The exact four-step method solar installers use to size a battery bank correctly — daily load, depth of discharge, and backup days — with worked examples and a free sizing calculator.
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📋 In This Guide
Get battery sizing wrong, and you usually don't find out for months — right up until a cloudy week or a string of power cuts pushes the bank past its limit. Undersize it, and the lights go out exactly when backup is needed most. Oversize it, and lakhs go into capacity that never gets used, while the client (or your own project budget) absorbs the unnecessary upfront cost.
This guide walks through the exact four-step method professional installers use to size a battery bank — daily load, depth of discharge, backup days, and the final capacity formula — with a worked example at every step. Whether this is your first sizing calculation as a student, part of a broader solar installation you're designing, or the start of a battery-sizing service you want to offer clients, the underlying math doesn't change. Only how confidently you do it does.
Before sizing anything, list every appliance the battery bank will support, its running wattage, and how many hours a day it actually runs. This is your daily load — the single most important number in the entire calculation, because every later step multiplies off of it.
| Appliance | Power (W) | Hours/Day | Daily Load (Wh) |
|---|---|---|---|
| LED Shop Lighting (6×18W) | 108 W | 10 hrs | 1,080 Wh |
| Desktop PC + Billing Printer | 150 W | 9 hrs | 1,350 Wh |
| CCTV System (4 cameras + DVR) | 40 W | 24 hrs | 960 Wh |
| Water Purifier | 25 W | 24 hrs | 600 Wh |
| Total Daily Load | — | — | 3,990 Wh |
Example: a small shop's load above adds up to 3,990 Wh/day. Add a 20% safety buffer for system losses, inverter inefficiency, and future load growth: 3,990 × 1.20 ≈ 4,790 Wh design load.
Depth of Discharge (DoD) is the percentage of a battery's total capacity you can safely use before recharging. Push past it regularly, and you accelerate degradation — sometimes drastically. Every chemistry has a different safe ceiling:
| Chemistry | Safe DoD |
|---|---|
| Flooded Lead-Acid | 50% |
| VRLA / AGM | 50–60% |
| Gel | 60% |
| Li-ion NMC | 80–90% |
| LiFePO4 | 90–95% |
This number plugs directly into the capacity formula in Step 4 — the full comparison further down covers cost and lifespan trade-offs for each chemistry.
Backup days (autonomy) is how many consecutive days the bank needs to cover with zero solar input — a run of monsoon clouds, an extended grid outage, or both. This number is driven entirely by how reliable the grid and weather actually are at the install site, not by a generic rule of thumb.
With all three inputs in hand — design load, DoD, and backup days — the final formula gives you the battery bank size in both Ah and kWh. Continuing the shop example from Step 1, sized on a 48V LiFePO4 system with 2 days of backup at 90% DoD:
Enter your numbers from Steps 1–3 to get a recommended battery bank size.
The DoD figures from Step 2 are only half the story — cost and cycle life decide whether the chemistry actually pays off over the system's lifetime. For the full sizing methodology applied project by project, IISE's Battery & Storage courses walk through each chemistry in depth.
| Chemistry | DoD | Cycle Life | Cost/kWh | Best Use Case |
|---|---|---|---|---|
| Flooded Lead-Acid | 50% | 300–500 | [VERIFY] | Lowest upfront budget, easy local servicing |
| VRLA / AGM | 50–60% | 400–600 | [VERIFY] | Indoor backup, tight spaces |
| Gel | 60% | 500–700 | [VERIFY] | Deep, slow discharge cycles |
| Li-ion NMC | 80–90% | 800–1,200 | [VERIFY] | Space-constrained installs prioritizing density |
| LiFePO4 Recommended | 90–95% | 3,000–6,000 | [VERIFY] | Most new installations — best lifetime cost |
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