Solar is one of the best upgrades for a sailing yacht, dayboat or fishing boat on a mooring. It keeps the batteries topped up between trips, runs instruments and lights at anchor, and saves you motoring just to charge. The key is to size it for what you actually use, choose a panel that suits where it will go, and use a proper MPPT controller. This guide is for sea and estuary boats - if you live on the canals, see our narrowboat solar guide.
What a boat actually needs
| Load | Typical draw | Daily energy (example) |
|---|---|---|
| Bilge pump (automatic, occasional) | 3-5A when running | Small, but must never go flat |
| Chartplotter and instruments | 1-3A | ~15-40Ah on a sailing day |
| VHF radio (listening) | ~0.5A | ~5Ah |
| LED navigation and anchor lights | ~0.5-1A | ~5-10Ah overnight |
| Fridge (if fitted) | 3-5A cycling | ~30-50Ah |
| Phone and tablet charging | — | ~5Ah |
Two very different situations come out of that table. A boat on a mooring between weekends mainly needs its battery kept healthy and the bilge pump always able to run - a small panel does that. A boat that cruises for days, with a fridge and instruments on, needs a much bigger system, and is where solar really earns its keep.
Sizing the panel
| Use | Panel size | Notes |
|---|---|---|
| Mooring maintenance, small battery | 10-20W | Keeps the battery topped up, runs the bilge pump |
| Weekend sailing, instruments and lights | 50-100W | Replaces what a day on the water uses |
| Cruising with a fridge | 150-300W+ | Space is the limit - more panels, or wind as well |
As a rough guide, a UK panel produces about 4 times its rated wattage in watt-hours on a good summer day, and much less in winter or when the sails shade it. A 100W panel might give 30Ah on a sunny June day at 12V, but a fraction of that on a grey October one. Size for the season you actually sail.
Flexible or rigid?
Semi-flexible panels are light, low-profile and can be stuck or screwed to a curved deck, a coachroof or a sprayhood. They are the usual choice on yachts where there is no room for a rigid frame. The trade-off is that they run hotter and generally don't last as long as glass panels, and you shouldn't walk on them. Rigid panels last longer and cope better with heat, but need a mounting point such as a stern arch, a guardrail bracket or a frame over the cockpit.
The charge controller
Use an MPPT controller, not the cheap PWM sort. MPPT controllers get more out of a panel in poor or partly shaded light - which on a boat, with sails, booms and rigging casting shadows, is most of the time. Bluetooth models let you check the battery from your phone. If you have a lithium battery, make sure the controller has a lithium charging profile. For more on controllers and batteries, see our solar accessories guide.
Fitting it on a boat
- Keep the panel clear of shade as far as you can - even a shroud shadow cuts output noticeably
- Seal every deck penetration with a proper cable entry gland and marine sealant
- Fuse the battery cable close to the battery, and use tinned marine cable where you can
- Protect the bilge pump circuit - it should always be able to run, even if other loads are switched off
- Check your insurer and marina rules if you're making permanent changes
Recommended kit
Everything below was in stock on Amazon UK when this guide was written.
Renogy 100W extremely flexible solar panel
A light, low-profile panel that can go on a coachroof, deck or sprayhood where a rigid frame won't fit. The ETFE surface is tougher than cheaper PET-coated panels. Rated 4.3 stars from over 900 Amazon UK reviews. Don't stand on it, and allow some airflow underneath if you can.
amazon Check Price on Amazon →Renogy 50W flexible solar panel
Enough to keep a mooring battery healthy and cover a day of instruments and lights on a smaller boat. Rated 4.3 stars from 288 reviews.
amazon Check Price on Amazon →Victron SmartSolar MPPT 75/15
The controller most UK boat owners end up with. It's small, efficient, has built-in Bluetooth so you can check charging from your phone, and supports lithium batteries. Rated 4.8 stars from over 5,600 reviews. For larger arrays, the 100/20 version (~£66) handles more panel power.
amazon Check Price on Amazon →DCHOUSE 12V 100Ah LiFePO4
Lithium is about half the weight of an equivalent lead-acid battery and can be used much more deeply, which matters on a boat. Low-temperature protection stops it charging below freezing over winter. Rated 4.6 stars from 338 reviews. Check your charging system, including the engine alternator, is suitable for lithium before switching.
amazon Check Price on Amazon →QWORK IP68 solar cable entry gland (2 pack)
A proper cable entry gland keeps water out where the panel cable passes through the deck. Bed it on marine sealant. Rated 4.7 stars from over 1,400 reviews.
amazon Check Price on Amazon →Summary
- A 10-20W panel keeps a moored boat's battery and bilge pump healthy
- 50-100W covers a sailing day of instruments and lights
- Semi-flexible panels suit decks and sprayhoods; rigid panels last longer on an arch
- Always use an MPPT controller - boats are full of shadows
- Seal deck penetrations and fuse the battery cable
For a small trickle charger to keep a dayboat or outboard battery topped up on its mooring, see our solar trickle charger guide. For the canals, see narrowboat solar panels.
Frequently Asked Questions
What size solar panel do I need for my boat?
For keeping a battery topped up on a mooring, 10-20W. For a day of instruments and lights, 50-100W. For cruising with a fridge, 150W or more, often with wind or engine charging as well.
Are flexible solar panels good on boats?
Yes, where there is no room for a rigid frame. They are light and low-profile but run hotter and usually do not last as long as glass panels, and you should not walk on them.
Do I need a charge controller on a boat?
Yes, for anything above a small trickle panel. An MPPT controller gets more from a panel in shade and poor light, which suits boats with sails and rigging.
Can solar keep my bilge pump running on the mooring?
Yes. A small panel keeping the battery topped up means the automatic bilge pump can always run. Make sure the bilge pump circuit cannot be switched off with other loads.
Can I use a lithium battery on a boat?
Many boat owners do, for the weight saving and deeper use. Check your charging system, including the engine alternator, is suitable, and choose a battery with low-temperature charge protection.