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.
How to Use This Calculator
Comprehensive Battery Guide
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 When the Grid Is Not There
An off-grid bank has to cover several sunless days at the Depth of Discharge you actually intend to use, not the nameplate on the case. A lithium bank that can use about 90% of nameplate still buys less hardware than a lead-acid bank limited to about 50% for the same usable kilowatt-hours — and on an off-grid design those extra nameplate kilowatt-hours are days of autonomy, not a slightly smaller wall unit. This calculator multiplies daily load by days of autonomy before it applies Depth of Discharge, which is the off-grid order of operations.
Round-Trip Efficiency and Winter Recharge
Off-grid, every percent lost as heat is a percent the array must put back during short winter days. Lithium chemistries typically return about 95–98%; lead-acid typically returns about 75–85%. That gap compounds with a shallow Depth of Discharge: a chemistry that is both inefficient and limited to half its nameplate needs a larger array to recharge the bank in one decent sun day. This tool sizes the PV array and the battery together so that mismatch is visible in the result, not discovered after purchase.
Unheated Sheds and the Coldest Night
Off-grid batteries often sit in a shed or equipment room that tracks outdoor temperature. Capacity is rated near 77°F (25°C). Lithium Iron Phosphate commonly keeps roughly 90% of rated capacity at freezing and around 65% at -20°F; lead-acid can fall to roughly 80% at freezing and 40% at -20°F. Size for the coldest temperature the batteries will actually sit in during the longest autonomy stretch, not a mild annual average. A bank sized only on summer nights is the usual winter failure.
Off-Grid Chemistries — What This Page Is For
Lithium Iron Phosphate is the usual sealed choice for a modern off-grid system: long cycle life, deep usable Depth of Discharge, and no watering. Flooded lead-acid is still common on cabins and farms where upfront cost matters and a vented battery room already exists; Absorbent Glass Mat and Gel avoid watering but keep the shallow Depth of Discharge and shorter cycle life. Nickel Manganese Cobalt, Lithium Titanate, and sodium-ion are here as comparison rows, not as the default cabin bank. Home wall-unit framing, evening peak shifting, and optional cost/payback for a grid-tied house live on the Solar Battery Storage Calculator, not on this page.
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.