Reading the three spec sheets

Designing a system is mostly a matter of making three datasheets agree with each other: the module, the inverter, and (if you have one) the battery. Once you can read these, the sizing math in the next modules is just plugging numbers in. Let’s decode each one.

1. The module (panel) datasheet

The numbers that matter for design, all quoted at STC (Standard Test Conditions — 25 °C, full sun):

• Pmax — rated power in watts (e.g. 440 W). Multiply by panel count for array size.
• Vmp / Imp — voltage and current at the maximum-power point (the panel’s normal operating point).
• Voc / Isc — open-circuit voltage and short-circuit current: the maximums the panel can hit. These set the safety limits your strings must stay under.
• Temperature coefficients — how much voltage/power change per °C. Voltage rises as panels get cold and falls as they get hot. This is the whole reason string sizing is temperature-dependent (Module 4).
• NOCT — a more realistic cell temperature than STC; useful for sanity-checking real-world output.
• Efficiency & dimensions — how much power per square meter, and whether the panels fit your roof.

Reality check: STC is a lab. On a hot Texas roof your panels run well above 25 °C, so they produce a bit less than the sticker — exactly what the simulator models as temperature loss.

2. The inverter datasheet

This is where most sizing constraints live:

• MPPT voltage range — the min–max input voltage each MPPT will operate in (plus a higher startup voltage). Your string’s voltage must stay inside this window in cold and hot weather.
• Max input current per MPPT — caps how many strings you can parallel onto one input.
• Number of MPPTs — how many independent groups (e.g. roof faces) you can run separately.
• Max DC input / max short-circuit current — the array-side ceilings.
• AC output (kW) — the inverter’s rated output. The ratio of array DC to this is your DC/AC ratio (Module 6).
• Efficiency (CEC/peak) and hybrid/battery support — needed if you’re adding storage.

3. The battery datasheet

• Usable capacity (kWh) — how much energy you can actually draw (already accounts for depth of discharge).
• Continuous / peak power (kW) — how much it can deliver at once (can it start your AC?).
• Nominal voltage — most home units are a fixed system voltage; you scale capacity by adding units in parallel (Module 7).
• Round-trip efficiency — energy out ÷ energy in; some is always lost in the cycle.
• Chemistry & warranty cycles — LFP vs. NMC, and how many cycles it’s rated for.

How they fit together (preview)

The design game is making three things true at once: the string’s voltage stays inside the inverter’s MPPT window across all temperatures (Module 4), the paralleled strings’ current stays under the MPPT’s limit (Module 5), and the array’s total power sits at a sensible ratio to the inverter’s AC output (Module 6). Every number above feeds one of those three checks.

Next up: panel groups — why you split panels by roof face, tilt, and shade before you size a single string.