How to Size a Solar Battery Bank (Step-by-Step Formula)

Undersize a solar battery bank and you’ll be running your generator every other night. Oversize it, and you’ve spent thousands of dollars on capacity that never gets used.

Getting the number right is the single most consequential calculation in any off-grid or backup solar system, more important than panel wattage, because the battery bank is what determines whether your lights stay on at 2 a.m.

This guide walks through the exact formula I use when sizing battery banks in the field, with a worked example, a chemistry comparison table, and the mistakes that trip up most first-time system designers.

How to Size a Solar Battery Bank?

To size a solar battery bank, calculate your daily energy consumption in watt-hours, multiply by your desired days of autonomy, divide by your battery’s usable depth of discharge, and adjust for inverter and temperature losses.

The result is your required battery bank capacity in watt-hours, which you then convert to amp-hours at your system voltage.

The formula looks like this

Battery Bank Capacity (Wh) = (Daily Load (Wh) × Days of Autonomy) ÷ (Depth of Discharge × Inverter Efficiency × Temperature Derate)

Below, each variable gets its own section so you understand exactly what you’re plugging in and why.

Step 1: Calculate Your Daily Energy Consumption

Every sizing exercise starts with an honest load audit. List every device you intend to power, its wattage, and the hours per day you’ll run it.

A few notes from field experience:

• Refrigerators and freezers don’t run their compressors continuously. It’s usually 30–50% of the hour, so multiply nameplate wattage by 8–12 effective hours, not 24.
• Motors and pumps draw a surge current at startup that can be 3–5x their running wattage. This matters for inverter sizing, not battery capacity, but note it for later.
• Add a 20% buffer to your total for loads you forgot or will add later. In the example above, 2,570 Wh becomes roughly 3,080 Wh/day.

Step 2: Decide Your Days of Autonomy

“Days of autonomy” is the number of consecutive cloudy days your battery bank can carry the full load with zero solar input. This is a judgment call based on climate and risk tolerance, not a fixed number.

• Grid-tied backup systems (occasional outages): 1–2 days is usually sufficient.
• Off-grid cabins in sunny climates (northern Mexico, southwestern US): 2–3 days.
• Off-grid primary residences or areas with extended cloudy seasons: 3–5 days.
• Critical loads (medical equipment, refrigeration for medication): err toward 4–5 days regardless of climate.

For our example, we’ll use 2 days of autonomy.

Step 3: Account for Depth of Discharge (DoD)

Depth of discharge is the percentage of total battery capacity you can actually use without damaging the battery or shortening its lifespan.

This is where battery chemistry makes the biggest difference in your final sizing math and your budget.

This is the number that separates two batteries with the same nameplate capacity into very different real-world systems.

A 200 Ah lead-acid battery only gives you 100 Ah of usable capacity at 50% DoD. A 200 Ah LiFePO4 battery gives you 160–200 Ah.

If you size based on nameplate capacity instead of usable capacity, you’ll end up with a bank that fails your autonomy target the first time it’s tested.

For this example, we’ll use LiFePO₄ at 90% usable DoD.

Step 4: Factor in Inverter and System Losses

No conversion is lossless. Budget for it.

Inverter efficiency

90–95% for a quality pure sine wave inverter. Use 0.92 as a safe planning figure.

Temperature derating

Battery capacity drops in cold weather, and cycle life shortens in extreme heat. If your battery bank lives outside a climate-controlled space, apply a 5–10% derating.

LiFePO₄ is more temperature-tolerant than lead-acid but isn’t immune. Most units also throttle or stop charging below freezing unless they have built-in heaters.

Wiring and connection losses

Typically 1–3%, often folded into the inverter efficiency figure for simplicity.

For our example: 0.92 (inverter) × 0.95 (temperature derate) ≈ 0.87 combined efficiency factor.

Step 5: Run the Full Calculation

Using our example numbers.

• Daily load: 3,080 Wh
• Days of autonomy: 2
• Usable DoD: 90% (0.90)
• Combined efficiency factor: 0.87

Battery Bank Capacity = (3,080 × 2) ÷ (0.90 × 0.87)

Battery Bank Capacity = 6,160 ÷ 0.783 = 7,867 Wh

Round up to 8,000 Wh (8 kWh) for margin.

Step 6: Convert Watt-Hours to Amp-Hours

Battery banks are usually specified in amp-hours (Ah) at a given system voltage. Divide total watt-hours by your system voltage.

Amp-Hours = Watt-Hours ÷ System Voltage

At a 48V system voltage: 8,000 Wh ÷ 48V = 166.7 Ah

At a 24V system voltage: 8,000 Wh ÷ 24V = 333.3 Ah

At a 12V system voltage: 8,000 Wh ÷ 12V = 666.7 Ah

This is why higher system voltages are standard for anything beyond a small cabin. The same energy capacity requires far less current, which means thinner, cheaper cabling and lower resistive losses.

As a rule of thumb: 12V for small systems under 2 kWh, 24V for mid-size systems, and 48V for anything approaching or exceeding 5 kWh of daily storage.

Quick Reference Sizing Chart

Figures include the 0.87 combined efficiency factor used above; adjust proportionally for lead-acid chemistries by roughly doubling the result (50% DoD vs. 90%).

Common Sizing Mistakes

Sizing off nameplate capacity instead of usable capacity

Covered above, but worth repeating because it’s the single most common error I see in DIY systems. It silently cuts your real autonomy in half or worse.

Ignoring inverter surge requirements

Battery capacity (Wh) and inverter power rating (W) are different specs. A battery bank sized correctly for daily energy might still pair with an undersized inverter that can’t handle your refrigerator’s startup surge or your well pump kicking on.

Forgetting temperature derating

A battery bank that performs fine in a lab spec sheet at 25°C can lose 20%+ of usable capacity at freezing temperatures if it’s in an unheated shed or garage.

Sizing for average load instead of realistic load

Averages hide peak days. Laundry day, guests visiting, a heat wave running extra fans. Build in the 20% buffer from Step 1.

Skipping charge controller and solar array sizing

A correctly sized battery bank that never fully recharges because the solar array or charge controller is undersized will degrade faster and never actually deliver its rated autonomy.

FAQ

How many batteries do I need for a 5kW solar system?

Battery count depends on daily consumption, not array size. A 5 kW array can pair with anywhere from a 5 kWh to a 20+ kWh battery bank depending on autonomy needs and usage patterns. Run the load audit in Step 1 rather than sizing the array wattage alone.

Should I size my battery bank around lithium or lead-acid?

Lithium (LiFePO₄) costs more upfront, but its higher DoD, longer cycle life, and lower long-term cost per usable kWh make it the better choice for most systems designed to last more than 5 years. Lead-acid remains viable for low-budget, low-cycle backup applications.

Is it better to oversize a solar battery bank?

Modest oversizing (10–20% margin) is good practice and extends battery lifespan by reducing average DoD per cycle.

Significant oversizing wastes capital that could go toward panels, an inverter upgrade, or a second bank added later as needs grow.

How do I size a battery bank for a specific appliance, like a CPAP machine or refrigerator?

Use the same watt-hours-per-day method from Step 1, isolating just that appliance’s consumption, then apply the same DoD and efficiency adjustments. Critical medical loads generally warrant sizing toward the higher end of the autonomy range.

Seki Hudson is an ANCE-certified solar technician and industrial automation engineer. This guide reflects field-tested sizing methodology used across off-grid and backup solar installations.