to be sized appropriately for your application, which starts with the demand you expect to have in your system. Start by listing all of the loads that will be on your system, and classify them in different ways: on anchor vs. on passage and define the minimum critical requirements. Then figure out how many amps and how many hours each draw will take on a given day – either on passage or at anchor. For example, we don’t need to run our autopilot while at anchor, and it’s one of the biggest amp draws we have on our boat.
Once you understand your overall system needs, you can play with sizing your solar panels and battery bank. This is the generation side of the equation. You want to make sure you’re accounting for usable sunlight, clouds, and a safety factor when making your assumptions. You also want to make sure you’ve got the ability to weather a few days of low (or zero) power generation. This all gets calculated at the bottom of the excel file where it shows the final calculations of how long you’d be able to last (theoretically) in each scenario, and what your excess/deficit would be.
What do you think? I’d love to get some feedback on the file/calculations.
3 replies on “Simple Guide for Calculating Your Sailboat’s Energy Budget”
Hey guys , lovely to read your blog regarding your ericson. I too own a 39b here in New Zealand and slowly going through a Refit. My understanding is only 19 to 20 of these were built. yours looks in great condition. regards Kyle
Thanks for the comment! Great to hear about other 39B’s out there! Overall it is in great condition, however we do have some wet spots we’re currently re-coring in the cockpit & foredeck. We’d like to hear more about your refit – we’re pretty active on Instagram @gerbersunderway if you want to message us there
Only just noticed your reply, sorry we don’t have Instagram but will try looking you up. regards Kyle