Article: How Many Solar Panels Do I Need for My Boat?
How Many Solar Panels Do I Need for My Boat?
"How much solar do I need?" is the question we get more than almost any other. And the honest answer is: it depends — but it's not complicated to figure out once you know the three numbers you're working with.
This guide walks through the math step by step, with real examples for the most common boat setups. By the end, you'll have a specific wattage target you can shop against.
The Formula (It's Simpler Than It Looks)
Sizing solar for a boat comes down to one equation:
Daily energy use (Wh) ÷ Peak Sun Hours ÷ 0.8 = Required panel watts
The 0.8 accounts for real-world system losses — heat, wiring resistance, charge controller efficiency, and the fact that panels rarely operate at their rated output in practice. It's a standard efficiency factor for marine solar sizing.
You need three inputs: how much energy your boat uses each day, how many peak sun hours you get in your typical cruising area, and the efficiency factor. Let's work through each one.
Step 1: Calculate Your Daily Energy Use
List every electrical load on your boat and how long it runs each day. Here are the most common ones:
| Load | Typical Draw | Hours/Day | Daily Use (Wh) |
|---|---|---|---|
| 12V refrigerator (marine) | 40–60W average | 24 | 960–1,440 |
| LED cabin lights (4 fixtures) | 20W total | 4 | 80 |
| VHF radio (standby) | 5W | 12 | 60 |
| Chartplotter / MFD | 25–40W | 6 | 150–240 |
| AIS transponder | 5W | 24 | 120 |
| Autopilot | 30–60W | 6 | 180–360 |
| Anchor light | 5W | 10 | 50 |
| Phone/tablet charging | 20W total | 2 | 40 |
| Water pump | 60W | 0.5 | 30 |
Typical totals:
- Weekend sailor, modest loads: 400–700 Wh/day
- Cruising sailboat, no A/C: 900–1,500 Wh/day
- Cruising sailboat with inverter use: 1,500–2,500 Wh/day
- Powerboat or liveaboard with A/C: 3,000–6,000+ Wh/day
Your fridge is almost always the dominant load. If you're unsure where to start, budget 1,200 Wh/day for a typical 30–40 foot cruiser without air conditioning — it's a reliable baseline.
Step 2: Find Your Peak Sun Hours
Peak Sun Hours (PSH) is not simply the number of hours of daylight. It's a measure of how much usable solar energy is available in a given location — standardized to how long you'd need to run panels at their rated output to collect that energy.
| Region | Peak Sun Hours |
|---|---|
| Pacific Northwest / British Columbia | 3.5–4.5 |
| California / Pacific Coast | 5–6 |
| Gulf Coast / Caribbean | 5–6.5 |
| East Coast (New England) | 4–5 |
| Florida / Bahamas | 5.5–6.5 |
| Pacific crossing / tropics | 5.5–7 |
Use the lower end if you're often in cloudy conditions or sailing in high latitudes. Use the higher end if you're doing a Caribbean season or Pacific crossing. When in doubt, use 5 — it's a reasonable middle-ground for most US coastal cruising.
Step 3: Run the Math
With daily energy use and peak sun hours in hand, the calculation is straightforward.
Example: Coastal cruising sailboat
Daily energy use: 1,200 Wh • Peak sun hours: 5
Required panel watts: 1,200 ÷ 5 ÷ 0.8 = 300W
Example: Liveaboard with inverter use
Daily energy use: 2,500 Wh • Peak sun hours: 5
Required panel watts: 2,500 ÷ 5 ÷ 0.8 = 625W
Example: Weekend sailor, minimal loads
Daily energy use: 500 Wh • Peak sun hours: 4.5
Required panel watts: 500 ÷ 4.5 ÷ 0.8 = 139W (round up to 150–200W)
Quick Reference: Solar by Boat Type
| Boat Type | Typical Loads | Recommended Solar |
|---|---|---|
| 25–32 ft sailboat, weekend use | Lights, chartplotter, VHF, no fridge | 100–200W |
| 32–40 ft cruiser, no A/C | Fridge, lights, instruments, autopilot | 250–400W |
| 40–50 ft cruiser, no A/C | Same + watermaker, bigger fridge | 400–600W |
| Liveaboard, occasional inverter | Full cruiser loads + laptop, cooking | 500–800W |
| Powerboat with A/C | High loads, short anchor time | 800W+ (supplement — solar alone rarely covers A/C) |
What Actually Limits Panel Output on a Boat
Rated panel wattage is measured under ideal laboratory conditions — full sun, perfect angle, 77°F. On a boat, you'll rarely hit that.
Shading. Even partial shading from a boom, mast, or antenna can cut output significantly. A single shaded cell in a series string drags down the whole panel. The 0.8 efficiency factor helps account for this, but layout matters.
Panel angle. Fixed flat panels on a deck won't always face the sun at the optimal angle. Tilting mounts recover 10–20% compared to flat deck installs. If you're installing fixed flat panels, bump your total wattage up slightly to compensate.
Heat. Solar panels lose roughly 0.4% of output for every degree Celsius above 25°C. On a dark-colored deck in the tropics, panels can run 20–30°C above air temperature — that's an 8–12% efficiency loss. Leaving an air gap beneath the panel helps.
Panel type. Monocrystalline panels are the standard recommendation for marine use — more efficient per square foot than polycrystalline, which matters when deck space is limited.
Don't Skip the Charge Controller
Your panels connect to your batteries through a charge controller — and the type matters. An MPPT (Maximum Power Point Tracking) controller extracts 15–30% more energy from your panels than a basic PWM controller, especially in variable sunlight. On a boat where conditions are always changing, that efficiency gain is real.
For any system over 100W, MPPT is the right call. Victron's SmartSolar MPPT line integrates with the Victron ecosystem, has Bluetooth monitoring built in, and is available in sizes from 75/15 up to 250/100 to match virtually any panel configuration.
Our Recommendations by System Size
100–200W (small boat, weekend use): One or two 100W rigid panels, Victron SmartSolar MPPT 75/15. Simple, handles a modest load profile without overcomplicating the system.
300–400W (most cruising sailboats): Two 200W panels or three 150W panels, Victron SmartSolar MPPT 100/30 or 100/50. This is the most common setup we spec for 35–42 foot cruisers. Covers daily loads with margin to spare in good sun.
500W+ (liveaboards, high-load boats): Multiple panels across deck and arch/rail mounts, Victron SmartSolar MPPT 150/70 or 150/100. At this level you'll also want a Cerbo GX for centralized monitoring so you can see exactly what's coming in and going out in real time.
Ready to Size Your System?
Browse the full solar panel collection and MPPT charge controllers at Blue Marine, or schedule a free system consultation with our ABYC-certified team. Solar sizing works best when you look at the whole picture — panel layout, battery bank size, existing charging sources, and how you actually use the boat — and we do that kind of sizing every day.
