Solar Battery Size Calculator
Size a home battery system from daily usage, solar production, and backup hours needed. Enter values for instant results with step-by-step formulas.
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer
Solar Battery Size Calculator
Calculator
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Formula: Actual Capacity = Usable Energy / (DoD x Round-Trip Efficiency)
Worked example — 18.5 kWh battery needed | 385 Ah at 48V | 2 Powerwalls
Formula
Actual Capacity = Usable Energy / (DoD x Round-Trip Efficiency)
Usable energy is the larger of your net daily need (usage minus solar) or backup energy required (hourly usage times backup hours). Actual battery capacity must account for depth of discharge limits and energy losses during charge/discharge cycles (round-trip efficiency).
Worked Examples
Example 1: Average US Home with Backup
Problem:A home uses 30 kWh/day, solar produces 25 kWh/day, and needs 12 hours of backup. Battery has 90% DoD and 90% round-trip efficiency at 48V.
Solution:Net daily need = 30 - 25 = 5 kWh Hourly usage = 30 / 24 = 1.25 kWh/hr Backup energy = 1.25 x 12 = 15 kWh Usable capacity needed = max(15, 5) = 15 kWh Actual capacity = 15 / (0.90 x 0.90) = 18.5 kWh Capacity in Ah = 18,500 / 48 = 385 Ah Powerwalls needed = ceil(18.5 / 13.5) = 2
Result:18.5 kWh battery needed | 385 Ah at 48V | 2 Powerwalls
Example 2: Off-Grid Cabin
Problem:A cabin uses 10 kWh/day with 8 kWh solar. Needs 24-hour backup with 80% DoD lead-acid batteries at 24V.
Solution:Net daily need = 10 - 8 = 2 kWh Backup energy = (10/24) x 24 = 10 kWh Usable capacity = max(10, 2) = 10 kWh Actual capacity = 10 / (0.80 x 0.85) = 14.7 kWh Capacity in Ah = 14,700 / 24 = 613 Ah
Result:14.7 kWh battery needed | 613 Ah at 24V | 3 x 5kWh batteries
Frequently Asked Questions
How do I determine the right battery size for my home solar system?
The right battery size depends on three key factors: your daily energy consumption, solar panel production, and desired backup duration. Start by checking your electricity bill for daily kilowatt-hour usage, typically 20 to 40 kWh for an average US home. Then determine how much your solar panels produce daily, which varies by system size, location, and season. The difference between consumption and production is what the battery needs to cover. Additionally, decide how many hours of backup you want during a grid outage. For critical loads only (refrigerator, lights, internet), 5 to 10 kWh may suffice. For whole-home backup during extended outages, you may need 20 to 40 kWh or more of usable capacity.
What is depth of discharge and why does it affect battery sizing?
Depth of discharge is the percentage of a battery capacity that can actually be used before recharging. Most lithium-ion solar batteries have a recommended depth of discharge of 80 to 95 percent. A 10 kWh battery with 90% DoD provides 9 kWh of usable energy. Using a battery beyond its recommended DoD dramatically reduces its lifespan. Lead-acid batteries typically have only 50% recommended DoD, meaning you need twice the rated capacity. This is why lithium-ion batteries, despite higher upfront cost per kWh, often provide better value since you can use nearly all their rated capacity. When sizing your system, always divide your energy needs by the DoD to determine the actual battery capacity required.
How does round-trip efficiency impact my battery system?
Round-trip efficiency measures how much energy you get back compared to what you put in. If a battery has 90% round-trip efficiency, storing 10 kWh of solar energy yields only 9 kWh when discharged. The 10% loss occurs as heat during the charging and discharging chemical processes. Lithium-ion batteries typically achieve 85 to 95 percent round-trip efficiency, while lead-acid batteries are around 70 to 85 percent. This efficiency loss means you need a larger battery and more solar panels to meet your energy goals. Over a year, a 5% difference in round-trip efficiency on a 30 kWh daily cycling system wastes about 548 kWh, equivalent to roughly $82 at average electricity rates.
How many Tesla Powerwalls or equivalent batteries do I need?
The Tesla Powerwall has 13.5 kWh of usable capacity with a continuous power output of 5 kW and peak of 7 kW. To determine how many you need, divide your required battery capacity by 13.5 kWh. For a typical home using 30 kWh daily with 12 hours of backup needed, you would require about 15 kWh of usable capacity, which equals two Powerwalls (providing 27 kWh total). However, also verify the power rating, not just capacity. If your home can draw 8 kW simultaneously from air conditioning, electric oven, and other loads, one Powerwall at 5 kW continuous would be insufficient even if the energy capacity is adequate. Multiple units connected in parallel increase both capacity and power output.
What is the typical payback period for a home battery system?
The payback period for residential battery storage varies widely based on electricity rates, solar production, battery cost, and available incentives. At current prices of $400 to $600 per kWh installed, a 10 kWh battery costs $4,000 to $6,000 before incentives. The US federal tax credit of 30% under the Inflation Reduction Act reduces this substantially. With time-of-use electricity rates where peak prices exceed $0.30 per kWh, the payback period can be 7 to 10 years. In areas with flat rates below $0.12 per kWh, payback may exceed 15 years, making it primarily a backup power investment. States with strong net metering may reduce the financial benefit of storage since you can effectively use the grid as a free battery.
How do I size a residential solar panel system?
Divide your annual kWh usage by your location's peak sun hours per day times 365. For example, 10,000 kWh/year with 5 peak sun hours = 10,000/(5*365) = 5.5 kW system. Account for system losses (about 20%) by dividing by 0.80, giving approximately 6.8 kW. Each 400W panel produces about 1.6 kWh/day.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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