Daily Energy Need (Wh) How much energy you want solar to replace each day. If you want it to fully cover daily consumption, use the same number from your battery sizing.

Peak Sun Hours per Day "Peak sun hours" is daily kWh/m² — not total daylight. Look up your location at Global Solar Atlas for accuracy.

System Losses Real-world panels never deliver nameplate. Marine installs lose about 25% to MPPT efficiency, panel temperature, mast/rigging shadows, salt spray, and panel angle.

Charge Controller

How to Use This

Enter daily Wh need. Same number you used for battery sizing. If you want solar to fully recharge the bank each day, that's the answer. If you're okay running the engine or generator part of the time, use a smaller number.

Peak sun hours, not daylight hours. This is the most-misunderstood input. "Peak sun hours" means the equivalent number of hours of full 1000 W/m² sun. Caribbean noon is full sun. Caribbean 8 AM is maybe 30% of full sun. The total energy delivered across the day is rolled up into "peak sun hours." A summer day in the tropics gives 5–6 peak sun hours, even though there are 13 hours of daylight.

Losses are real and big. Marine installs lose 20–30% between nameplate and what hits the battery: MPPT controller efficiency (~95%), panel temperature derating (panels get hot, output drops), shading from rigging or biminis, salt and dirt on glass, and panel angle (most marine panels are mounted flat, missing 10–15% vs optimum tilt). The 25% default is realistic.

MPPT vs PWM. Use MPPT. The 30% efficiency hit from PWM is rarely worth the small cost savings. Victron SmartSolar MPPT controllers are the gold standard.

Real-World Example

On Yoto we have ~2,600W of panels in the tropics. Math:

2,600W × 5 sun hours × 75% efficiency = 9,750 Wh/day
We typically see 7,500–10,000 Wh/day in clean conditions, dropping to 3,000–5,000 Wh on cloudy days

That covers our typical 6,000–8,000 Wh daily consumption with surplus most of the time. On heavy cloud days we either run the watermaker only when the sun's out, or top up with the engine alternators.

This calculator's defaults are calibrated to roughly match what we actually see.

What This Doesn't Account For

Latitude and seasonal variation. Peak sun hours change by month and location. The selector is a rough average. For accuracy, look up your specific location on Global Solar Atlas and pick the worst-case month for your usage pattern.

Shading patterns. A panel is only as good as its worst-shaded cell on most strings. If you have rigging shadows that move across panels through the day, model the worst case, not the average.

Heat derating in detail. Panels get derated about 0.4% per °C above 25°C cell temperature. In tropical summer, panel surface temperature can hit 60–70°C. That's another 15–18% loss baked into the typical 25% loss factor.

Cloudy-day reality. This calculator assumes the average sun day. Your week of overcast might deliver 30% of expected output. Size your battery for the autonomy days; size your solar for the average day.

Common Mistakes

Confusing daylight hours with peak sun hours. "13 hours of sunlight" doesn't mean 13 peak sun hours. The actual number is closer to 4–6 even in the tropics.

Trusting nameplate. A "400W panel" delivers 400W only at standard test conditions: 25°C, 1000 W/m², no losses, perpendicular sun. You will not see 400W in real use. The calculator's loss factor accounts for this.

Skipping the MPPT. People save $80 buying a PWM controller and lose 30% of their solar output for the life of the system. Pay for MPPT.

Sizing solar to fully charge a too-big bank. If you have 10,000 Wh of solar capacity per day but a 25,000 Wh bank, you'll never fully recharge during the days you actually need it. Size battery and solar together — solar should be capable of fully replenishing the bank in 1–2 days of normal sun.