Calculate boat solar system
Sizes the on-board system for lying at anchor: a 24/7 base load instead of a daily profile, rigging shading as a quantified deduction, and sun hours from the hourly grid of your cruising area — for the horizontal deck. Plus the galvanic traffic light, the most expensive question on board: an aluminium hull on shore power without an isolator is critical.
- CRITICAL galvanic combination: aluminium hull on shore power without a galvanic isolator. Through the protective conductor your hull is electrically bonded to every neighbouring boat on the dock — the most anodic metal sacrifices itself first. Retrofit an isolator or isolation transformer, check the anodes.
GALVANIC_RISK_CRITICAL
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
Safety-relevant calculation. This is an estimate based on the stated assumptions. The final design must be checked by a qualified electrician against the rules that apply where you are.
Data as of: 2026-06-15 · The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Solar yield with rigging cut | P*PSH*eta_Kette*f_Takelage | 246.09 Wh/d | assumed |
| Required bank capacity | E*d/(U*DoD*f_alter*eta_Pfad) | 580.13 Ah | exact |
- Formula
E_solar = P·PSH·η_chain·f_rigging · C_req = E·d/(U·DoD·f_age·η_path) · galvanic: hull × shore power × isolator × grounding concept- Valid for
- Sailing or motor boat with horizontal deck mounting (β = 0°), one design month, peak sun hours from the hourly TMY grid (1,155 DACH grid points, PVGIS-SARAH3 2005–2023). Rule facts from US federal law 33 CFR 183 Subpart I, which is public domain; the ISO edition is referenced, not reproduced.
- Not covered
- Movement of the boat (heel, swinging at anchor) and the changing orientation that follows, salt spray soiling, tilted or rail-mounted panels, wind and tow generators, and any concrete design to ISO 13297 — for that the standard text governs, which this calculator does not reproduce.
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
How much solar yield does the rigging really cost?
More than any land calculation suggests: mast, boom, shrouds and radar arch cast moving shadows that sweep every deck panel several times a day. The calculator carries this as a visible rigging factor (default 0.65 = 35% cut, range 0.5–0.85) and quantifies the loss in Wh: in the example, of a theoretical 945 Wh only 614 remain — 331 Wh go to the rigging. Position beats area: a stern arch, davits or bimini top usually sit outside the shadow tracks.
Why is anchoring different from any land system?
Because the load never sleeps: fridge, anchor light, instruments and bilge pump run 24/7 — there is no “pull the plug at night”. The battery carries the full baseline every night, and one day’s solar yield must cover day AND night consumption. Hence the calculator computes coverage against the full 24/7 demand and sizes the bank with real autonomy days for the weather front at anchor.
What exactly does the galvanic warning check?
The combination of four inputs: hull material, shore power, galvanic isolator and grounding concept. Critical is the pattern aluminium hull + shore power + no isolator: through the protective conductor your underwater body is then galvanically bonded to the dock and all neighbouring boats, and aluminium is the least noble common hull metal — it sacrifices itself first, before the anodes. The damage quickly exceeds the value of the entire solar system; an isolator costs a fraction.
I have a GRP hull — does galvanic corrosion still concern me?
Yes, through the metal parts in the water: shaft, propeller, saildrive, rudder fittings and their anodes hang on the ship’s negative. On shore power without an isolator they join the same galvanic bond as a metal hull — the most common finding is rapidly vanishing anodes and pitted saildrives. The calculator therefore rates GRP on shore power without an isolator as elevated risk, not critical.
Which rules govern electrics on board?
Governing is ISO 13297:2020 + Amd 1:2022 (unifying AC and DC since 2020; the old ISO 10133 merged into it — edition verified by us), ABYC E-11 in the USA and 33 CFR 183 for gasoline-engine boats. The calculator quotes the public-domain CFR facts (battery fuse ≤ 72 inches from the terminal, minimum sizes) and refers ISO sizing to the standard and a professional — it deliberately does not reproduce standard content.
Are 300 Wp enough on a sailboat?
Not quite for the example demand (1,500 Wh/day with a fridge): with the rigging cut, 300 Wp deliver about 614 Wh — 41% coverage; the rest comes from the bank, which needs about 580 Ah for that (3 autonomy days, LiFePO4). The honest order on a boat: cut consumption first (fridge insulation, LED), then optimise panel position, then add area — and count the alternator as the second source when under way (the camper calculator shows the principle).
How much solar does a boat really need?
Considerably more than the usual rules of thumb suggest, because two things combine: the anchor load runs around the clock, and the deck lies flat. Example Berlin cruising area, 300 W, 1,500 Wh daily demand, 35 % rigging deduction: in July the system covers 47 %, in October 16 %, in December 4 %. Anyone wanting to lie at anchor year-round cannot manage on solar alone — that is not a calculation error but the consequence of a 24/7 load on a horizontal deck.
How much do the rig and mast shade the panels?
The calculator carries it as a visible deduction instead of hiding it: the default is 65 % remaining yield, so a 35 % loss — for 300 W in July that is 382 of 1,091 Wh a day. The figure is an assumption with a range (50 to 85 % remaining), because it depends on rig type, panel position and heading. What matters is the order of magnitude: a mast casts a travelling shadow across the day, and with series-wired panels it costs more than its area.
Why does the calculator ask for a design month?
Because it decides half the system. On a horizontal deck the same boat in the Berlin area gets 709 Wh a day in July and 246 Wh in October — identical hardware. The widespread default of 4.5 peak sun hours, which this calculator itself used to assume, is effectively a midsummer figure. Design with it and still be out in September, and you are left with a system delivering less than half.
When does galvanic corrosion become dangerous on a boat?
When three things coincide: a less noble hull, a shore power connection, and no isolator in the protective earth. The calculator assesses exactly that combination — an aluminium hull plus shore power without a galvanic isolator is critical, because the protective earth connects every boat on the pontoon electrically and the hull becomes the sacrificial anode for the neighbours. The damage exceeds the cost of the whole solar system. The traffic light does not replace expert review; it names the case.
Which standards apply to boat electrical systems?
In Europe, ISO 13297 in its 2020 edition with Amendment 1:2022 — it absorbed the former ISO 10133, so DC and AC now sit together. This calculator does not reproduce the standard text; it references it and works from the rule facts of 33 CFR 183 Subpart I, the US federal regulation for boat electrics, which is public domain: overcurrent protection within 72 inches of the battery terminal, minimum conductor sizes, and ampacity per conductor size and temperature rating.