By Tom Burden
Before you start replacing batteries and adding solar panels, start by analyzing your energy requirements. There are two ways to calculate how much energy is used in a given time period (we recommend that you use 24 hours).
Cruising Sailors generally plan on being “engineless” for a day at a time, hoping to operate their boats with their engine running only one or two hours each day. This allows other engine-driven functions (refrigeration, watermaking) to be performed while charging batteries with the engine. Their battery storage requirements are 100% related to the time between engine-driven charges, plus the added input from other sources they utilize: solar, wind, etc.
Powerboaters generally have very modest battery capacity and charging needs until they anchor in some nice spot and want to be free of engine noise and exhaust odor. Under these circumstances, they operate in “sailboat mode,” discharging their batteries deeply and periodically recharging them.
Loads like inverters, refrigeration and lighting have a large impact on your energy budget, so it is difficult to estimate your needs without actually adding up the loads and duty cycles to find the total consumption. Most boaters who operate on battery power (no AC generator) end up using from 60-200 amp-hours (Ah) per day. Calculate your own needs using one of the two methods above.
A proven rule of thumb is to have 3-4 times your daily energy consumption in battery capacity. That means that boats that consume 120Ah per day of energy should consider having 360-480Ah of capacity. This rule allows you to avoid damaging deep discharges, and it reduces your recharging time.
Your alternator should be sized appropriately to product the highest amount of current your batteries can accept (their charge acceptance rate, which ranges from 25 to 40% of the battery bank’s total amp hour capacity). This gets the bank recharged as quickly as possible, minimizing the engine running time and the fuel needed to replenish the batteries. Thus, a boat with a 200Ah battery bank should be paired with an alternator that maxes-out at 50-80A.
|Extremely Approximate Battery Bank Size Rule|
|30' boat||200Ah house bank|
|35' boat||300Ah house bank|
|40' boat||400Ah house bank|
|45' boat||500Ah house bank|
|50' boat||600Ah house bank|