Solar sizing
Marine Solar Calculator
Plan a marine solar array that actually keeps up with your loads. The calculator combines panel watts, peak sun hours, MPPT efficiency and realistic marine derating to estimate the daily Wh you'll harvest — and how it feeds your battery bank.
How marine solar differs from rooftop solar
Boats heel, sails throw shadows, panels bake at 60°C, salt dulls glass, and you almost never get a clean tilt to the sun. That's why a 400Wp array on a boat sustains around 220W, not 400W. Marine Power Designer applies a sustained-output factor (default 0.55) and a diversity factor when combined with alternator or generator charging.
What the calculator returns
Recommended array size in Wp, estimated daily harvest in Wh, peak charging current into your bank, and a chemistry-aware check that your batteries can absorb the current comfortably (not above the conservative C-rate for LiFePO4, AGM, Gel or Flooded).
Inputs
- Daily energy budget (Wh or kWh)
- Peak sun hours for your cruising area (typical: 5h tropics, 3.5h Med, 2h NW Europe)
- System voltage and battery chemistry
- Other charging sources running in parallel (alternator, shore, generator)
Frequently asked questions
How many watts of solar do I need on my boat?
A rough rule: required Wp ≈ daily Wh / (peak sun hours × 0.6). The 0.6 factor accounts for marine derating — shading from rigging, heel angle, dirt, heat and panel ageing. Marine Power Designer models this explicitly.
Will solar alone keep my batteries full?
In the tropics, often yes for cruising loads up to ~3kWh/day with 600–800Wp on a mid-size cruiser. In higher latitudes or winter, plan for a backup (alternator, generator, shore).
MPPT or PWM controller?
MPPT is the default for any serious install — it recovers 15–30% more energy, especially in partial shade or cold conditions. PWM only makes sense for tiny trickle-charge setups.
Why is realistic solar output so much lower than panel rating?
Marine panels rarely hit STC conditions. Between heat derating, partial shading, oblique sun angle and panel ageing, sustained output is around 55% of rated peak — the calculator uses this as the default sustained factor.
Ready to size your system?
Marine Power Designer runs in your browser. No sign-up needed to try the guided wizard.
Open the calculator