How much solar do I need for a 400 Ah battery bank?

For a 12V 400 Ah LiFePO4 bank used daily with 150 Ah/day drawdown (1,800 Wh), you need about 600W of panel in the PNW (3.5 PSH) or 350W in Arizona (6.5 PSH). Oversize 20% for cloudy weeks.

MPPT vs PWM — when does PWM make sense?

PWM is fine for arrays under 200W where the cost savings ($40-80) matter and panel voltage closely matches battery voltage. Above 200W, or for any 24V/48V bank, MPPT is the correct choice — the 10-30% energy recovery pays back in under a year.

Does panel temperature affect output?

Yes — panels lose about 0.4% of output per °C above 25°C cell temp. A panel hitting 60°C on a still summer day produces ~15% less than its rated watts. Ventilation gap under the panel makes a measurable difference.

Solar Sizing Calculator

Size a solar array for a boat, RV, van, or off-grid cabin. Accounts for peak sun hours, system voltage, and real-world losses — not just nameplate watts.

Your inputs

Daily consumption (Wh)e.g., fridge 600Wh + lights 200Wh + nav 200Wh + misc 500Wh = 1500WhPeak sun hours (per day)System voltagePanel wattage (each)Battery chemistryAdjusts the required-solar-Wh upstream by the chemistry’s round-trip efficiency (RTE): LiFePO4 96%, Gel 85%, AGM 83%, Flooded 78%. Lead-acid banks need ~17–28% more solar than LiFePO4 to deliver the same daily Wh. Sources: Battery University BU-808, manufacturer datasheets.

Recommendation

Recommended array

600W (3 × 200W)

Daily charge

168.8 Ah/day

Charge controller

63 A

Cost range (CAD)

$915–$2,445

MPPT recommended — higher efficiency at this array size

Your daily consumption is 1500Wh. With 4.5 peak sun hours, a 75% system efficiency factor, and LiFePO4 (Lithium Iron Phosphate) round-trip efficiency of 96%, you need 463W of solar panels. We recommend 3 × 200W panels (600W total). This array will produce approximately 169Ah per day at 12V. You'll need a charge controller rated for at least 63A. MPPT recommended — higher efficiency at this array size.

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Why “just buy 400W” fails

A 400W array in the Pacific Northwest produces about 1,050 Wh on a good day. In Arizona, the same array produces 2,400 Wh. Peak sun hours — not panel wattage — are usually the gap between “runs my fridge” and “dead battery by Wednesday.”

PowerLab applies a 25% loss factor (wire, heat, angle, controller efficiency) by default. If your panels are flat on a van roof in July, real losses can be much higher — you can tune the loss factor inside the full app after signup.

MPPT vs PWM — when it matters

MPPT recovers 10–30% more energy than PWM, especially in cool weather or when panel voltage doesn’t match battery voltage. Below 200W of panels, the cost of MPPT rarely pays back. Above 400W, MPPT is almost always the right call.

Want loads, batteries, wire sizing, and a shareable PDF with this array in it? Design the full system.

Code references

The Solar Sizing math above sits inside a regulatory framework. Inspectors and underwriters look at the citations below — your installation has to satisfy them, not just produce a number that looks reasonable.

NEC 690.7(A) (2023 / 2026)
Maximum PV system DC voltage is the sum of module Voc × the temperature-correction factor for the lowest expected ambient temperature (use 690.7(A)(1) table or manufacturer datasheet — typically ~1.20× at -25 °C).
Why it matters: The recommended-watts output of this calculator informs the array layout downstream; verify Voc-corrected string voltage stays under your charge controller's max DC input (commonly 100 V, 150 V, or 250 V).
NEC 690.8(A)(1) + 690.8(B)(1) (2023 / 2026)
Maximum PV source/output circuit current = module Isc × 1.25 (irradiance factor). Conductor ampacity must be ≥1.25× of that for continuous duty (so cumulative 1.25 × 1.25 = 1.5625× module Isc).
Why it matters: When you wire the array this calculator recommends, the panel-to-controller cable must clear 1.5625× total Isc. Use the Wire Sizing tool with this number; do not size from nameplate Imp.
NEC 690.9 (Overcurrent protection, 2023 / 2026)
Each ungrounded PV source/output conductor needs an OCPD when more than two parallel strings are connected, or when the array source-circuit current can flow backward into a fault. Series fuse rating ≤ module max-series-fuse rating (per module datasheet).
Why it matters: For arrays at the recommended-watts output of this calculator, any 3+ parallel string layout requires source-circuit fusing — budget for a combiner box even if the BOM here doesn't name one.
ABYC E-11 §11.6 (2018) — PV on boats
Marine PV installations follow E-11 conductor and OCPD rules with adjustments: source-circuit conductors must be marine-rated insulation, panels must be mechanically secured against the marine environment's 1g+ dynamic loading, and DC OCPD must lie within E-11 §11.10.1.3.1 distance limits of the battery.
Why it matters: Boat installs sized with this calculator inherit ABYC marine wiring rules on top of NEC; the difference is non-trivial when picking PV cable insulation (BC-5W2 / BC-7W2 / BC-8W2) and combiner-to-controller routing.
CSA C22.1 §50 (Solar PV systems, 2024 edition)
Off-grid PV in Canada must comply with §50 — module rapid-shutdown for rooftop arrays, working space, and labelling requirements that mirror NEC 690 with CSA-specific labelling text and bilingual (EN/FR) signage where required by jurisdiction.
Why it matters: Canadian off-grid installs sized here need CSA-compliant labels at the array, combiner, and controller — a small but mandatory line item that the watts-only output here doesn't surface.

Citations are paraphrased for brevity. Always work from the most recent published edition of each standard for permit submittals.