Bottom Line: Worthwhile — Conditionally
The three walls offer almost exactly as much area as the roof (~728 vs ~738 usable ft²). Vertical mounting in the tropics yields about 58% of what the horizontal roof gets, so the walls add roughly +40 kWh/day on average — a ~58% increase over roof-only. Your 120° wall layout is close to ideal for a fixed installation: at every sun bearing at least one wall is illuminated, never more than two, and the set averages ~95% of a perfectly sun-tracking vertical wall.
+14.6 kWp
added array (walls)
+40 kWh/day
average extra energy
+58%
vs roof-only (~68 kWh/d)
~1,900 lb
added weight (860 kg)
$18–27k
installed cost
~4 yr
payback vs diesel genset
Build the side arrays if:
- Demand is, or will grow to, roughly ≥70 kWh/day (air conditioning, watermaker, induction cooking, RIB/e-mobility charging, a second connected seastead).
- You can spare ~1,900 lb of the 27,500-lb waterline budget — this is the real cost, bigger than the dollars.
- You want morning/evening generation to flatten the charging curve into the big LFP bank (lower peak charge rates, less midday clipping).
If the roof array already covers demand with big surpluses, the walls are a luxury — run a 6-month demand audit first, then decide.
1. Available Area
| Surface | Gross (ft²) | Usable (ft²) | Usability notes |
|---|---|---|---|
| Roof (equilateral triangle, 44′ side) | 838 | 738 | 88% — hatches, vents, mount setbacks, walkway access clearance |
| Walls, gross (3 × 44′ × 7′) | 924 | — | Full-height envelope |
| Walls, panel band (y = 1′–7′, above walkway) | 792 | — | Bottom 1′ strip excluded: shaded by the 3′ walkway whenever sun elevation > ~18°, and it is the splash zone |
| Walls, after doors & framing margins | — | 728 | −26 ft² two stern doors; −5% frame edges/sealant joints. Assumes no windows (see §9) |
Convenient coincidence: wall area ≈ roof area, so the side-array question is essentially “do we want +100% area at ~58% efficiency?”
2. How the Numbers WereComputed
Sun geometry on three walls at 120°
Beam irradiance on a vertical wall whose outward normal points at azimuth φn:
Ewall = DNI · max( 0, cos(altitude) · cos(φsun − φn) )Because the triangle is equilateral, the three normals are 120° apart. Summing the positive projections over all possible sun bearings:
| Sun bearing relative to wall A | Walls lit | Summed projection factor |
|---|---|---|
| 0° (square to A) | 1 | 1.00 |
| 30° | 2 | 1.24 |
| 60° | 2 | 1.00 |
| 90° | 1 | 0.87 |
| 120° | 1 | 1.00 |
| 150° | 2 | 1.24 |
| 180° (square to B+C split) | 2 | 1.00 |
- Never zero, never more than two — confirms your intuition.
- Azimuth-average capture = 3/π ≈ 0.955: the fixed 120° set collects ~95% of what one ideally-rotated vertical wall would. Heading-independent — valuable for a home that swings on moorings.
Resource conversion (Caribbean baseline, ~15°N)
POAwall ≈ 0.55 · GHI + (ρ/2)·GHI = 0.55·5.8 + 0.20 ≈ 3.4 kWh/m²/dayYield = POA × PR(0.80) ≈ 2.7 kWh/kWp/day (roof: 5.8 × 0.80 ≈ 4.6)
The 0.55 factor is the classic low-latitude result for vertical surfaces (beam geometry + diffuse sky), averaged over orientation; the second term is ocean reflection (§5). PR = 0.80 covers temperature (vertical walls actually run cooler), wiring, mismatch, railing/walkway shading (~2–3%), and salt soiling.
3. Energy Results
Module density assumed 20 W/ft² (215 W/m²; e.g., a 420 W module ≈ 20.9 ft²).
| Array | Area (ft²) | Size (kWp) | Avg kWh/day | MWh/yr |
|---|---|---|---|---|
| Roof only (baseline) | 738 | 14.8 | 68 | 25.1 |
| Side walls (added) | 728 | 14.6 | 40 | 14.5 |
| Total with sides | 1,466 | 29.4 | 108 | 39.6 |
Average kWh/day, Caribbean baseline. Walls deliver ~58 kWh/day per 100 ft² less than roof per unit area, but there is nearly equal area available.
Battery synergy: your ~25%-of-displacement LFP bank (≈6,900 lb ≈ 350–450 kWh)
cycles only ~25–30% on a 108 kWh day. Because wall output arrives in the morning/evening shoulders,
it raises the marginal value of every panel: less midday surplus that would otherwise go unused
when the bank is already full, and gentler charge rates on the three per-leg inverters.
4. Daily Production Shape (illustrative, clear-sky, near-equinox)
5. Ocean-Surface Reflection (Albedo) Contribution
Light bouncing off the sea onto a vertical surface is handled with the standard ground-reflected term. For a surface tilted at β from horizontal, reflected irradiance is:
R = ρ · GHI · (1 − cos β) / 2 → for a vertical wall (β=90°): R = ρ · GHI / 2Seawater broadband albedo ρ ≈ 0.03–0.05 at high sun, rising steeply at low sun angles (Fresnel glint off wave facets, 0.2–0.4 near sunrise/sunset); a daytime-effective ρ ≈ 0.07 is a fair annual average, and whitecaps push diffuse upwelling a bit higher.
| Term | kWh/m²/day | Note |
|---|---|---|
| Beam + diffuse on vertical (orientation-averaged) | 3.19 | 0.55 × GHI 5.8 |
| Ocean-reflected (albedo) bonus | +0.20 | ρ/2 × GHI ≈ +6% |
| Total plane-of-array, walls | 3.4 | used throughout this analysis |
- Dawn/dusk sun-glitter adds short bursts beyond the simple model — a freebie at the same hours the east/west walls are already angled toward the sun.
- Bifacial caveat: flush wall panels face the cabin with their rear side, so bifacial gain is not available here (unlike ground-mounted vertical fences). Stick with monofacial unless you ever build standoff-mounted wing panels.
6. Weight Budget Impact
| Item | Weight (lb) | Basis |
|---|---|---|
| Modules (glass/backsheet, salt-mist certified) | 1,460 | 728 ft² × ~2.0 lb/ft² (~35 modules of ~420 W) |
| Mounting: SS316 rails, bonded standoffs, fasteners | 330 | ~0.45 lb/ft² |
| Added wiring, conduit, junction boxes | 110 | extension of the three per-leg power chains |
| Total | ~1,900 | (860 kg) |
- = 3.1% of the 62,000-lb container shipping limit — not a constraint.
- = ~6.9% of the 27,500-lb waterline budget — this competes directly with “humans and their stuff.” This is the true price of side solar.
- Mass sits high (band centroid ~4′ above the walkway). Symmetric port/starboard, so list is neutral, but have the naval architect check the KG/GM change against the low-mounted battery ballast.
- Shipping volume: ~35 modules crate to roughly 70–90 ft³ — a noticeable but manageable slice of the container’s center space alongside the other kit.
- Lightweight flexible panels (~0.9 lb/ft², ~800 lb total) exist but run hot, fade faster, and cost 20–30% of their rating within a few years — not recommended for a primary array.
7. Cost Estimate (marine-grade, 2025 USD)
| Line item | Low | High | Notes |
|---|---|---|---|
| Modules, 14.6 kWp, IEC 61701 salt-mist | $7,300 | $9,500 | $0.50–0.65/W small-quantity |
| Mounting hardware & sealants | $2,000 | $3,000 | bonded standoffs, 316SS fasteners |
| Electrical: MPPT expansion (4–6 ch), breakers, DC surge protection | $2,000 | $3,500 | keep wall strings mapped to the same 3 independent leg chains |
| Shipyard labor | $4,000 | $7,000 | 60–80 hr; near-zero if self-installed |
| Contingency (~15%) | $2,300 | $3,500 | |
| Total installed | $18k | $27k | self-build: $13k–17k |
- Unit cost: $1.25–1.85 per Watt installed.
- 20-year output ≈ 260 MWh (incl. degradation) → simple energy cost ≈$0.09/kWh.
- If it displaces diesel genset runtime at $0.35–0.50/kWh (fuel + maintenance), savings are $5–7k/yr → payback ~3.5–5 years.
- If it merely pads surplus into an already-full battery, marginal value is much lower — hence the conditional verdict.
8. Sensitivity (walls array, average kWh/day)
| Scenario | Walls kWh/day | Comment |
|---|---|---|
| Baseline — 15°N, GHI 5.8 | 40 | central Caribbean |
| Cloudier anchorage (GHI 5.0) | 34 | eastern Caribbean trades |
| Sunnier (GHI 6.3) | 43 | ABC islands |
| Latitude 10°N | 42 | lower latitudes slightly favor verticals |
| Latitude 20°N | 37 | |
| Albedo 0.04 vs 0.10 | 39 / 41 | reflection is a modest, stable bonus |
| Reserve 60 ft² for windows | −3 | ~1.1 kWp displaced |
| No freshwater rinsing (salt film) | −2 | vertical panels shed rain well; rinse monthly anyway |
Realistic operating band: ~34–43 kWh/day. The conclusion is robust across the Caribbean — the walls always add roughly half again as much energy as the roof alone.
9. Design Integration, Risks & Trade-offs
- Windows vs. watts — decide now. Panels and windows want the same 6-ft band. Every 10 ft² of window costs ~200 Wp (~0.55 kWh/day). Recommendation: reserve modest cutouts (e.g., 40–80 ft² total, high on the walls) even if panelling immediately — a windowless steel triangle interior would be grim, and retrofitting cutouts into a paneled wall is painful. Roof skylights can offset some of this without touching wall area.
- Walkway interaction: start the array at walkway level (y=1′). The strip below is shaded for most sun angles and lives in the splash zone. Railing posts/top-rail cast narrow moving shadows at low sun — already priced in at 2–3%.
- Hurricane survivability: flush-mount (proud <3″), through-bolt to frame members at ~24″ centers with structural adhesive backup, 5 mm+ tempered glass, impact-rated modules. Flush panels add essentially no windage over the bare wall. Treat them as permanent fixtures, not quick-release.
- Corrosion & fouling: IEC 61701 salt-mist certification, IP68 junction boxes, isolated stainless fasteners (no dissimilar-metal contact with aluminum frames). Vertical orientation is self-cleaning in rain; schedule a monthly freshwater rinse.
- Wave slap: in storm conditions green water crosses the walkway; seal panel bottom edges and provide drain paths.
- Lightning: bond module frames and rails into the hull bonding system; surge protection on every MPPT input.
- Electrical architecture fit: ~4.9 kWp added per leg keeps the elegant triple-redundant per-leg chain (own controller/inverter/thruster feed). Verify MPPT headroom per channel.
- Tension-leg heading: when parked, rotate so one wall faces south — worth ~3–5% annually (winter sun is low and loves vertical south glass). Underway, heading varies and the 120° layout averages it out.
- Connected-pair transits: the lead seastead will partially shade the follower’s two forward walls at some sun angles. Acceptable; stagger headings if it bothers you.
- Future expansion paths: a roofed walkway (+~350 ft², +7 kWp) or a flexible panel on the RIB (+~0.3 kWp) remain available later; the walls are the big lever now.
10. Recommendations
- Do it, in phases: West + East walls first (maximum curve-flattening), South wall next, North wall last (poorest performer in the northern tropics — diffuse and glint only).
- Monofacial, flush, salt-mist certified, 20 W/ft²-class modules; skip bifacial on flush walls.
- Map wall strings onto the existing three per-leg power chains to preserve the triple-redundant failure-isolation philosophy.
- Reserve window cutouts in the structural design now, even if glass goes in later.
- Run a 6-month demand audit on the roof-only build. If average consumption is under ~65 kWh/day and the bank refills by noon, defer the North wall and revisit.
- Have the naval architect sign off the KG change from ~1,900 lb at wall height against the low battery ballast.
11. Key Assumptions
- Location ~15°N Caribbean; GHI 5.8 kWh/m²/day annual average; diffuse fraction ~42%.
- Heading-averaged (isotropic) wall exposure — valid for a vessel that swings on moorings or moves; a permanently fixed favorable heading shifts a few percent between walls, not the total.
- Performance ratio 0.80 (temperature, wiring, mismatch, railing/walkway shading, salt soiling).
- Module density 20 W/ft²; usable areas per §1; two stern doors (26 ft²) excluded; no windows in the base case.
- Seawater albedo 0.07 effective; reflected term ρ·GHI/2 for vertical surfaces.
- Degradation (~0.5%/yr) excluded from daily figures; included in the 20-year LCOE estimate.
- Costs are ballpark 2025 USD for small-lot marine-grade equipment; quotes will vary.