Will your solar array actually recharge the bank?
An array sized only to match your daily load can run your devices but never fully refill the battery — and chronic undercharging, not overdischarge, is the most common way off-grid banks die early. Check whether a proposed array has enough surplus to keep your bank healthy.
Why matching the load isn't enough
Your array has to do two jobs on the same solar day: power everything you use that day, and put back the energy you drew from the battery overnight and during cloudy spells. If it is sized only for the first job, there is no surplus for the second, so the bank drifts down and never returns to full. The fix is headroom — a daily harvest comfortably above your daily load.
Lead-acid is least forgiving
Lead-acid (flooded, AGM, gel) must reach a full charge regularly. Left partially charged, the plates sulfate and lose capacity permanently — the single most common cause of early lead-acid failure off-grid. That is why this tool asks for around 1.25x your daily load in harvest for lead-acid, versus about 1.10x for lithium, which tolerates partial-state-of-charge cycling far better.
What to do with the result
If the array is too small, increase the panel wattage to the suggested minimum or beyond, or reduce your daily load. To size a complete system from scratch instead, use the full calculator, which sizes the array with this headroom built in, or the solar array calculator for the bare requirement.
Frequently asked questions
Why is my solar battery never fully charged?
The most common reason is an undersized array: if your panels only produce about as much as you use each day, there is no surplus left to refill the battery, so it never reaches full. Increasing array wattage so daily harvest exceeds daily load fixes it.
How much bigger than my load should my solar array be?
Aim for daily harvest of roughly 1.25x your daily energy use for lead-acid banks (which must reach full charge to avoid sulfation) and about 1.10x for lithium. Cloudier climates need more headroom.
Does chronic undercharging really damage batteries?
Yes. For lead-acid it causes sulfation, a permanent capacity loss and the leading cause of early failure off-grid. Lithium is more tolerant but still benefits from regularly reaching a full, balanced charge.
Looking for a definition or a quick answer? See the solar & battery glossary for terms used on this page, or the FAQ for common sizing questions.
Sources & standards
- NREL PVWatts Calculator — NREL's solar-production model; its default system-loss breakdown underlies the ~0.75 array loss factor used here.
- NREL PVWatts Version 5 Technical Reference — Documents the individual loss terms (soiling, wiring, temperature, availability) that make up the system derate.
- IEEE 1013 — Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems — Widely used engineering practice for days-of-autonomy and depth-of-discharge sizing methods, referenced here for the general sizing approach.
Educational estimate. Real recharge also depends on charge-controller behaviour, cloudy-day patterns and how deeply you cycle. See the methodology.