Sizing an off-grid solar system by hand means juggling four interconnected calculations at once — and a mistake in the first one (energy use) throws off every calculation that follows it. This wizard walks through the same logic a designer would use, in order, so each component is sized against your actual numbers rather than a generic assumption.
The sizing logic, step by step
- Daily energy use sets the foundation — it determines how much the panel array needs to generate and how much the battery bank needs to store.
- Peak simultaneous load is independent of daily energy — a system can have modest daily use but a high peak if several things run at once, and that peak is what the inverter has to survive.
- Sun hours and battery autonomy together determine how oversized the panel array and battery bank need to be to comfortably ride out real-world weather, not just a perfect sunny day.
- Panel array size then drives the charge controller size, since the controller has to handle the full array's output current.
How each number is calculated
| Component | Formula |
|---|---|
| Panel array (W) | Daily Wh ÷ (peak sun hours × 0.75 system efficiency) |
| Battery bank (Ah) | (Daily Wh × autonomy days) ÷ (depth of discharge × battery voltage) |
| Inverter (W) | Peak simultaneous load × 1.25 safety margin, rounded to a standard size |
| Charge controller (A) | (Panel array W ÷ battery voltage) × 1.25 safety margin, rounded to a standard size |
The 0.75 system efficiency factor for the panel array accounts for real-world losses — wiring resistance, temperature effects, dust, and inverter/charge controller conversion losses — that always separate a panel's rated wattage from what actually reaches your battery bank.
What this wizard doesn't replace
- Site-specific shading and orientation analysis — sun hours here are a regional average, not a measurement of your specific roof or ground-mount location.
- Local code requirements — grounding, disconnects, and wiring methods still need to follow your local electrical code regardless of what this wizard recommends.
- Professional design review for larger systems — this is a solid starting point for planning and budgeting, not a substitute for a licensed designer's sign-off on a system that will be permitted or inspected.
To fine-tune any single part of the recommendation, use the dedicated tools: daily load, panel, battery, charge controller, and inverter calculators. For background on system types, see our grid-tied vs off-grid vs hybrid guide.
Frequently asked questions
Why does battery type change the recommended capacity so much?
Depth of discharge is the practical reason — a lead-acid battery bank needs roughly 60% more total capacity than an equivalent LiFePO4 bank to deliver the same usable energy, since only about half of a lead-acid bank's capacity can be used regularly without shortening its life.
Should I size for my average day or my worst day?
For an off-grid system, size for your highest-usage season and factor in realistic autonomy days for your local weather patterns — designing around an unusually good day is the most common way off-grid systems end up under-sized.
Can I use this for a grid-tied hybrid system too?
The panel and inverter math still applies, but battery autonomy matters less on a hybrid system since the grid can cover gaps — you may reasonably choose fewer autonomy days than a true off-grid design would need.