Solar + Battery Storage Calculator

Why Add Battery Storage to Your Solar Panel System

Solar panels alone are a powerful investment, but they have a fundamental limitation: they only generate electricity when the sun is shining. For most homeowners, peak electricity consumption happens in the evening hours when families return home, cook dinner, run laundry, and use heating or air conditioning. Without a battery, much of the solar energy produced during midday is exported to the grid at a reduced credit rate while expensive grid electricity is purchased during the evening peak.

Adding a home battery storage system fundamentally changes this equation. A battery allows you to store excess solar energy generated during the day and use it during the evening and nighttime hours when your electricity demand is highest. This shift from grid consumption to stored solar consumption is called increased self-consumption, and it is the primary financial driver behind residential battery storage.

Beyond the pure economics, batteries provide resilience. Grid outages are increasing across the United States due to extreme weather events, aging infrastructure, and public safety power shutoffs. A solar-plus-battery system provides reliable backup power that can keep your essential appliances running for hours or even days when paired with solar recharging. For homeowners in areas prone to hurricanes, ice storms, wildfires, or rolling blackouts, this peace of mind has significant value that goes beyond dollars and cents.

The economics of battery storage have improved dramatically in recent years. Lithium-ion battery prices have fallen by over 80% since 2013, and competition among manufacturers like Tesla, Enphase, SolarEdge, Generac, and Panasonic continues to drive prices lower and performance higher. While the federal residential solar tax credit (Section 25D) expired at the end of 2025 for homeowner-owned systems, many states offer their own battery incentives, and third-party-owned systems through leases or power purchase agreements can still access the Section 48E Clean Electricity Investment Tax Credit through the end of 2027.

Understanding Time-of-Use Rates and Battery Arbitrage

Time-of-use (TOU) electricity pricing is becoming the standard rate structure across the United States. Under TOU pricing, electricity costs more during peak demand periods (typically 4 PM to 9 PM on weekdays) and less during off-peak hours (overnight and midday). The price differential between peak and off-peak rates can range from 50% to over 200% depending on your utility.

Battery arbitrage is the practice of charging your battery when electricity is cheap and discharging it when electricity is expensive. With a solar-plus-battery system, you have the best of both worlds: your panels charge the battery for free during the sunny midday hours (which are typically off-peak under modern TOU schedules), and you discharge that stored energy during the expensive evening peak hours.

For example, consider a homeowner in California with a TOU plan that charges 50 cents per kWh during peak hours and 15 cents per kWh during off-peak hours. If their 13.5 kWh battery shifts that entire capacity from off-peak solar to peak consumption each day, they save approximately $4.73 per day, or about $1,725 per year, in TOU arbitrage alone. This does not even include the value of self-consumed solar energy that displaces grid purchases entirely.

Our calculator models these TOU savings by comparing your peak and off-peak rates and applying the differential to the additional energy your battery allows you to consume from solar rather than purchasing from the grid during peak hours. The results account for the fact that real-world round-trip efficiency of lithium-ion batteries is approximately 90%, meaning 10% of stored energy is lost in the charge-discharge cycle.

Battery Sizing Guide: How Much Storage Do You Need?

The right battery size depends on your energy consumption patterns, solar system size, electricity rate structure, and backup power requirements. Here are general guidelines to help you determine the optimal battery capacity for your home.

A single battery unit in the 10-15 kWh range (such as the Tesla Powerwall 3 at 13.5 kWh or the Enphase IQ Battery system at 5 kWh per unit) is sufficient for most homes looking to maximize self-consumption and TOU arbitrage. This capacity can store approximately 4-6 hours of average evening consumption, which covers the typical peak pricing window.

For backup power, consider your essential loads. A refrigerator uses about 0.15 kW continuously, LED lighting runs at 0.1-0.3 kW, a Wi-Fi router needs 0.01 kW, and phone and laptop chargers collectively use about 0.05 kW. Total essential load for most homes is approximately 0.5-1.0 kW, meaning a 13.5 kWh battery provides 13-27 hours of essential backup. If you want to run your HVAC system during an outage, add 2-5 kW to your load requirements, which significantly reduces backup duration unless you install multiple batteries.

The optimal battery size relative to your solar system is typically a ratio of 1.5 to 2.0 kWh of storage per kW of solar. For an 8 kW solar system, 12-16 kWh of storage is ideal. Going larger than this ratio offers diminishing returns because there is not enough excess solar energy to fully charge a larger battery each day, especially during shorter winter days.

Backup Power and Grid Independence

One of the most compelling reasons to add battery storage is energy resilience. Unlike a traditional backup generator that requires fuel and maintenance, a solar-plus-battery system provides clean, quiet, automatic backup power that recharges itself every day from your solar panels. When the grid goes down, a properly configured system seamlessly switches to battery power in milliseconds, keeping your essential circuits running without interruption.

Grid independence is measured as the percentage of your total electricity consumption that comes from your own solar-plus-battery system rather than the utility grid. Without a battery, a typical solar homeowner achieves 25-35% self-consumption because most solar production occurs during the midday hours when consumption is lowest. Adding a right-sized battery increases self-consumption to 60-80%, meaning you can meet the majority of your electricity needs from your own rooftop.

Achieving 100% grid independence (going fully off-grid) is technically possible but economically impractical for most homes. It would require a very large solar array and multiple battery units to cover winter days, extended cloudy periods, and high-draw appliances. The most cost-effective approach is a grid-tied system with battery backup, where you use the grid as a safety net while meeting 60-80% of your needs from your own clean energy system.

The value of backup power extends beyond financial calculations. For families with medical equipment, home offices, or young children, the ability to maintain power during an outage is invaluable. For homeowners in regions with frequent natural disasters, a solar battery can mean the difference between sheltering safely at home and being forced to evacuate.

Popular Home Battery Systems Compared

The residential battery market has matured significantly, with several well-established manufacturers offering reliable, warrantied products. Here is an overview of the most popular options available in 2026.

The Tesla Powerwall 3 remains the market leader with 13.5 kWh of usable capacity, an integrated solar inverter, continuous power output of 11.5 kW, and a 10-year warranty guaranteeing 70% capacity retention. Installed pricing typically ranges from $12,000 to $14,000. Its integrated design simplifies installation and its mobile app provides excellent monitoring and storm-watch features that automatically charge the battery before predicted severe weather.

Enphase IQ Battery 5P offers a modular approach with 5 kWh per unit, allowing homeowners to start small and add capacity over time. Each unit costs approximately $6,000-$7,500 installed. Enphase batteries work exclusively with Enphase microinverters, making them ideal for Enphase-based solar systems. The modular design provides flexibility, but costs can be higher per kWh than single-unit solutions.

SolarEdge Home Battery provides 9.7 kWh per unit and integrates seamlessly with SolarEdge optimizers and inverters. Pricing runs approximately $9,000-$11,000 installed. It is a strong choice for homeowners already using SolarEdge equipment. The Generac PWRcell offers 9-18 kWh of modular capacity with a focus on whole-home backup capability, priced at $10,000-$15,000. Panasonic EverVolt provides 11.4-17.1 kWh options at $12,000-$16,000, with the distinction of working with both AC and DC coupled solar systems.

Battery Degradation and Lifespan

Like all lithium-ion devices, home batteries experience gradual capacity degradation over time. Understanding degradation is important for accurately projecting long-term savings and planning for eventual replacement.

Modern lithium iron phosphate (LFP) batteries, which are now the dominant chemistry in home storage systems, degrade more slowly than the older lithium nickel manganese cobalt (NMC) chemistry. Typical degradation rates are 1.5-3% per year, with most manufacturers guaranteeing at least 70% of original capacity at the 10-year warranty mark. In practice, many LFP batteries retain 80-85% capacity after 10 years under normal residential cycling conditions.

Several factors influence degradation rate. Depth of discharge matters: regularly draining the battery to 0% wears it faster than cycling between 20% and 80%. Temperature plays a significant role, with extreme heat being the primary enemy of battery longevity. Batteries installed in climate-controlled spaces or shaded outdoor locations last longer than those exposed to direct sunlight and high ambient temperatures. Cycling frequency also affects lifespan: a battery cycled once per day will last longer than one cycled two or three times daily.

Our calculator incorporates a conservative 0.5% annual degradation factor on overall system performance. Over 25 years, this represents approximately 12% total degradation, which accounts for both panel and battery aging. If you plan to keep your solar system for its full 25-year lifespan, budget for one battery replacement around year 12-15. The replacement cost will likely be significantly lower than today's prices due to continued technology improvements and manufacturing scale.

Is a Solar Battery Worth It? Key Decision Factors

The decision to add battery storage to your solar system comes down to several key factors that vary by location, lifestyle, and financial priorities. Here is a framework to help you evaluate whether a battery makes sense for your situation.

Electricity rate structure is the most important financial factor. If your utility uses time-of-use pricing with a large peak-to-off-peak differential (30% or more), batteries provide strong arbitrage savings. If you have a flat rate structure, the arbitrage benefit is minimal and the battery investment becomes harder to justify on pure economics alone.

Net metering policy matters significantly. In states with full retail net metering, where you receive full credit for exported solar energy, the financial case for batteries is weaker because you can effectively use the grid as a free battery. However, many states are moving to reduced export compensation (like California's NEM 3.0), where exported solar earns only a fraction of the retail rate. In these markets, batteries become essential for maximizing the value of your solar production.

Power outage frequency adds value that is difficult to quantify. If your area experiences frequent outages lasting several hours or more, the backup value of a battery can be substantial. Consider the cost of spoiled food, lost productivity from home office downtime, the inconvenience of no lighting or climate control, and the potential health risks if you rely on powered medical equipment.

Local incentives can dramatically change the equation. Some states and utilities offer specific battery incentives worth $2,000-$5,000 or more. California's SGIP program, Oregon's Solar+Storage rebate, and Massachusetts' ConnectedSolutions program are examples of programs that can reduce battery costs by 20-40%. Check the DSIRE database at dsireusa.org for current incentives in your area.

Your payback tolerance is the final factor. If you need a payback period under 5 years, batteries may not meet that threshold in most markets. However, if you are comfortable with an 8-12 year payback and value the combination of long-term savings, backup power, and energy independence, a solar battery is a sound investment that will pay for itself well within its useful life.

Our Solar + Battery Storage Calculator above helps you model these scenarios with your specific inputs. By adjusting system size, battery capacity, electricity rates, and self-consumption percentages, you can see exactly how the numbers play out for your unique situation and make an informed decision about whether battery storage is the right addition to your solar energy system.