Solar Battery Storage Calculator
Battery storage lets you use solar energy after the sun goes down, provides backup during grid outages, and can reduce demand charges on time-of-use rate plans. This calculator estimates the battery capacity you need based on your energy usage, desired backup duration, and which loads you consider critical.
Estimate Your Battery Storage Needs
Battery sizing depends on simultaneous load (peak kW draw), not just total energy (kWh). A 200A panel with heavy loads may require multiple batteries for adequate peak power even if energy capacity is sufficient. Consult a certified installer for load panel analysis and proper system design. Prices are approximate 2024–2025 installed costs and vary by region, installer, and electrical panel requirements.
Battery Storage Fundamentals
Capacity vs. Power
Battery specs include two key numbers: capacity (kWh) and power output (kW). Capacity determines how long the battery can run — a 13.5 kWh battery at 90% depth of discharge provides 12.15 kWh of usable energy. Power output determines how many appliances can run simultaneously. A battery rated at 5 kW continuous can power loads totaling up to 5,000 watts at once. If your critical loads exceed the battery’s power rating, it will shut down regardless of remaining capacity.
Battery Chemistry Comparison
| Feature | LFP (Lithium Iron Phosphate) | NMC (Nickel Manganese Cobalt) |
|---|---|---|
| Cycle Life | 4,000–6,000+ cycles | 2,000–3,000 cycles |
| Calendar Life | 12–15+ years | 10–12 years |
| Depth of Discharge | 90–100% | 80–90% |
| Energy Density | Lower (heavier per kWh) | Higher (lighter per kWh) |
| Thermal Stability | Excellent (no thermal runaway) | Good (requires active cooling) |
| Cost per kWh | $800–$1,100 installed | $900–$1,200 installed |
| Notable Products | Tesla Powerwall 3, Enphase IQ | LG RESU, older Powerwalls |
Popular Battery Systems
| Product | Usable Capacity | Power Output | Approx. Installed Cost |
|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 11.5 kW continuous | $12,000–$15,000 |
| Enphase IQ Battery 5P | 5 kWh | 3.84 kW continuous | $6,000–$8,000 |
| Franklin WH aPower | 13.6 kWh | 5 kW continuous | $11,000–$14,000 |
| SolarEdge Home Battery | 9.7 kWh | 5 kW continuous | $10,000–$13,000 |
| Generac PWRcell | 9–18 kWh | 4.5–9 kW | $12,000–$20,000 |
Frequently Asked Questions
How many batteries do I need for whole-home backup?
The average U.S. home uses 28–30 kWh per day. For 24-hour whole-home backup at 90% depth of discharge, you need approximately 33 kWh of installed capacity — about 2.5 Powerwall-class batteries. However, most homeowners opt for critical-load backup (50–70% of usage) which requires fewer batteries and significantly lower cost.
Can batteries power my air conditioner?
Central A/C units draw 3,000–5,000 watts during operation with a startup surge of up to 3x. A single Powerwall-class battery can typically run a central A/C unit, but it will drain quickly. A 13.5 kWh battery running a 3,500W A/C unit would last roughly 3–4 hours at full load. Window units (500–1,500W) are much more battery-friendly for outage scenarios.
Do batteries qualify for the federal tax credit?
Yes. Under the Inflation Reduction Act, standalone battery storage systems (not paired with solar) qualify for the 30% ITC starting in 2023. The battery must have at least 3 kWh of capacity. When paired with solar, the battery has always been eligible for the ITC as part of the solar system cost.
How long do home batteries last?
Modern LFP batteries are warranted for 10–15 years or 4,000+ cycles, whichever comes first. With daily cycling, this translates to roughly 12–15 years of useful life before capacity drops below 70–80% of original. NMC batteries typically have shorter cycle life but may last 10–12 years with moderate use.