Solar Battery Size Calculator
Size a home battery system from daily usage, solar production, and backup hours needed. Compare Powerwall equivalents and estimate costs.
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.
What is the difference between a battery's kWh capacity and its kW power rating?
Capacity in kWh is how much energy the battery holds; power in kW is how fast it can deliver that energy. They are independent specifications, and a system can be adequate on one and short on the other. Solar Battery Size Calculator flags the mismatch by assuming a C/2 continuous discharge rate, meaning a pack can sustain about half its capacity in kW — an 18.5 kWh bank supports roughly 9.25 kW. Enter your realistic simultaneous draw as the peak load: an electric oven, a well pump and air conditioning starting together set the kW requirement, not your daily kWh.
Why does the amp-hour figure change when I change the system voltage?
Because amp-hours are energy divided by voltage: Ah = Wh / V. An 18.5 kWh bank is 385 Ah at a 48 V nominal DC bus but 1,542 Ah at 12 V — the same stored energy, four times the current. That matters physically, since resistive losses rise with the square of current, so low-voltage banks need much thicker cabling and larger fuses to move the same power without overheating. This is why whole-home systems standardise on 48 V while 12 V is reserved for small RV and boat installations.
How do LFP and NMC lithium chemistries differ for home storage?
Lithium iron phosphate (LFP) and nickel manganese cobalt (NMC) are both lithium-ion but behave differently. LFP tolerates deeper discharge, so you can enter a higher depth of discharge, and it typically delivers more full cycles before capacity fades, with better thermal stability. NMC packs more energy into a given volume and weight, which is why it dominates in vehicles where space is tight. For a wall-mounted or garage-floor home battery, physical size is rarely the binding constraint, so LFP's cycle life and thermal margin usually matter more than NMC's density.
Should I size for outage backup or for daily self-consumption?
They are different targets, which is why the formula takes the larger of the two. Self-consumption sizing covers only the net daily need — usage minus solar production — so a home using 30 kWh against 25 kWh of solar needs just 5 kWh to shift evening load off the grid. Backup sizing covers hourly usage times the outage hours you want to ride through, which for 12 hours at 1.25 kWh per hour is 15 kWh. If outage resilience is your goal, backup will normally dominate; if you are chasing time-of-use arbitrage, net daily need is the number to optimise.
Will my battery power the house if the grid goes down?
Only if the system was designed to island. Grid-tied inverters are required to shut down when the grid fails, so that they cannot energise lines that utility crews believe are dead. Backup operation needs additional hardware — a transfer switch or gateway that disconnects the home from the grid and forms its own stable voltage and frequency. Most installations pair this with a critical-loads subpanel carrying the refrigerator, lights and networking gear, which is also what keeps your backup-hours figure achievable rather than draining the pack into whole-home loads.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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