Choose Quick Entry or the Detailed Appliance List below, then set your system requirements. Formula: PV Array = (Daily Load ÷ Inverter Efficiency) ÷ Peak Sun Hours × (1 + Safety Margin); Battery Bank = (Daily Load × Days of Autonomy) ÷ Depth of Discharge ÷ Efficiency ÷ Cold-Weather Factor.
Your Daily Energy Use
Not sure what a field means? Tap the ⓘ next to its label.
Pick a preset to auto-fill typical wattage, or choose Custom Appliance and enter your own. Totals update live.
Your System Setup
Choosing a battery chemistry auto-fills a typical Depth of Discharge and efficiency — either field stays editable.
With a generator able to recharge the bank or carry the load, the battery only needs to be sized for about half your entered Days of Autonomy — this calculator applies a 50% reduction to Days of Autonomy for the battery formula only when this is on.
The most common lithium chemistry for home solar storage. Long cycle life, stable, and safe, with a high usable Depth of Discharge.
A lithium chemistry common in electric vehicles and some home batteries. Slightly higher energy density than LFP, with a somewhat shorter cycle life.
A premium lithium chemistry with an exceptionally long cycle life and strong cold-weather performance, at a higher cost than LFP or NMC.
An emerging, cobalt-free chemistry with lower energy density than lithium options but good cold-weather tolerance and lower raw material cost.
The traditional, lowest-upfront-cost option. Limited to a much shallower Depth of Discharge and shorter cycle life than lithium chemistries. Choose a subtype below.
Your Results
Results are estimates for planning purposes only and do not replace a licensed solar installer's site assessment.
A deeper look at what goes into correctly sizing an off-grid battery bank, and how to choose between chemistries — written for first-time buyers and job-site reference alike.
Depth of Discharge — Why It Drives Bank Size More Than Anything Else
Depth of Discharge is the share of a battery's total, nameplate capacity you're allowed to use before recharging. A lithium bank at 90% Depth of Discharge needs far less nameplate capacity than a lead-acid bank at 50% Depth of Discharge to deliver the same usable energy — this single number is usually the biggest reason lithium banks are physically smaller than lead-acid banks with the same real-world backup capability.
Round-Trip Efficiency
No battery returns 100% of the energy put into it — some is lost as heat during charging and discharging. Lithium chemistries typically return 95–98%; lead-acid chemistries typically return 75–85%. This loss compounds with Depth of Discharge, so a chemistry with both a shallow Depth of Discharge and low efficiency needs meaningfully more nameplate capacity for the same job.
Why Cold Weather Matters
Battery capacity is rated at a standard temperature, typically 77°F (25°C), and drops as the battery gets colder. Lithium chemistries hold up better in cold than lead-acid — Lithium Iron Phosphate can retain roughly 90% of rated capacity at freezing and around 65% at -20°F, while lead-acid can drop to roughly 80% at freezing and 40% at -20°F. Size for the coldest temperature the batteries will actually sit in, not your regional average.
Choosing a Chemistry — The Real Trade-off
Lithium Iron Phosphate is the standard choice for most off-grid solar storage today: long cycle life, a stable and safe chemistry, and a deep usable Depth of Discharge. Nickel Manganese Cobalt offers slightly higher energy density at somewhat shorter cycle life. Lithium Titanate costs more but leads on cycle life and cold-weather performance. Sodium-ion is a newer, cobalt-free option worth watching for lower material cost and strong cold tolerance, at lower energy density. Lead-Acid — Flooded, Absorbent Glass Mat, or Gel — remains the lowest upfront cost, at the expense of a shallow Depth of Discharge, a shorter cycle life, and, for Flooded specifically, a ventilation requirement under electrical code.
Generator Backup — How It Changes Sizing
A generator that can recharge the battery bank or carry the load directly during an extended low-sun stretch means the battery no longer has to be the sole backstop — many off-grid designs size the bank for roughly half the autonomy they'd otherwise plan for. This calculator's Generator Backup toggle, next to Days of Autonomy, applies that reduction to the battery formula only — your solar array is still sized for your full daily load either way.
Common Sizing Mistakes
The most frequent errors are: using a single generic Depth of Discharge number for every chemistry instead of the chemistry actually being purchased, ignoring cold-weather capacity loss for a bank that will sit in an unheated space, and sizing the battery bank without checking that the paired solar array can actually recharge it within a single sunny day. This tool sizes both together for exactly that reason — check the PV Array and Battery Bank results against each other before buying.
This guide is provided for educational and reference purposes. Always confirm system specifications with a licensed solar installer or electrician before purchasing or installing equipment.