Sailboat 400W 12V (marine)
12V system, designed for a -5C (23F) cold snap - saved 2026-09-06
Compatibility checks
12 passing checks - expand to see the math
passBattery matches the 12V systemSystem voltage coherence
SOK SK12V206P 12V 206Ah Marine (12.8V nominal) is a 12V-class battery.
battery nominal 12.8V is in the 12V class == system 12V
passInverter matches the 12V systemSystem voltage coherence
Renogy 1000W 12V Pure Sine Wave Inverter (RNG-INVT-1000-12V-P2) DC input is 12V-class.
inverter DC input 12V is in the 12V class == system 12V
passCharge controller supports 12V banksSystem voltage coherence
Victron Energy SmartSolar MPPT 100/50 supports 12/24V battery banks.
controller battery voltages [12, 24] include 12V
passString Voc stays at 49V at -5C - cannot reach the 100V limit at any temperatureCold-temperature string voltage
2 x Newpowa 100W 12V Monocrystalline Solar Panel (9BB) in series stays below the Victron Energy SmartSolar MPPT 100/50 max PV input voltage at your design low temperature. This string cannot reach 100V at any physically possible temperature.
Why this matters: Panel voltage rises as temperature falls. A string that measures fine in summer can exceed the controller's absolute maximum input voltage on the first cold clear morning, which can permanently destroy the controller - and at these DC power levels, failures can start fires. This is the mistake experienced builders catch most often in DIY parts lists.
Voc_string = Voc x N_series x (1 + tempCoeff/100 x (T_design - 25))
= 22.12V x 2 x (1 + (-0.36%/100) x (-5 - 25))
= 22.12V x 2 x 1.1080 = 49V
Controller max PV input: 100V
Crossover: string Voc reaches 100V at T = 25 + (100/44.2 - 1) x 100/-0.36 = -325.1C (below absolute zero - never reached)passString Vmp 38.5V has healthy margin over the 12V bankMPPT voltage margin
Clears the start floor cold and the hot-weather floor with sag included.
Vmp_string = 19.23V x 2 = 38.5V Start floor = V_batt + 5V = 12 + 5 = 17V Hot Vmp (~11% sag) = 38.5V x 0.89 = 34.2V vs hot floor 20V
passArray short-circuit current 11.00A is within the 60A limitArray current vs controller input limit
Within the Victron Energy SmartSolar MPPT 100/50 published max PV short-circuit current.
Isc_array = 5.50A x 2 parallel = 11.00A Controller max PV short-circuit current: 60A = 60A (manufacturer-specified, compared directly)
passController can pass the array's full output (31.3A of 50A rated)Array size vs controller charge output
No clipping at rated conditions.
Array = 100W x 4 panels = 400W Potential charge current = 400W / 12.8V = 31.3A Controller rated charge current: 50A
passArray 400W is within the controller's 700W rating at 12VMax array wattage for battery voltage
Within the published maximum array wattage.
Array = 100W x 4 panels = 400W Controller max array wattage at 12V: 700W
passVictron Energy SmartSolar MPPT 100/50 supports LiFePO4 chargingBattery chemistry support
Controller has a lithium profile or user-adjustable charge voltages.
battery chemistry = lifepo4; controller lifepo4_profile = true
passBank BMS limit 170A comfortably covers the inverter's 78.1A drawInverter draw vs battery BMS limit
Healthy headroom at full inverter load.
Inverter draw = 1000W / 12.8V = 78.1A Bank BMS limit = 170A x 1 batteries = 170A Headroom band starts at 80% of limit = 136A
passBank BMS peak 200A covers the inverter's 156.3A surgeInverter surge vs battery peak limit
Surge loads within the bank's published peak rating.
Surge draw = 2000W / 12.8V = 156.3A Bank BMS peak = 200A x 1 batteries = 200A
passCharge current 50A is within the bank's 70A capacityCharge current vs bank capacity
Charge rate within limits.
Controller rated charge = 50A Bank max charge = manufacturer max charge 70A x 1 = 70A
Parts list
| Part | Qty | Est. price | Where to buy |
|---|---|---|---|
Newpowa 100W 12V Monocrystalline Solar Panel (9BB) | 4 | $300 | |
Victron Energy SmartSolar MPPT 100/50 controllerdatasheet | 1 | $300 | |
SOK SK12V206P 12V 206Ah Marine batterydatasheet | 1 | n/a | |
Renogy 1000W 12V Pure Sine Wave Inverter (RNG-INVT-1000-12V-P2) inverterdatasheet | 1 | $175.99 | |
| Estimated total | $775.99(some prices unknown) |
Wire and fuse guidance
| Segment | Continuous | Run (one way) | Copper wire | V-drop | Fuse/breaker |
|---|---|---|---|---|---|
| Array to charge controller | 11.0A | 10 ft | 14 AWG | 1.44% | 15APV-rated (gPV) fuse or DC breaker |
| Charge controller to battery bank | 50.0A | 4 ft | 6 AWG | 1.23% | 70AClass T |
| Battery bank to inverter | 78.1A | 3 ft | 3 AWG | 0.72% | 100AClass T |
Array to charge controller: show the math
Continuous current = 11A Design current = 11A x 1.25 (NEC continuous) = 13.8A Wire: 14 AWG copper (75C ampacity 20A); voltage drop 1.44% over 20ft round trip at 38.5V (target <= 3%) Fuse/breaker: 15A - two criteria, both required: carry the load: fuse >= design 13.8A -> next standard size = 15A protect the wire: fuse 15A <= 14 AWG ampacity 20A - OK construction: PV-rated (gPV) fuse or DC breaker
Charge controller to battery bank: show the math
Continuous current = 50A Design current = 50A x 1.25 (NEC continuous) = 62.5A Wire: 6 AWG copper (75C ampacity 65A); voltage drop 1.23% over 8ft round trip at 12.8V (target <= 3%) Fuse/breaker: 70A - two criteria, both required: carry the load: fuse >= design 62.5A -> next standard size = 70A protect the wire: fuse 70A <= 6 AWG ampacity 65A - NOT satisfied, upsize the wire bank BMS continuous 170A >= fuse 70A: the bank can hold the fuse's rated load construction: Class T (interrupt rating matters on battery mains)
Battery bank to inverter: show the math
Continuous current = 78.1A Design current = 78.1A x 1.25 (NEC continuous) = 97.7A Wire: 3 AWG copper (75C ampacity 100A); voltage drop 0.72% over 6ft round trip at 12.8V (target <= 3%) Fuse/breaker: 100A - two criteria, both required: carry the load: fuse >= design 97.7A -> next standard size = 100A protect the wire: fuse 100A <= 3 AWG ampacity 100A - OK bank BMS continuous 170A >= fuse 100A: the bank can hold the fuse's rated load construction: Class T (interrupt rating matters on battery mains)
Mount the fuse close to the battery positive terminal - it protects the wire run, so the unprotected stub should be as short as possible.
Sized for 75C copper ampacity with the NEC 1.25 continuous factor and a 3% voltage-drop target. Round up when between sizes and verify against local code.
Balance of system - shopping list
| Item | Qty | For |
|---|---|---|
| 14 AWG copper wire | 1 | ~25 ft total (10 ft one way, round trip + slack) - Array to charge controller |
| 15A PV-rated (gPV) fuse or DC breaker | 1 | Array to charge controller |
| 6 AWG copper wire | 1 | ~10 ft total (4 ft one way, round trip + slack) - Charge controller to battery bank |
| 70A Class T | 1 | Charge controller to battery bank - include the matching fuse holder/block |
| 3 AWG copper wire | 1 | ~10 ft total (3 ft one way, round trip + slack) - Battery bank to inverter |
| 100A Class T | 1 | Battery bank to inverter - include the matching fuse holder/block |
Derived from the wire and fuse guidance above. Wire lengths include round trip plus slack - verify against your actual layout, and verify ratings against local code. Buy fuses and wire from reputable electrical suppliers; marketplace listings for these items are a common source of counterfeit or mislabeled parts.
About this build
Boats are the harshest place solar equipment lives: salt air that corrodes every connection, constant motion, and shading that sweeps across the panels with every change of heading. This build answers with four smaller panels in two strings rather than one or two large ones - when the boom shades half the array at anchor, the other string keeps working - and a marine-grade battery with a sealed case, vibration-rated mounting, and a BMS whose discharge limits are actually published, so the inverter's surge demand can be checked against them instead of guessed.
The 206Ah house bank runs the boat natively at 12V throughout - navigation electronics, lights, pumps, and fridge all draw direct from the bank - with the inverter reserved for the odd household load at the nav station. The generously rated controller takes the whole array without clipping and leaves input headroom for a rail-mount panel later. Everything conductive gets marine-rated terminations; ordinary automotive crimps are the first thing the salt eats.
Common questions
Why marine-grade batteries instead of ordinary lithium?
The cells are similar; the packaging is not. Marine-grade means a sealed enclosure rated for spray, terminals and mounts designed for vibration and heel, published BMS discharge limits you can size an inverter against, and charge behavior documented for the sources boats actually have. A drop-in built for a garage floor is a gamble in a bilge-adjacent locker.
How does shading at anchor affect the array?
Severely and constantly - mast, boom, and rigging shadows sweep the deck as the boat swings. Splitting the array into parallel strings limits the damage to the shaded string, and mounting panels where the boom does not park over them matters more than any electrical decision.
Can this charge from the engine too?
Most cruising boats add a DC-DC charger from the engine's alternator circuit, sized conservatively - sailboat alternators are small and lithium banks will take everything they offer. Add it in the builder as a second charge source and check it against the battery's charge current limit.
Is 400W enough for a liveaboard?
For lights, instruments, phones, and an efficient DC fridge in a sunny cruising ground, often yes. Add an autopilot working hard, a freezer, or cloudy-latitude cruising and the budget tightens fast. Deck and rail space set the ceiling; meter your real loads before deciding you need more.