System Design.

Why design must come before purchase

Every component in an off-grid system is sized relative to the others. The battery bank determines how many panels you need. The panels determine what size charge controller you need. The inverter is determined by your peak load. Start with the wrong number at any point and the error propagates through every downstream purchase.

Vendors sell systems in round numbers because round numbers are easy to market. A “10kW system” means nothing without knowing your location, your load profile, your seasonal variation, and how many days of autonomy you require. The Solar Calculator exists precisely because the round-number approach destroys real money in the field.

The engineer in East Tennessee who designed his own cabin but handed system sizing to a contractor and watched $22,000 disappear in three years of failures. The farmer in Nebraska who was told a 6kW system would handle his well pump and grain auger — it could not, and he found out in fall harvest. The retiree in New Mexico who overbought panels but undersized his battery bank and spent the first winter watching his system throw charge all afternoon with nothing to store it in. The couple in Montana who bought a kit because it looked complete and spent two seasons adding to it piece by piece, paying retail every time. This guide is the design process that prevents every one of those situations.

Load calculation — the foundation of every system

A load calculation is a complete inventory of everything your system needs to power, how many watts each item draws, and how many hours per day it runs. The result is your total daily watt-hour consumption — the single most important number in your system design.

Add 20% to your calculated load to account for inefficiency losses — inverter conversion losses, wire resistance, and the fact that appliances often draw more than their rated wattage during startup. This gives your design load: the number your battery bank and panel array must reliably serve every day.

Do not average your load. Design for your peak season consumption. A system that handles winter is oversized for summer — and that is exactly right.

Peak sun hours — location matters more than you think

Peak sun hours are not total daylight hours. They are the equivalent number of hours per day at which solar irradiance equals 1,000 watts per square meter — the standard test condition for panel ratings. A location receiving 5 peak sun hours per day generates twice the energy from the same panel array as a location receiving 2.5 peak sun hours.

The National Renewable Energy Laboratory (NREL) publishes peak sun hour data for every US location. Design to your worst-month value — December in Montana, not July. A system sized for your worst month will exceed your needs every other month of the year.

The U.S. Department of Energy data shows that solar panel output in December at 45°N latitude can be 40–60% lower than July output for the same array. If you design to the summer number, your system will fail every winter.

Battery bank design — capacity, voltage, and autonomy

Your battery bank must store enough energy to power your home through the night plus your designed number of days without solar input. The formula is straightforward, but the inputs must be correct — garbage load calculation in means undersized battery bank out.

Example: 3,048Wh daily load (2,540Wh + 20% inefficiency buffer) × 3 days autonomy = 9,144Wh ÷ 0.8 (LiFePO4) = 11,430Wh needed. At 48V: 11,430 ÷ 48 = 238Ah. Round up to the next standard battery configuration — in practice, 280Ah or 300Ah at 48V.

Always round battery bank sizing up, never down. A 10% larger bank lasts years longer. A 10% smaller bank hits its depth-of-discharge limit every night and fails years early.

Solar array sizing — panels, tilt, and string configuration

The solar array must refill the battery bank in a single day's production — your worst-month production day. Array size is determined by your daily load and your location's peak sun hours.

Example: 3,048Wh daily load ÷ 3.5 PSH (Montana December) ÷ 0.85 = 1,026W array minimum. Round up to 1,200W — four panels at 300W each, or three at 400W. Always oversize the array slightly; panels degrade 0.5–0.8% per year and production losses from dust, temperature, and wiring resistance are real.

Panel tilt: Optimal tilt angle equals your latitude. At 45°N, tilt panels at 45°. For winter optimization in northern climates, increase tilt by 10–15° — this maximizes December output when the sun is low in the sky. Fixed-tilt ground mounts outperform rooftop mounts in most off-grid applications because adjustment is easier and shading is controllable.

Never mix panel brands, wattages, or ages in the same string. A single weak panel reduces the output of every other panel in its string to its level.

Charge controller sizing — MPPT math

The charge controller is sized to handle the maximum current output from your panel array. With MPPT controllers, this calculation accounts for the voltage conversion between panel open-circuit voltage (Voc) and battery bank voltage.

Never spec a charge controller at its rated maximum. Size up to the next standard rating — a 40A controller on a 31A calculated load, not a 30A controller at 99% of its capacity. Controllers running at their limit run hot and degrade faster.

Inverter selection — surge capacity and continuous load

The inverter is sized to the peak load it must handle — not your average load, not your typical daily draw, but the maximum simultaneous load any combination of your appliances can produce. This includes surge current for motors on startup.

Motor-driven loads — well pumps, refrigerator compressors, air conditioning, power tools — draw two to five times their rated wattage for the first few seconds at startup. Your inverter's surge rating must cover this. An inverter rated 3,000W continuous / 6,000W surge will handle a well pump that draws 750W continuously but 2,500W at startup.

Pure sine wave only. Modified sine wave inverters damage sensitive electronics, degrade motor efficiency, and produce audible noise in audio equipment. There is no legitimate trade-off here.

Wire sizing and system layout

Wire sizing in a DC off-grid system has two goals: safe current capacity and acceptable voltage drop. Undersized wire does not just fail dramatically — it fails slowly, degrading efficiency and creating heat in connections over months or years before anything obvious happens.

DC systems carry higher current than equivalent AC systems for the same power — Ohm's Law. A 48V system at 3,000W carries 62.5 amps. At 12V, the same 3,000W means 250 amps. This is why 48V matters: the wire and connection hardware for 62.5A is practical. For 250A, it is expensive, heavy, and complicated.

Use the NEC Article 690 wire sizing tables for DC solar circuits. Target 1% or lower voltage drop on all DC runs — battery to inverter, charge controller to battery, panel array to charge controller. Use an online voltage drop calculator for every run and document the results in your design file.

Minimize cable run length. Every foot of wire adds resistance and heat. Place the charge controller as close to the battery bank as physically possible, and the battery bank as close to the inverter as the installation allows.

Planning for expansion — build what you can grow

Off-grid energy needs grow. You add a second refrigerator, a chest freezer, a power tool shop, a second well pump. Design the original system to accommodate expansion without replacing core hardware.

The U.S. Department of Energy notes that residential energy consumption tends to increase 1–2% annually even in households actively managing usage. A system designed with a 20–30% headroom buffer stays viable for a decade without forced replacement of core hardware.

The most expensive design mistakes

These are not theoretical. Every one of these destroyed real money in the field.