1. Reference geometry used
Walls already contain doors, a ladder on each leg, and the dinghy bay. Practical usable wall area for PV is taken as 80 % of gross, i.e. ≈ 694 ft² (64.5 m²). Roof usable area taken as 85 % ≈ 630 ft² (58.5 m²).
2. How much sun the sides actually catch
Because the three walls face 120° apart, at any solar azimuth exactly one (sometimes two) of the walls has a normal within ±60° of the sun. That has a useful geometric consequence:
- The projected width of the triangle on a vertical plane, averaged over all horizontal viewing angles, equals
Perimeter / π = 123.9 / π = 39.4 ft. - Mean projected collecting area of the walls =
39.4 × 7 = 276 ft² (25.6 m²)— independent of heading. - Compare to the roof's full 741 ft² projected horizontally. So purely geometrically, the walls present about 37 % as much “target” to the sun as the roof does.
Real-world irradiance (≈ 25°N, trade-wind latitudes)
| Surface | Mean daily insolation (kWh/m²/day) | Notes |
|---|---|---|
| Horizontal roof | 5.5 | Tropical marine annual mean. |
| Vertical wall, any orientation averaged | 2.8 | Rule of thumb ≈ 45–55 % of horizontal at low latitudes. |
| + ocean-reflected component (albedo ≈ 6–10 % specular boost) | +0.4 | Fresnel reflectance over water rises steeply below 25° sun elevation. Adds ~10–15 % to sides. |
| Total effective for side walls | 3.2 | Beam + diffuse sky + sea-glare bounce. |
3. Energy yield — roof only vs. roof + sides
Assumptions: 22 % panel efficiency, 0.80 system derate (wiring, MPPT, soiling/salt-film, heat).
58.5 m² × 5.5 kWh/m² × 0.22 × 0.80 × 365 ≈ 20 700 kWh / yr
64.5 m² × 3.2 kWh/m² × 0.22 × 0.80 × 365 ≈ 15 800 kWh / yr
36 500 kWh / yr — almost double the roof-only number.
Relative to roof-only. The gain is biggest when the sun is low — i.e. exactly when you most want every watt.
4. Weight budget
| Item (sides only) | Qty | Unit | Subtotal |
|---|---|---|---|
| Flexible / marine-grade PV sheets | 694 ft² | 2.3 lb/ft² | 1 600 lb |
| Marine bonding + framing (al backing rails, adhesive, structural bond) | 694 ft² | 0.9 lb/ft² | 625 lb |
| Cabling, junction boxes, connectors | — | — | 180 lb |
| Extra MPPT / DC breakers | 3 strings | — | 90 lb |
| Total added | ≈ 2 500 lb (1 134 kg) |
Container max weight is 62 000 lb, so this is ~4 % of the shipping/float budget — easily absorbed. Note the panels sit on the 7 ft walls, so the CG only rises modestly compared to roof panels, which helps trim.
5. Cost budget (sides only)
| Item | Qty / size | Unit cost | Subtotal |
|---|---|---|---|
| Marine PV sheets (22 % flexible) | 14.2 kWp | $0.85/W | $12 100 |
| Mounting / bonding / framing | 694 ft² | $4/ft² | $2 800 |
| 3 × marine MPPT charge controllers | 3 | $1 400 | $4 200 |
| Cable, breakers, combiner boxes, conduit | — | — | $2 500 |
| Install labor / sealing | — | — | $3 800 |
| Contingency (10 %) | $2 540 | ||
| Total | ≈ $28 000 |
6. How the extra 43 kWh/day actually gets used
A seastead of this size plausibly needs 35–60 kWh/day for:
- Propulsion (6 × RIM drives, 1–8 kt cruising)
- 3 active stabilizers
- Watermaker (≈ 3–5 kWh/m³)
- Refrigeration + cooking
- Electronics, lighting, comms
With the roof alone, you are near break-even on a cloudy week. With the sides, you:
- Cut your battery bank by roughly half for the same autonomy, or
- Double cruise range under sun (the sides generate almost as much as the roof), or
- Make water freely on sun days — a real game-changer at sea.
Time-of-day smoothing (a real hidden win)
The east wall peaks in the morning, the west wall in the afternoon. Combined with the roof hitting at midday, your daily power curve flattens — less stress on batteries, less need for oversized inverter capacity, and much better match to a watermaker run 10 hours/day.
7. Risks & design impacts
- Salt film Flexible marine panels lose output fast when crusted. Plan a fresh-water rinse cycle (a few litres per panel per week) or hydrophobic top-coat. Budget 5–10 % output loss between washes.
- Windage Wall PV adds almost no wind drag (it is flush or near-flush). But bonding must survive 60 kt gusts → use Sikaflex 292 + mechanical fasteners at edges.
- Windows Covering 100 % of the walls kills daylight. The 80 % figure deliberately leaves room for portholes on the non-leg walls; you can push it to 90 % if you accept few windows.
- Aesthetics / glare Dark PV around the whole triangle changes the look. Two-tone (dark lower, lighter upper window strip) reads well.
- Redundancy Three independent wall strings (one per face) mesh naturally with your existing “one inverter, one charge-controller, one battery bank per leg” philosophy — each face of PV can feed its own leg’s bank. Triple-redundant, fault-isolatable, no single-point failure.
- Maintenance access Side panels on the waterline wall get hit by spray, and occasional replacement will mean climbing the leg ladder. Plan for removable/sectional panels, not one monolithic sheet.
8. Verdict
Recommendation
YES — put PV on the sides.
For a seastead (not a stationary house) the walls are an unusually valuable second solar array because:
- The equilateral triangle is geometrically kind to vertical collection — the walls together present ~37 % of the roof’s collecting area, independent of heading.
- Ocean-reflected / low-angle sun adds a meaningful +10…15 % on top of land-based vertical-PV rules of thumb.
- The 120° wall spacing flattens the diurnal curve, which relaxes battery and inverter sizing.
- The energy system is already split into 3 independent leg-based buses — side PV fits that topology perfectly.
- At ~$0.09/kWh LCOE and 18× cheaper than the diesel baseline, the economics dominate any other “next kWh” option available to you.
Suggested build order
- Start by tiling the two faces that do not contain the dinghy bay / rear deck — cleanest geometry, fewest cutouts.
- Leave ~20 % of the rear face free for the walkway/dinghy, ladders, and future mooring screw lines.
- Wire each face to its own leg’s charge-controller + LiFePO₄ bank, matching the redundant power already spec’d for thrusters and stabilizers.
- Add hydrophobic top-sheet and a salt-rinse loop fed from the watermaker’s reject stream.
9. Quick-reference numbers
| Roof PV (58.5 m², 22 %) | 12.9 kWp | 57 kWh/d | $ — in base build |
| Side PV (64.5 m², 22 %) | 14.2 kWp | 43 kWh/d | $28k / 2 500 lb |
| Total | 27.1 kWp | 100 kWh/d | 36.5 MWh/yr |