Systems & how-tos
Solar panel sizing for cruising yachts
How to size solar for Nordic and Mediterranean cruising — daily Ah budget, panel watts, controller choice, and realistic Baltic yields.
Overview
Solar panels recharge house batteries at anchor without running the engine. Sizing starts with daily consumption in amp-hours (Ah), not with "how much panel fits on the arch".
Build your load model in electrical budget at anchor before buying hardware.
Sizing steps
- List loads — fridge, instruments, autopilot at anchor, phones, inverter if used
- Sum Ah per day — include inverter losses (×1.15) and cloudy-day margin (×1.3)
- Match harvest — in Nordic summer expect 3–4 peak sun hours effective; Mediterranean 5–6
- Panel watts ≈ (Ah × 12.8 V) ÷ sun hours ÷ controller efficiency (~0.85)
Example: 80 Ah/day need in the Baltic ≈ (80 × 12.8) ÷ 3.5 ÷ 0.85 ≈ 340 W — often two panels plus alternator backup.
Installation choices
| Choice | Trade-off |
|---|---|
| Rigid glass | Best €/W; walkable decks need careful mounting |
| Semi-flexible | Easy on bimini; heat and shading reduce life |
| MPPT controller | Worth it above ~200 W — better partial shade recovery |
| Series vs parallel | Series raises voltage for MPPT; one shaded cell hurts — use bypass diodes |
Baltic reality
June 18 h daylight does not mean 18 h full charge — low sun angle, bird dirt, and mast shadow cut yield. Budget engine or shore charge every third day if you run a large fridge.
Pair solar with battery chemistry choice — see AGM vs lithium.
FAQ
Is 100 W enough?
For LED lights and phones yes; for continuous fridge + instruments on a 10 m cruiser, rarely.
Next steps
Model loads in energy budget at anchor or book advisory for a refit plan.