Boat Solar Calculator
Marine solar sizing — enter your navigation electronics, fridge, and anchor days.
How to Use This Calculator
Navigation electronics as the baseline
Unlike RVs, boats have a set of always-on electronics that consume power continuously at sea and at anchor. The VHF radio draws 6W in standby but 25W when transmitting. The chart plotter/MFD runs 8-12 hours per day. The autopilot is the wildcard — it can draw 30-150W depending on sea state. Enter average draw, not peak draw.
Marine fridge vs. standard fridge
A well-insulated marine fridge (Isotherm, Waeco, Engel) draws 30-55W on average. Poor insulation or warm ambient temperatures can double this. The 24-hour figure is the compressor cycle average, not constant draw — a 45W fridge uses about 1,080 Wh (90 Ah) per day.
Days at anchor
This is the most important battery sizing input. A boat that goes marina-to-marina needs 1-2 days of battery. An ocean cruiser anchoring for a week between ports needs 5-7 days. This drives the battery bank requirement far more than solar output.
Marine efficiency factor
The calculator uses 75% efficiency instead of the residential 86%. Marine installations suffer more losses: panels are often at flat angles (poor in early morning / late afternoon), covered in salt spray, and partially shaded by rigging or the boom. Over-spec your solar by 25% versus what the math suggests.
The Formula
Marine systems run at 12V (most boats under 45 ft) or 24V (larger vessels). The battery bank uses 50% depth of discharge assuming lead-acid AGM — the standard for marine use. If you use lithium (increasingly popular on bluewater boats), divide battery Ah by 1.6 to get equivalent lithium capacity needed.
Example
SV Meridian — 40' sloop, Caribbean cruise
Mark and Lisa are liveaboards planning a Caribbean circuit. They anchor most nights, run a Garmin plotter, Simrad autopilot, a Waeco fridge, and a watermaker. They want 5 days of battery autonomy.
Result
The Caribbean gets 5.5-6.5 peak sun hours — excellent for solar. A 570W arch-mounted array with two Victron 100/30 controllers can fully replenish the daily draw by early afternoon. The battery bank (six 300Ah AGM batteries) provides the needed 5-day reserve. Many bluewater sailors upgrade to lithium to cut weight by 60% and halve the bank size.