Solar Battery Calculator

How much battery storage does your solar system need? Enter your usage and backup hours — get your answer.

Your energy needs

Daily energy usage?

kWh/day

Backup duration?

Depth of discharge?

Battery bank voltage?

Round-trip efficiency?

Battery storage needed

13.2 kWh (274 Ah at 48V)

Energy for backup10.0 kWh

100Ah batteries needed3

200Ah batteries needed2

Capacity at 80% DoD274 Ah

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How to Use This Calculator

Enter your daily energy usage

The daily energy usage field is the foundation of battery sizing. Find it on your utility bill — take your monthly kWh and divide by 30. The US average home uses about 30 kWh/day. If you're sizing for essential loads only (lights, fridge, phone charging), a realistic number is 3–8 kWh/day.

Choose backup duration

The backup duration controls how many hours your battery needs to power your home without solar input or grid. 8 hours covers an overnight period. 12 hours gets you through cloudy mornings. 24 hours means full off-grid independence for one day — pair it with the off-grid calculator to size the solar array too.

Set depth of discharge

Depth of Discharge (DoD) is the percentage of battery capacity you actually use before recharging. Lithium iron phosphate (LiFePO4) batteries can safely discharge to 80–95%. Lead-acid batteries should only go to 50% to preserve cycle life. Using a higher DoD means each battery does more work — but pushing lead-acid past 50% degrades it fast.

Pick your system voltage

The battery bank voltage affects how many amp-hours (Ah) your bank needs. Higher voltage means lower current for the same power — 48V systems use thinner, cheaper wiring and have lower resistive losses. Most modern home battery systems run at 48V. Small RV or cabin setups often use 12V or 24V.

Round-trip efficiency

Round-trip efficiency accounts for energy lost during the charge-discharge cycle. LiFePO4 batteries are 95–98% efficient. Lead-acid: 80–85%. This means if you store 10 kWh in a lead-acid battery, you only get 8–8.5 kWh back out.

The Formula

Battery storage is sized from your energy need, adjusted for DoD and efficiency losses:

Power per hour = Daily kWh ÷ 24 hours Energy for backup = Power per hour × Backup hours Required kWh = Energy for backup ÷ (DoD × Round-trip efficiency) Required Ah = (Required kWh × 1000) ÷ System voltage Batteries needed = Required Ah ÷ Battery Ah rating (100Ah or 200Ah)

The key insight: your battery bank must hold more capacity than you actually use, because DoD and efficiency prevent you from accessing 100% of the rated capacity. A 100Ah battery at 80% DoD and 95% efficiency only delivers 76Ah of usable energy.

Example

The Martinez family — Phoenix, AZ

The Martinez family uses 35 kWh/day and wants 12 hours of whole-home backup during monsoon outages. They're installing a 48V lithium (LiFePO4) system with 80% DoD and 95% round-trip efficiency.

Daily usage35 kWh/day

Backup hours12 hours

Depth of discharge80%

System voltage48V

Round-trip efficiency95%

Result

Average power draw1.46 kW

Energy for 12-hr backup17.5 kWh

Required battery capacity23.0 kWh

Required at 48V479 Ah

100Ah batteries needed5 batteries

200Ah batteries needed3 batteries

Five 100Ah 48V LiFePO4 batteries would provide the Martinez family with 12 hours of whole-home backup. At roughly $800–$1,200 per 100Ah 48V battery, the bank costs $4,000–$6,000 before installation. Combined with a solar array, this system qualifies for the 30% federal ITC.

FAQ

What is depth of discharge and why does it matter? ▼

Depth of Discharge (DoD) is the percentage of a battery's total capacity that you discharge before recharging. If a 100Ah battery is discharged to 80% DoD, you've used 80Ah. Lithium iron phosphate (LiFePO4) batteries tolerate 80–95% DoD with minimal degradation — some are rated at 100% DoD. Lead-acid batteries should be kept above 50% remaining charge; repeatedly discharging deeper cuts cycle life dramatically, from 500 cycles to under 200.

How many kWh of battery storage does a home need? ▼

For an average US home (30 kWh/day), 8 hours of whole-home backup requires roughly 15–20 kWh of battery storage. Essential-loads-only backup (fridge, lights, router, phone) needs only 3–5 kWh for 8 hours. A Tesla Powerwall holds 13.5 kWh — one unit covers most essential loads overnight. Two cover most whole-home needs. Enter your actual daily usage above for a precise number.

What voltage should my battery bank be — 12V, 24V, or 48V? ▼

For home solar systems above 2 kW, 48V is the right choice. Higher voltage means lower current for the same power (P = V × I), which means thinner wiring, smaller charge controllers, and lower resistive losses. 12V is fine for small off-grid setups under 600W — RV fridges, cabin lighting. 24V bridges the gap for 600W–2kW systems. Most modern all-in-one inverter/chargers (Victron, EG4, Growatt) are 48V.

LiFePO4 vs lead-acid: which battery should I use? ▼

LiFePO4 (lithium iron phosphate) costs 2–3x more upfront but lasts 5–10x longer (3,000–6,000 cycles vs 300–600 for lead-acid). Over a 10-year period, LiFePO4 is typically cheaper per kWh delivered. LiFePO4 is also lighter, tolerates deeper discharge, charges faster, and has higher round-trip efficiency (95–98% vs 80–85%). Lead-acid makes sense for low-budget, low-cycle applications or very cold climates where lithium performance drops. Use our Battery Sizing Calculator for a full cost-of-ownership comparison.

Can I add batteries to an existing solar system? ▼

Yes, with the right equipment. If your existing system has a hybrid inverter (Victron, SolarEdge StorEdge, Enphase IQ), adding batteries is straightforward. If you have a standard string inverter, you'll need to add an AC-coupled battery system (like the Tesla Powerwall) or replace the inverter with a hybrid unit. The battery addition qualifies for the 30% federal ITC if it's charged at least 90% by solar.