Enter your daily energy use and backup days below, then choose a battery chemistry and voltage. Formula: Battery Capacity = (Daily Use × Backup Days) ÷ Depth of Discharge ÷ Round-Trip Efficiency ÷ Cold-Weather Capacity Factor.
Your Energy & Backup Needs
Not sure what a field means? Tap the ⓘ next to its label.
Battery Chemistry & Voltage
Choose a chemistry to auto-fill a typical Depth of Discharge and round-trip efficiency, or edit either field yourself.
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.
Cold-Weather Adjustment
Battery capacity drops as temperature drops. Enter the coldest temperature your batteries will actually sit in.
Your Results
This chart shows the nominal battery capacity your same daily use and backup days would require under each chemistry's typical Depth of Discharge and efficiency, holding your entered temperature constant. Your currently selected chemistry is highlighted — useful for weighing upfront battery size against cost and cycle life before you shop.
Results are for reference and estimation only. Verify all values before use in any solar system design or purchase. See full disclaimer below.
Calculate a battery bank size above to enable this section.
Solar arrays and batteries are often purchased and tracked separately. Enter what you're paying for batteries, plus your electric rates, to estimate total investment and a simple Time-of-Use payback. For a full whole-system cost and incentive analysis including your solar array, use the Solar Savings Calculator.
Time-of-Use savings assume one full daily charge/discharge cycle at your entered rates and do not include incentives, demand charges, or battery degradation. For a full analysis including your solar array and available incentives, use the Solar Savings Calculator. Estimates only — verify with your utility's actual rate schedule.
Common daily energy-use levels mapped to an approximate battery bank size, assuming Lithium Iron Phosphate chemistry (90% Depth of Discharge, 96% round-trip efficiency), 1 day of backup, and 77°F installation temperature — the standard battery rating temperature, with no cold-weather capacity loss. Use this to sanity-check a calculated result at a glance — your own inputs above will differ from these defaults.
| Daily Use (kWh) | Battery Capacity | Modules (5 kWh) | Typical Household |
|---|---|---|---|
| 10 | 11.6 kWh | 3 | Small apartment, minimal AC |
| 15 | 17.4 kWh | 4 | Small home, gas heat |
| 20 | 23.1 kWh | 5 | Average home, moderate AC |
| 25 | 28.9 kWh | 6 | Larger home, central AC |
| 30 | 34.7 kWh | 7 | Home with EV charging |
| 40 | 46.3 kWh | 10 | Large home, electric heat |
| 50 | 57.9 kWh | 12 | Large home, pool, EV charging |
A deeper look at what goes into correctly sizing a 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 home 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.
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. Always size the array and the battery bank together — use the Solar Panel Size Calculator alongside this tool to confirm both numbers are compatible.
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.