Solar Battery Calculator
Determine how many batteries you need for home backup power. Factor in daily energy usage, desired backup duration, and depth of discharge.
US average: ~30 kWh/day
Duration of backup power
Capacity of each battery unit
Typical: 80-90% for lithium-ion
How Solar Batteries Work
A solar battery stores excess electricity generated by your solar panels during the day so you can use it at night, during peak rate hours, or during power outages. The battery connects to your solar system through a hybrid inverter or a dedicated battery inverter that manages the flow of electricity between the panels, battery, home loads, and the grid.
During daylight hours, your solar panels generate electricity. Any energy your home does not consume immediately flows into the battery until it is full. Once the battery reaches capacity, additional surplus is exported to the grid (if net metering is available). When the sun goes down or your panels cannot meet demand, the battery discharges to power your home before you draw from the grid.
Most modern home batteries use lithium-ion chemistry, the same fundamental technology found in smartphones and electric vehicles. These batteries are compact, efficient (round-trip efficiency of 90-95%), and capable of thousands of charge-discharge cycles over their lifespan. The battery management system (BMS) inside each unit monitors cell voltage, temperature, and state of charge to ensure safe and optimal operation.
How to Size Your Solar Battery System
Proper battery sizing depends on three key factors: your daily energy consumption, the amount of backup time you want, and the battery's usable capacity (depth of discharge). Here is the step-by-step process:
Find your daily energy usage
Check your utility bill for monthly kWh, then divide by 30. The US average is about 30 kWh per day, but homes in hot climates with AC may use 40-50 kWh.
Decide your backup coverage
Whole-home backup requires matching your full daily usage. Essential loads only (lights, fridge, internet, medical devices) typically need 10-15 kWh per day.
Calculate required storage
Multiply your hourly usage (daily kWh / 24) by the number of backup hours you need. Divide by the battery's depth of discharge (typically 90%) to get the required rated capacity.
Batteries Needed = (Daily kWh / 24 x Backup Hours) / (Battery Capacity x DoD)
For a home using 30 kWh per day that wants 24 hours of backup with 13.5 kWh batteries at 90% DoD: (30/24 x 24) / (13.5 x 0.9) = 30 / 12.15 = 2.5, rounded up to 3 batteries. For essentials-only backup (10 kWh), one battery is often sufficient.
Backup Duration Calculation Examples
| Scenario | Daily Usage | Batteries | Usable kWh | Backup Time |
|---|---|---|---|---|
| Essentials only | 10 kWh | 1 x 13.5 kWh | 12.15 kWh | 29 hours |
| Average home | 30 kWh | 2 x 13.5 kWh | 24.3 kWh | 19 hours |
| Large home | 45 kWh | 3 x 13.5 kWh | 36.45 kWh | 19 hours |
| Full-day whole-home | 30 kWh | 3 x 13.5 kWh | 36.45 kWh | 29 hours |
* Based on Tesla Powerwall 3 (13.5 kWh, 90% DoD). Actual backup time varies with real-time usage patterns.
Solar Battery Types Compared
The two dominant battery chemistries for home energy storage are NMC (nickel manganese cobalt) and LFP (lithium iron phosphate). Each has distinct advantages that affect performance, safety, and longevity.
| Feature | NMC (Li-ion) | LFP (LiFePO4) |
|---|---|---|
| Energy Density | Higher (more compact) | Lower (larger footprint) |
| Cycle Life | 3,000-5,000 cycles | 5,000-10,000 cycles |
| Depth of Discharge | 80-90% | 95-100% |
| Thermal Stability | Good (requires cooling) | Excellent (inherently safer) |
| Cost per kWh | $400-$600 | $350-$500 |
| Popular Examples | Tesla Powerwall 2, LG RESU | Tesla Powerwall 3, Enphase, BYD |
The industry is rapidly shifting toward LFP chemistry due to its superior cycle life, better safety profile, and lower cost. Tesla's Powerwall 3, released in 2024, switched to LFP cells. For homeowners, LFP batteries offer longer warranties (often 10-15 years) and can handle daily cycling without significant degradation.
Top Home Battery Brands in 2026
| Brand / Model | Capacity | Power Output | Chemistry | Price (Installed) |
|---|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 11.5 kW continuous | LFP | $9,200-$12,500 |
| Enphase IQ 5P | 5 kWh | 3.84 kW continuous | LFP | $7,000-$9,000 |
| Franklin WholePower | 13.6 kWh | 10 kW continuous | LFP | $10,000-$14,000 |
| SolarEdge Home Battery | 9.7 kWh | 5 kW continuous | LFP | $8,500-$11,000 |
| Generac PWRcell | 9-18 kWh | 9 kW continuous | NMC | $12,000-$18,000 |
* Prices include typical installation costs and vary by region. All qualify for the 30% federal tax credit when installed with solar.
Battery Cost, ROI, and Incentives
Home batteries cost between $8,000 and $18,000 installed, depending on capacity and brand. When paired with a solar installation, the 30% federal ITC reduces your effective cost significantly. Some states offer additional battery incentives: California's SGIP program, Oregon's battery rebate ($2,500-$5,000), and Massachusetts' ConnectedSolutions pay-for-performance program.
The financial ROI of a battery depends heavily on your utility's rate structure. With time-of-use (TOU) rates, batteries can save $50-$150 per month by shifting solar energy to expensive peak hours. In states with poor net metering (like California post-NEM 3.0), batteries are especially valuable because they let you self-consume solar energy rather than exporting it at low wholesale rates.
Beyond financial savings, batteries provide peace of mind during grid outages. With extreme weather events increasing in frequency, blackout protection is a significant non-financial benefit. A single 13.5 kWh battery can keep essential loads running for 24+ hours, and solar panels recharge the battery each day for potentially indefinite off-grid operation.
Whole-Home vs. Partial Backup
When designing a battery backup system, you must choose between whole-home backup and partial (essential loads) backup. This decision significantly affects cost and the number of batteries needed.
Whole-Home Backup
- + Powers everything including HVAC, dryer, EV charger
- + Seamless transition during outages
- - Requires 2-4 batteries ($18,000-$40,000)
- - May need electrical panel upgrade
Essential Loads Only
- + 1 battery is usually sufficient ($9,000-$13,000)
- + Longer backup duration per battery
- - Cannot run large loads (AC, dryer, EV)
- - Requires a dedicated essential loads subpanel
Most homeowners start with essential loads backup and add batteries over time. The Tesla Powerwall 3 simplifies whole-home backup with its high 11.5 kW continuous output, capable of running most loads from a single unit. To plan your complete solar and battery system, start with the Solar Panel Calculator to size your panels, then use the Solar Payback Calculator to evaluate ROI.
Time-of-Use Rate Optimization
Many utilities now charge different electricity rates depending on the time of day. Time-of-use (TOU) rates typically have three tiers: off-peak (cheapest, usually overnight), mid-peak, and on-peak (most expensive, typically 4 PM to 9 PM). Solar batteries excel at TOU arbitrage: charging from solar during cheap midday hours and discharging during expensive evening peak hours.
For example, if your utility charges $0.12/kWh off-peak and $0.45/kWh on-peak, a 13.5 kWh battery cycling daily saves you approximately $0.33 x 12.15 kWh = $4.01 per day, or about $1,464 per year. Over 10 years, that is $14,640 in savings from rate optimization alone, not counting backup power value or solar self-consumption benefits.
Smart battery systems automatically learn your usage patterns and TOU rate schedules to maximize savings. Combined with solar panels, a battery can reduce your grid purchases during peak hours to near zero. Use our Electricity Cost Calculator to understand your current costs, and explore our Home Electrification Planner to see how electrifying your entire home with solar and batteries compares to fossil fuel alternatives.
Frequently Asked Questions
How many batteries do I need for my house?
The average US home uses about 30 kWh per day. For a full day of backup with a 13.5 kWh battery (like the Tesla Powerwall) at 90% depth of discharge, you would need about 3 batteries. For essential loads only (10-15 kWh), 1-2 batteries are usually enough.
What is depth of discharge (DoD)?
Depth of discharge is the percentage of battery capacity that can be used before recharging. Most lithium-ion batteries have a DoD of 80-90%. A 13.5 kWh battery with 90% DoD provides 12.15 kWh of usable energy.
How long will a solar battery last?
Most home solar batteries come with a 10-year warranty and can last 10-15 years. Lithium-ion batteries typically retain 70-80% of their original capacity after 10 years of regular cycling.