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Contents
A solar panel is the quietest and cheapest way to have electricity aboard: it tops up your batteries at the dock, lets you spend days at anchor without starting the engine, and extends battery life by keeping them full. But to get it right you need to do the math. This guide shows you how to work out your 12V power budget, how many watts to install, how to choose between rigid and flexible panels, how to size the charge controller and how to wire it without losses or risks.
Quick summary
Work out your daily amp-hours, install enough panel to replace them in the season you boat the most, use a quality MPPT charge controller, and mount rigid panels whenever you have a radar arch or hard top to fix them to. Otherwise, use well-ventilated flexible panels. And don't forget the fuse between the controller and the battery.
Step 1: work out your 12V power budget
It all starts with knowing how much you use. Multiply each device's current draw in amps by its daily hours of use and add it up. The result, in amp-hours (Ah) per day, is what the panels have to put back. Typical values for a recreational boat:
| 12V device | Current draw | Hours per day | Ah per day |
|---|---|---|---|
| 40 L (about 1.4 cu ft) compressor fridge (summer) | Average 1.5 to 2 A | 24 | 30 to 45 |
| Tiller autopilot | 1 to 3 A | 6 | 6 to 18 |
| 7-inch chartplotter or fishfinder | 1 A | 6 | 6 |
| Fixed VHF on standby | 0.4 A | 8 | 3 |
| LED anchor light | 0.15 A | 10 | 1.5 |
| Interior LED lighting (4 fixtures) | 1 A | 3 | 3 |
| Pressure water pump | 5 A | 0.3 | 1.5 |
| 12V fan | 0.5 A | 8 | 4 |
| Phone and tablet charging | 2 A | 3 | 6 |
| Example total (weekend sailboat) | 60 to 90 |
The fridge is almost always the biggest load: choosing a good compressor cooler or fridge and ventilating it properly saves more energy than any extra panel. Lighting works the same way: switching to interior LEDs and LED navigation lights cuts consumption to a fraction.
Step 2: how many watts of panel to install
Output depends on peak sun hours (roughly 5 to 6 in summer along sunny coasts and 2 to 3 in winter) and on real losses aboard: nearly flat panels, shade from rigging and boom, heat and wiring. With a realistic loss factor of 25-30%, each installed wattage produces about:
| Installed power | Summer (Ah/day) | Spring and fall (Ah/day) | Winter (Ah/day) |
|---|---|---|---|
| 50 W | 12 to 17 | 8 to 12 | 5 to 7 |
| 100 W | 25 to 35 | 16 to 24 | 10 to 15 |
| 200 W | 50 to 70 | 32 to 48 | 20 to 30 |
| 300 W | 75 to 105 | 48 to 72 | 30 to 45 |
| 400 W | 100 to 140 | 64 to 96 | 40 to 60 |
With that, the most common setups look like this:
- Powerboat at the dock or day anchoring: 20 to 50 W to keep the battery topped up.
- Weekend sailboat with a fridge: 150 to 200 W.
- Cruising sailboat living at anchor: 300 to 500 W and lithium batteries.
Your battery bank should store at least a day and a half of use. With lead-acid or AGM you should only use half the capacity; with lithium batteries (LiFePO4) you can use 80-90%, which in practice doubles usable capacity in the same space.
Step 3: rigid, semi-flexible or flexible panels
| Type | Pros | Cons | Where it goes |
|---|---|---|---|
| Rigid (glass and aluminum frame) | Most durable, best heat dissipation, cheapest per watt | Heavier; needs a structure | Radar arch, hard top, rails, bow pulpit |
| Semi-flexible ETFE | Light, follows gentle curves, stands up well to weather | Runs hot; shorter life than rigid | Cabin top, bimini, dodger |
| Budget flexible PET | Very cheap and light | Degrades sooner in sun and salt | Occasional or portable use |
The installation matters as much as the panel: a rigid panel well mounted on an arch with stainless hardware will last many years, while a flexible one glued straight to the deck, with no ventilation, loses output to heat. If you go flexible, leave a small air gap or fix it with grommets on the bimini fabric. See the options for boat solar panels.
Step 4: the MPPT charge controller
The controller protects the battery from overcharge and applies the right charge stages (bulk, absorption and float). An MPPT constantly tracks the panel's maximum power point and converts excess voltage into charging current, typically yielding 10 to 30% more than a PWM. In the naming used by several makers, a 75/15 accepts up to 75 V from the panel and delivers up to 15 A to the battery.
| Panel power (12V battery) | Approx. current | Recommended controller |
|---|---|---|
| Up to 130 W | Up to 10 A | MPPT 75/10 |
| Up to 200 W | Up to 15 A | MPPT 75/15 or 100/15 |
| Up to 290 W | Up to 20 A | MPPT 100/20 |
| Up to 440 W | Up to 30 A | MPPT 100/30 |
| Up to 700 W | Up to 50 A | MPPT 100/50 |
If you wire panels in series, add up their open-circuit voltages (Voc) and leave a 10-15% margin for cold mornings: two panels at 22 V Voc add up to 44 V, fine for a 75 V controller. On sailboats with shade from the boom or rigging, wire panels in parallel or use one controller per panel. Models with Bluetooth let you check production from your phone.
Step 5: wiring, fuses and mounting
Voltage drop between panel, controller and battery should stay under 3%. Wire size is calculated as S = 2 × L × I × 0.0175 / ΔV (S in mm², L the one-way run in meters, I in amps and ΔV the allowed drop in volts). For 12V and 3%, ΔV = 0.36 V. Sizes below are given in metric with approximate AWG equivalents (4 mm² is about 12 AWG, 6 mm² about 10 AWG, 10 mm² about 8 AWG, 16 mm² about 6 AWG, 25 mm² about 4 AWG):
| Current | 10 ft run (3 m) | 16 ft run (5 m) | 26 ft run (8 m) |
|---|---|---|---|
| 10 A | 12 AWG | 10 AWG | 8 AWG |
| 15 A | 10 AWG | 8 AWG | 6 AWG |
| 20 A | 10 AWG | 8 AWG | 6 AWG |
| 30 A | 8 AWG | 6 AWG | 4 AWG |
Between the panel and the controller the voltage is higher (especially with panels in series), so the current is lower and the wire can be thinner. Between the controller and the battery all the charge current flows: keep that run short and thick.
- Use tinned marine-grade copper wire and terminals crimped with a quality crimping tool, never just soldered.
- Fit a fuse between the controller and the battery, as close as possible to the positive terminal.
- Run cables through watertight glands and protect MC4 connectors and terminals with corrosion inhibitor.
- Connect the battery to the controller first, then the panels. To disconnect, reverse the order.
- Install a battery monitor (shunt) to know how much is really going in and out.
Warning: never connect a panel straight to a battery without a controller. Overcharging destroys lead-acid and lithium batteries and can cause a fire.
Lithium, chargers and shore power: how they fit
If you run lithium, make sure the controller has a LiFePO4 profile or allows you to set voltages (absorption around 14.2-14.4 V and a low float). Some batteries won't charge below freezing: in cold climates look for models with built-in protection or a temperature sensor. At the dock, a good battery charger with the right profile complements solar when there are several cloudy days.
Common mistakes
- Sizing without doing a power budget and falling short in winter.
- Wiring all panels in series with shade from the boom or rigging: one shaded panel drags down the lot.
- Gluing a flexible panel straight to the deck with no ventilation.
- Using thin wire or long runs between controller and battery, wasting energy as heat.
- Forgetting the fuse, or fitting it far from the battery.
- Skimping on hardware: use proper stainless steel on any mount exposed to spray.
If you're fitting out a whole sailboat, this guide goes hand in hand with our list of sailboat equipment.
Recommended gear for this guide
Frequently asked questions
How many watts of solar panel does a boat need?
It depends on your daily use. As a reference, each 100 W of well-aimed panel produces about 25 to 35 Ah per day at 12V on a sunny summer day. A small boat that only keeps the battery topped up needs 20 to 50 W; a weekend sailboat with a fridge, 150 to 200 W; and a cruiser living at anchor, 300 to 500 W.
Rigid or flexible solar panels for a boat?
Rigid panels, with an aluminum frame and glass, last longer and keep their output better; they're the best choice if you have an arch or hard top to mount them on. Flexible panels fit curved surfaces and canvas biminis and weigh very little, but they run hotter and have a shorter life.
Do I need an MPPT controller, or is PWM enough?
A cheap PWM works with nominal 12V panels, but wastes part of the energy. An MPPT uses the panel's extra voltage and usually delivers 10 to 30% more, especially in cold weather, changing light or with panels in series. On a boat, where every watt counts, we recommend MPPT.
What MPPT controller do I need for 200 W?
Divide the power by battery voltage: 200 W over 12 V is about 17 A, so a 20 A controller is the right size. Also check that the open-circuit voltage of the panels, added in series with a margin for cold, doesn't exceed the controller's maximum.
How does shade affect a solar panel on a sailboat?
A lot. The shadow of the boom, a shroud or the rigging over just a few cells can cut a panel's output in half or more. That's why it's better to install several panels with independent controllers or in parallel, rather than several in series that all get hit at once.