How many Powerwalls do I need?

Divide your required capacity by 13.5 kWh (Powerwall usable capacity). A home needing 18-20 kWh of storage needs two Powerwalls. For whole-home backup during outages, you may need 2–3 depending on your consumption patterns.

What is depth of discharge?

Depth of discharge is the percentage of total battery capacity that can be used. A 20 kWh battery with 80% DoD provides 16 kWh of usable energy. Discharging beyond the rated DoD shortens battery life significantly.

Do I need a battery with solar?

Not necessarily. Grid-tied solar without batteries uses net metering for credit. Batteries add value if you have time-of-use rates, want outage backup, or your utility has reduced net metering credits. Batteries are not required for basic solar savings.

How long do solar batteries last?

Modern lithium solar batteries are warrantied for 10–15 years or a set number of cycles (typically 4,000–6,000). Most retain 60–80% of original capacity at warranty end. Actual lifespan depends on cycling depth, temperature, and usage patterns.

Can batteries power my entire home during an outage?

It depends on battery capacity and your consumption. A single Powerwall can power essential loads (lights, fridge, router, charging) for 12–24 hours. Powering everything, including HVAC and cooking, requires 2–4 batteries or load management.

What about lead-acid batteries?

Lead-acid batteries cost less per kWh upfront but have 50% DoD limits, shorter lifespans (5–7 years), and lower round-trip efficiency (80–85% vs 95% for lithium). For daily solar cycling, lithium is more cost-effective over the system lifetime.

Solar Battery Sizing Calculator

Calculate the optimal battery capacity for a solar-plus-storage system. Size your battery bank based on nighttime usage, backup needs, and depth of discharge.

Daily Energy Usage

kWh

Solar Array Size

Autonomy Days

days

Battery Chemistry

System Voltage

Temperature Derating

Safety Margin

Required Battery Capacity

57.1 kWh

After DoD, derating, and safety margin applied

Usable Energy Stored

46.9 kWh

1.5 days of autonomy at 30 kWh/day

Battery Bank (Ah)

1,190.00 Ah

At 48V system voltage

Batteries Needed

4.8 kWh units = 57.6 kWh total

Estimated Cost

$25,710.00

At ~$450.00/kWh for Lithium LFP

Daily Solar Production

50.0 kWh

Surplus: 20.0 kWh/day

Solar vs Consumption Balance

Solar Production: 50.0 kWhDaily Usage: 30 kWh

Battery Bank Sizing by Autonomy

Autonomy	Capacity (kWh)	Ah @ 48V	Est. Cost
0.5 days	19.0 kWh	396.00 Ah	$8,550.00
1 day	38.1 kWh	794.00 Ah	$17,145.00
1.5 days	57.1 kWh	1,190.00 Ah	$25,695.00
2 days	76.2 kWh	1,588.00 Ah	$34,290.00
3 days	114.3 kWh	2,381.00 Ah	$51,435.00

Chemistry Comparison

Chemistry	DoD	Efficiency	Capacity Needed	Cycle Life	Est. Cost
Lithium LFP	95%	96%	57.1 kWh	6,000.00	$25,710.00
Lithium NMC	90%	95%	60.9 kWh	4,000.00	$30,471.00
Lead-Acid (AGM/Gel)	50%	82%	127.1 kWh	1,200.00	$25,417.00
Saltwater	90%	85%	68.1 kWh	3,000.00	$37,461.00

Capacity by Chemistry

Lithium LFP57.1 kWh

Lithium NMC60.9 kWh

Lead-Acid (AGM/Gel)127.1 kWh

Saltwater68.1 kWh

Planning notes, formulas, and examples

About the Solar Battery Sizing Calculator

Adding battery storage to a solar system enables energy independence, backup power during outages, and maximized self-consumption of solar energy. But sizing the battery correctly is critical — too small and you still rely heavily on the grid, too large and you overspend on capacity you'll never use.

Battery sizing depends on your nighttime energy consumption (when solar isn't producing), desired backup hours during an outage, and the battery's depth of discharge (DoD). Modern lithium batteries can discharge to 80–90% of their rated capacity, while older lead-acid batteries should only be discharged to 50%.

This calculator determines the total battery capacity you need based on your nightly energy consumption, desired backup duration, and the DoD of your chosen battery technology. The result tells you the minimum rated capacity to purchase.

Understanding this metric in precise terms allows energy managers to evaluate investment options, forecast savings, and build compelling business cases for efficiency upgrades and retrofits.

When This Page Helps

Battery storage is the most expensive component of a solar-plus-storage system. Proper sizing ensures you get the reliability you need without overspending on excess capacity. Regular monitoring of this value helps energy teams detect usage anomalies early and address equipment malfunctions or operational issues before they drive utility costs higher.

How to Use the Inputs

Enter the kWh you consume during non-solar hours (evening through morning).
Enter desired backup hours during a grid outage.
Enter your average hourly consumption during an outage.
Enter the battery depth of discharge (80–90% for lithium, 50% for lead-acid).
Review the recommended total battery capacity.

Formula used

Nightly Storage = Nightly kWh / DoD
Backup Storage = Backup Hours × Hourly Load / DoD
Recommended Capacity = max(Nightly Storage, Backup Storage)

Example Calculation

Result: 18.75 kWh total battery capacity

Nightly storage: 12 / 0.80 = 15.0 kWh rated. Backup storage: 10 × 1.5 / 0.80 = 18.75 kWh rated. The backup scenario requires more capacity, so 18.75 kWh is the recommended minimum — roughly equivalent to 1.4 Tesla Powerwalls.

Tips & Best Practices

A Tesla Powerwall is 13.5 kWh usable; an Enphase IQ Battery 10 is 10.08 kWh.
Focus on nighttime consumption, not total daily usage — daytime loads are covered by solar.
Consider which circuits are essential during an outage to reduce backup requirements.
Lithium iron phosphate (LiFePO4) batteries last longer than NMC lithium in daily cycling.
In time-of-use rate areas, batteries can shift solar power to expensive evening hours for extra savings.
Most batteries degrade 2–3% per year — size slightly larger for year-10 performance.

Grid-Tied vs Off-Grid Battery Sizing

Grid-tied batteries typically cover nighttime consumption and outage backup. Off-grid batteries must handle all non-solar hours plus multiple days of autonomy. Grid-tied systems need 10–30 kWh; off-grid may need 40–100+ kWh.

Time-of-Use Optimization

In TOU rate areas, batteries charge from solar during cheap midday hours and discharge during expensive evening peaks. This arbitrage can save $30–60/month beyond what net metering provides alone.

Battery Chemistry Comparison

Lithium iron phosphate (LFP) batteries offer the best cycle life and safety for stationary storage. Nickel-manganese-cobalt (NMC) batteries are lighter and more energy-dense but have shorter cycle lives. Lead-acid is cheapest upfront but has the worst lifetime economics.

Sources & Methodology

Last updated: February 9, 2026

Frequently Asked Questions

Divide your required capacity by 13.5 kWh (Powerwall usable capacity). A home needing 18-20 kWh of storage needs two Powerwalls. For whole-home backup during outages, you may need 2–3 depending on your consumption patterns.