How many watts of solar for a boat? Work it out properly, before you buy panels.
The honest answer to how many watts solar for a boat has nothing to do with the length of the hull. It comes from what you actually use in a day, and from how much usable sun Long Island really gives you — which is less than most sizing charts assume.
Below is the same worksheet we use before quoting anyone an array. Work through it and you will know roughly what you need. Get it wrong and you either pay for panels you cannot use, or you spend real money on a system that still leaves you starting the engine to charge.
Set Expectations First
What marine solar really does — and what it does not.
Solar on a boat is very good at one thing: quietly carrying the loads that run all day and all night while you are away from a dock. The fridge, the electronics, the anchor light, the bilge pump, the phones. That is the job it does well, and doing it means you stop running the engine to charge and stop coming back to a flat bank.
It is not an air conditioner, and it is not a generator. A single marine air conditioner draws more continuously than a realistic boat array makes on its best day of the year. Being straight about that up front is the difference between a system you are happy with and an expensive disappointment.
Solar does this well
Fridge, chartplotter, lights, pumps, phones — the steady all-day loads that flatten a bank at anchor.
Solar does not do this
Air conditioning, electric cooking, a water heater. Those are shore power or a generator, full stop.
The shoulder season
An array sized on July sun is half a system in April. We size for when you actually use the boat.
The Worksheet
Step 1 — add up what you actually use in a day.
Everything starts with amp-hours per day. Take each DC item, multiply its draw in amps by the hours it really runs in 24 hours, and add them up. Typical marine figures, to get you started:
Draws
Per day
3–5 A running
40–60 Ah
1–2 A
6–12 Ah
0.5–2 A
5–12 Ah
2–5 A average
10–30 Ah
5 A intermittent
2–4 Ah
0.5 A
5–10 Ah
1–3 A
4–10 Ah
—
5–15 Ah
—
3–8 Ah
Add your own numbers, then add 30%. Every boat uses more than its owner estimates, gear gets added, and a bank that is never quite full ages faster. A typical Long Island weekender with a fridge lands around 80 to 120 amp-hours a day.
The Arithmetic
Step 2 — turn amp-hours into panel watts.
Two honest conversions. First, amp-hours into watt-hours: multiply by your system voltage. 100 Ah on a 12-volt system is 1,200 watt-hours.
Second, watt-hours into panel watts. Divide by the peak sun hours you actually get, then divide again by about 0.75 for real-world losses — heat, wiring, angle, dirt, a panel that is never pointed at the sun because it is bolted to a boat.
Long Island peak sun hours, honestly: 4 to 5 in June and July, about
3 in May and September, 1.5 to 2 in midwinter. "Peak sun hours" is not daylight — it is the equivalent hours at full rated output.
So: 1,200 Wh ÷ 4.5 sun hours ÷ 0.75 ≈ 355 watts of panel to cover that day in high summer. Want the same performance in May? Divide by 3 instead and you are at 530 watts. That gap is the whole reason to be honest about the season you actually boat in.
No fridge
30–50 Ah/day. Roughly 100–200 W of panel. A couple of panels holds it comfortably.
Weekender with a fridge
80–120 Ah/day. Roughly 300–600 W. The most common Long Island answer by far.
Extended cruising
150–250 Ah/day. 700 W and up, and the conversation turns to lithium and roof space.
Storage
Step 3 — size the battery bank behind the panels.
Panels make power when the sun is out. The bank is what gets you through the night, and through two grey days in a row. Aim to hold at least two days of your daily amp-hours in
usable capacity — and usable is the word that matters.
AGM gives you about 50% of its rating before you start shortening its life. A 200 Ah AGM bank is really 100 Ah of usable storage.
Lithium gives you 80 to 90%, charges much faster, and takes everything the panels can push at it. A 200 Ah lithium bank is 160 to 180 Ah usable — nearly double the AGM, in less weight and less space.
So for 100 Ah a day and two days of reserve, you are looking at roughly 400 Ah of AGM or around 240 Ah of lithium. If your existing bank is near the end of its life, replacing it at the same time as adding solar is cheaper than doing the same job twice.
The Last Number
Step 4 — size the charge controller.
Panel watts divided by battery voltage gives you roughly the output amps the controller has to pass. 600 watts on a 12-volt bank is about 50 amps, so a 150/70 has headroom. Then check the other number: the controller’s maximum PV voltage has to sit above your panel string’s open-circuit voltage
in the cold, because panel voltage rises as temperature falls.
Use an MPPT controller, not a PWM — on anything past a single trickle panel it is worth 20 to 30% more harvest from the panels you already paid for. There is a whole page on that:
MPPT charge controller installation.
Leave headroom on both numbers. A controller run at its ceiling runs hot, and heat is what kills them.
Ready to talk? Or keep reading below.
Hardware
Panel types, and where each one belongs on a boat.
Rigid on a hardtop or arch
Most watts per dollar, longest life, and air moves behind them so they run cooler and make more. First choice wherever there is a hardtop, arch or davit frame to carry them.
Walkable semi-rigid
For a deck or cabin top people stand on. Lower profile, takes foot traffic, and the right answer where a rigid frame would be in the way. More on walkable panels.
Thin flexible — carefully
Tidy when bonded to a curved surface, but they run hot, lose output as they do, and have the shortest life of the three. We use them where nothing else fits, not by default.
The Thing People Forget
Shade costs you more than roof space does.
On a house, shade is a fixed problem. On a boat it moves all day, and it moves again when you swing on the anchor. A mast, a radar arch, an outrigger, a boom, a furled sail, a dinghy on davits — each one drags a shadow across the panels on a schedule of its own.
In a series string, the shaded panel sets the pace for the whole string. Losing 10% of the panel area to a shadow can cost far more than 10% of the output. Wiring in parallel, or splitting the array across separate controller inputs, costs a little more and is far more forgiving.
This is why we look at the boat before specifying an array. Where the shadows fall through a day changes the wiring, and sometimes it changes where the panels go entirely.
What It Costs
Time and materials — never a fixed price.
The first hour is a flat $280 for one installer, covering travel to your boat up to 30 minutes each way, setup and the first hour of work. After that it is
$140/hr in 15-minute steps. No four-hour minimum, no separate trip or setup charge.
A modest array with a controller, isolation and proper fusing is generally a day. A larger system with a new bank, monitoring and distribution is more. Design and layout work off the boat bills at the CAD/design rate, not the field rate, and shows as its own line.
Equipment is supplied by us at cost plus markup and itemized on your receipt. Everything is billed on the time actually worked — you pay for the job that got done, never a padded fixed quote.
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$280 First Hour · $140/hr After
No Minimums
60-Day Labor Warranty
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FAQ
Sizing a marine solar system — what owners ask.
How many watts of solar do I need for a boat?It comes from what you use, not from the size of the boat. Add up the amp-hours your DC gear draws in a day, add about 30% for reality, then divide by the usable sun hours where you actually keep the boat. On Long Island, a weekender with a 12-volt fridge, chartplotter, lights and phone charging usually lands somewhere between 300 and 600 watts of panel. A boat with no fridge might be happy with 100 to 200. A liveaboard with a freezer and an inverter needs far more, and may need a different answer entirely.
It comes from what you use, not from the size of the boat. Add up the amp-hours your DC gear draws in a day, add about 30% for reality, then divide by the usable sun hours where you actually keep the boat. On Long Island, a weekender with a 12-volt fridge, chartplotter, lights and phone charging usually lands somewhere between 300 and 600 watts of panel. A boat with no fridge might be happy with 100 to 200. A liveaboard with a freezer and an inverter needs far more, and may need a different answer entirely.
How many hours of usable sun does Long Island actually get?
Far less than the brochures assume. Figure roughly 4 to 5 peak sun hours a day in June and July, about 3 in May and September, and 1.5 to 2 in the middle of winter. "Peak sun hours" is not daylight hours — it is the equivalent hours at full rated output. Sizing an array on 6 hours because that is what a spec sheet implies is the single most common reason a solar system disappoints its owner in the shoulder season.
Will solar run my air conditioning?
No, and anyone who tells you otherwise is selling something. A single marine air conditioner pulls in the region of 1,000 to 1,500 watts continuously. Covering that with panels alone would need more roof than a 40-foot boat has, plus a battery bank to match. Solar on a boat is for the fridge, the electronics, the lights, the pumps and the phones — the loads that run all day and quietly flatten a bank at anchor. Air conditioning is shore power or a generator.
Do I need lithium batteries to make solar worthwhile?
No, but lithium changes the arithmetic in solar’s favour. You can only use about half of an AGM bank before you damage it, while a lithium bank gives you 80 to 90% of what it says on the label, charges faster and accepts everything the panels can make. That means a lithium bank of the same nominal size does roughly twice the work. If your AGMs are near the end anyway, do both at once rather than twice.
What happens if one panel is shaded?
More than you would expect. In a series string the weakest panel sets the pace for the whole string, so a mast, a radar arch, an outrigger or a boom shadow crossing one panel can knock down the output of all of them. That is why shade, not roof space, often decides how panels get wired on a boat — parallel wiring or separate controller inputs cost a little more and are far more forgiving of a moving shadow.
Rigid, semi-rigid or flexible panels?
Rigid panels on a hardtop or an arch give the most watts per dollar and last longest, and they run cooler because air moves behind them. Walkable semi-rigid panels are the answer where the panel has to live on a deck or a cabin top people stand on. Thin flexible panels bonded straight to a surface look tidy but run hot, lose output, and have the shortest life of the three. We fit what suits the boat, not what is easiest to stick down.
Can you add solar to a boat that already has a charger and batteries?
Usually yes. The array and controller become another charging source alongside the shore charger and the alternator, feeding the same bank. What matters is that the charge profiles all agree, the new circuit is fused on both sides, and the bank is actually healthy enough to be worth charging. We check the bank before quoting panels.
What does a marine solar system cost on Long Island?
Time and materials, never a fixed price. The first hour is a flat $280 for one installer including travel up to 30 minutes each way and setup, then $140/hr in 15-minute steps with no four-hour minimum. A modest array with a controller and proper fusing is generally a day. Equipment is supplied by us at cost plus markup and itemized. We would rather size it properly and tell you the honest number than sell you an array that under-delivers.
Want us to run the numbers on your boat?
Send the boat, the marina, what you run in a day and a photo of the hardtop and the battery space. We will tell you the honest panel and bank size — including if the answer is that solar is not the right spend on your boat.