Solar Battery Storage Calculator
Calculate how much battery storage you need based on your solar generation, daily usage, and backup goals. Get an instant capacity estimate and installed cost range.
Find on your utility bill
Leave at 0 if no solar panels
100% of daily usage — all circuits
1–7 days
💡About this calculator▼
Whether you're pairing batteries with a new solar system or adding storage to an existing one, sizing the battery bank correctly is the first — and most consequential — decision you'll make. Too small and you run out of power before the sun comes up or the grid comes back. Too large and you've spent tens of thousands on capacity you'll never use.
This calculator estimates the battery capacity you need based on your daily energy usage, how much your solar system generates each day, how much of your home you want to back up, and how many days of autonomy you're targeting. It outputs the required storage in kilowatt-hours, an equivalent number of Tesla Powerwall 2 units for reference, and an installed cost range based on current national averages.
The calculator also works for battery-only systems with no solar — just leave the solar generation field at zero.
The calculator finds the net daily load your battery needs to cover, then applies a coverage ratio, multiplies by backup days, and adjusts upward by roughly 11% to account for battery round-trip efficiency losses.
When your solar generation is less than your daily usage, the battery covers the difference. When solar exceeds daily usage, the calculator applies an overnight floor equal to 50% of your daily usage — because solar generates nothing for roughly 12 hours each night, and your battery always needs to store enough to get you through until sunrise. This floor prevents the calculator from showing zero storage needed even when your panels produce more than you consume in a day.
Coverage ratios reflect real-world load shedding in an outage:
Whole Home (100%) covers all circuits with no changes to how you use power. Essential Loads (40%) covers lighting, refrigeration, HVAC fan, and key outlets — roughly what a dedicated transfer panel handles. Critical Only (20%) covers lighting and the refrigerator only, the bare minimum for livability.
The 90% round-trip efficiency factor accounts for energy lost converting AC to DC for storage and back to AC for use. Real-world efficiency varies by battery chemistry, but 90% is a reasonable planning assumption for most residential systems.
Cost estimates use $800–$1,200 per usable kWh installed, reflecting current ranges for whole-system installed costs including battery hardware, inverter, installation labor, and permitting.
📐How it's calculated▼
Overnight floor = Daily usage × 0.50 (minimum load regardless of solar)
Net daily load = max(Overnight floor, Daily usage − Solar generation)
Required capacity (kWh) = (Net daily load × Coverage ratio × Backup days) ÷ 0.90
Coverage ratios:
• Whole Home: 1.00 (100% of net daily load)
• Essential Loads: 0.40 (40% — lights, fridge, HVAC fan, outlets)
• Critical Only: 0.20 (20% — lights and refrigerator only)
Powerwall 2 equivalent = Required kWh ÷ 13.5, rounded up to nearest whole unit
Low installed cost = Required kWh × $800
High installed cost = Required kWh × $1,200
Example: 30 kWh/day household, 25 kWh solar generation, Whole Home coverage, 1 day of backup
→ Overnight floor: 30 × 0.50 = 15 kWh
→ Solar offset: 30 − 25 = 5 kWh
→ Net daily load: max(15, 5) = 15 kWh (floor wins — solar covers daytime but not overnight)
→ Raw need: 15 × 1.00 × 1 = 15 kWh
→ Efficiency-adjusted: 15 ÷ 0.90 = 16.7 kWh
→ Powerwall 2 equivalent: ⌈16.7 ÷ 13.5⌉ = 2 units
→ Installed cost range: $13,360 – $20,040
📎Source: U.S. Energy Information Administration — Residential Energy Consumption Survey
🔍Finding your inputs▼
Daily Energy Usage: Find this on your electric bill — most utilities report monthly kWh consumed, so divide by 30 to get a daily average. U.S. residential averages around 29–33 kWh/day, but varies significantly by home size, climate, and whether you have an EV or electric HVAC. If you have a smart meter, pull your actual daily average for a more accurate result.
Daily Solar Generation: Enter your system's average daily output in kWh. You can find this in your inverter's app or monitoring portal — most show a daily or monthly production total. Divide monthly production by 30 for a daily average. If you don't have solar yet, your installer can estimate daily production for your roof and location. Leave this at 0 for battery-only systems.
Backup Coverage: Whole Home covers everything — no load management needed. Essential Loads assumes you have a dedicated critical loads panel (or are willing to manually manage circuits) covering roughly 40% of your normal usage. Critical Only is the bare minimum — lights and refrigerator — around 20% of typical daily usage. If you don't have or plan a dedicated sub-panel, select Whole Home.
Days of Backup: How many days you want to run without solar recharging or grid power. One day covers most grid outages — the majority resolve within 24 hours. If your solar system is generating, your battery recharges daily and this number matters much less. For cloudy-climate or winter-scenario planning, three to five days is a common target.
⚠️Special situations▼
Solar generation equals or exceeds daily usage
Even when your panels produce more energy than you consume in a day, you still need storage to cover overnight hours when solar generates nothing. The calculator accounts for this with a minimum overnight floor of 50% of your daily usage — roughly 12 hours of nighttime consumption. So a household using 42 kWh/day with 73 kWh of solar generation still needs storage sized for at least 21 kWh of overnight load.
Battery-only system (no solar)
Leave solar generation at 0. The calculator applies directly — it sizes the battery based on your full daily usage, coverage goal, and backup duration. Battery-only systems are commonly used for time-of-use arbitrage (charge cheap at night, discharge during expensive peak hours) and grid outage backup.
Electric vehicle charging
If you charge an EV at home, its consumption can easily add 10–20 kWh/day to your usage total. During an extended outage you likely won't run the EV charger, so consider subtracting EV charging from your daily usage number when calculating backup storage needs.
Whole-home backup with heat pump or central AC
HVAC is the largest load in most homes — central AC can draw 3–5 kW continuously. Whole-home backup that includes running air conditioning or electric heat will require significantly more capacity than the daily usage average suggests, because HVAC runtime tends to increase during weather-driven outages. Consider upsizing by 20–30% if uninterrupted HVAC is a requirement.
Federal and state incentives
The federal Residential Clean Energy Credit covers 30% of battery storage costs through 2032. For systems installed in 2023 and later, standalone battery storage qualifies as long as the battery is charged at least 70% by renewable energy. If you're adding batteries alongside a solar system, the full installed cost qualifies. Many states offer additional rebates — California's SGIP, New York's NYSERDA, and utility-specific incentives can reduce net costs by an additional 10–30%.
❓Common questions▼
How do I find my daily energy usage and solar generation?
For energy usage, check your utility bill — it shows monthly kWh consumed. Divide by 30 for a daily average. For solar generation, check your inverter's monitoring app or portal, which shows daily and monthly production totals. Divide monthly solar production by 30 for a daily average. If you don't have solar yet, your installer will estimate daily output based on your roof orientation, pitch, shading, and local irradiance.
Why does adding solar reduce the battery size so much?
Because your battery only needs to cover the energy your solar panels don't produce. If your panels generate 15 kWh on a typical day and you use 30 kWh, your battery only needs to store 15 kWh per day — half as much as without solar. Add multiple days of backup and the effect multiplies. This is the core value of the solar-plus-storage combination: solar shrinks the battery you need, which reduces upfront cost significantly.
Do I need solar panels to install a home battery?
No. Home batteries work without solar. Common non-solar use cases include time-of-use arbitrage (charge on cheap overnight electricity, discharge during expensive peak hours), whole-home backup for grid outages, and demand charge reduction. That said, solar significantly extends backup capability by recharging the battery daily, so combining both is often the most effective setup for energy independence.
What's the difference between usable capacity and nameplate capacity?
Nameplate capacity is the total energy a battery can store. Usable capacity is what you can actually draw from it — manufacturers limit depth of discharge to protect battery longevity, typically reserving 5–10% on each end. Always compare systems on usable kWh, not nameplate. The calculator uses usable capacity throughout. When a contractor quotes you a system size, confirm whether they're citing nameplate or usable.
Does the 30% federal tax credit apply to battery storage?
The Residential Clean Energy Credit covers 30% of battery storage system costs through 2032. For systems installed in 2023 and later, standalone battery storage qualifies as long as the battery is charged at least 70% by renewable energy. If you're adding batteries to an existing or new solar system, the full installed cost qualifies. Consult a tax professional for your specific situation — IRS guidance has evolved and state-level rules vary.