Battery Size & Backup Time Calculator
Planning reliable power for your home, solar, car, EV planner, emergency outage, fire alarm, CCTV, PC, WiFi router, refrigerator, fan, lights or many appliances? This comprehensive battery size, backup time calculator helps you accurately determine capacity, runtime, energy consumption, usable storage and autonomy for any system.
Whether you need lithium ion, LiFePO4, deep cycle lead acid, AGM, gel, nickel or custom 12V Ah / mAh / Wh / kWh battery bank calculations — get instant results for load wattage, watt, amp, voltage, current, discharge rate, depth of discharge, efficiency, pure sine wave or modified inverter sizing, continuous & surge rating, charging, health, degradation, Peukert effect, temperature impact and reserve capacity.
Use the multi-appliance builder for real-world energy usage, daily consumption, peak demand and runtime prediction. Includes formula, wiring diagrams, chemistry comparison, weight estimates, nickel strip considerations and generator / grid / standby support. Perfect for electricity operation, renewable planning and maximizing battery life in UPS, Casio-style precision tools or full off-grid setups. Start calculating how long your power pack will last today.
🔋 Battery Size & Backup Time Calculator
Calculate battery capacity (Ah/kWh), runtime, inverter sizing, and energy storage for solar, UPS, home backup, lithium, lead-acid & more.
Results
Battery Status
| Load (W) | Backup Time | Current Draw (A) | Status |
|---|---|---|---|
| Run a calculation to see results | |||
| Battery Type | DoD | Efficiency | Runtime | Cycle Life | Weight (est.) |
|---|---|---|---|---|---|
| Run a calculation to see comparison | |||||
Formulas Used in Calculations
Backup Time (Hours)
\[ T = \frac{V \times Ah \times n_{\text{series}} \times n_{\text{parallel}} \times \text{DoD} \times \eta_{\text{inv}} \times \eta_{\text{temp}} \times \text{SoH}}{L_W} \]Where: T = Time (hours), V = Voltage, Ah = Amp-hours per battery, DoD = Depth of Discharge (decimal), ηinv = Inverter Efficiency (decimal), ηtemp = Temperature Derating Factor, SoH = State of Health (decimal), LW = Load in Watts.
Required Battery Capacity (Ah)
\[ Ah_{\text{required}} = \frac{L_W \times T}{\; V \times \text{DoD} \times \eta \times (1 - \text{Buffer})\;} \]Add safety buffer by dividing by (1 - Buffer%). Result is the minimum usable Ah; round up to next standard battery size.
Watt-hours, kWh, Current Draw
\[ Wh = Ah \times V \qquad kWh = \frac{Wh}{1000} \] \[ I_{\text{draw}} = \frac{L_W}{V \times \eta} \quad \text{(Amps)} \] \[ \text{Usable Wh} = Wh_{\text{total}} \times \text{DoD} \times \eta \]Peukert Corrected Capacity
\[ C_{\text{actual}} = C_{\text{rated}} \times \left(\frac{I_{\text{rated}}}{I_{\text{actual}}}\right)^{k-1} \] \[ t = H \left(\frac{C}{I \cdot H}\right)^k \]k = Peukert exponent (1.1–1.3 for lead-acid, ~1.05 for LiFePO4). Higher discharge current reduces effective capacity. Apply when discharge rate > 0.2C.
Temperature Correction Factor
\[ \eta_{\text{temp}} = 1 - k_T \times (25 - T_{\text{ambient}}) \]Where kT ≈ 0.008 per °C for lead-acid (0.004 for lithium). At 0°C, lead-acid loses ~20% capacity. Factor is capped at 0.6 (minimum) and 1.05 (maximum).
| Temp | Lead-Acid Factor | LiFePO4 Factor |
|---|---|---|
| -20°C | ~0.64 | ~0.82 |
| 0°C | ~0.80 | ~0.90 |
| 25°C | 1.00 | 1.00 |
| 40°C | ~1.04 | ~1.02 |
Battery Charging Duration
\[ T_{\text{charge}} = \frac{Ah_{\text{usable}}}{I_{\text{charger}} \times \eta_{\text{charge}}} \]A 100Ah battery at 50% DoD needs 50Ah replenished. With a 10A charger at 90% efficiency: T = 50 / (10 × 0.90) = 5.56 hours.
Battery Wiring Diagrams
🧪 Battery Chemistry Quick Reference
| Type | Safe DoD | Efficiency | Cycle Life | Best For | Cost |
|---|---|---|---|---|---|
| LiFePO4 Best | 80-90% | 95-98% | 2000-6000 | Solar, EV, off-grid | High |
| Lithium-Ion | 80% | 90-95% | 500-1000 | Portable, UPS, PC | Medium-High |
| AGM | 50-60% | 85-92% | 400-800 | UPS, solar, marine | Medium |
| Gel | 50-70% | 85-90% | 500-1000 | Deep cycle, telecom | Medium |
| Flooded Lead-Acid | 50% | 80-85% | 200-400 | Home inverter, car | Low |
| Deep-Cycle Lead-Acid | 50-60% | 80-88% | 300-600 | RV, solar, marine | Low-Medium |
| NiMH | 70% | 65-75% | 300-500 | Small electronics | Medium |
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Battery Size & Backup Time Calculator — Complete User Guide
Step-by-step guide to calculating battery capacity (Ah/kWh), backup time, runtime, and energy storage for solar, UPS, home inverter, car, lithium-ion, lead-acid, AGM, and EV systems. Includes every formula, worked example, and common mistake fix.
🔋 What This Battery Size & Backup Time Calculator Does
Whether you are sizing a home UPS inverter for power outages, building a solar battery bank for off-grid energy storage, planning a car or EV battery pack, or figuring out how long a single 12V deep-cycle lead-acid battery will last running your fan, lights, and refrigerator — this calculator covers every scenario in one place.
The tool answers four fundamental questions every power system planner must resolve before buying a single battery or inverter:
Enter your existing battery capacity in Ah, Wh, or kWh plus your total load in Watts and get an accurate backup time in hours and minutes.
Input your load (watts) and desired backup duration, and the calculator outputs the minimum battery capacity in Ah plus a recommended standard size to buy.
Enter daily consumption, peak sun hours, panel wattage, and autonomy days to size your solar battery bank and estimate renewable energy harvest vs. grid dependence.
Build a full appliance list — refrigerator, fan, WiFi router, PC, CCTV, lights — and auto-sum peak wattage demand before sizing your battery and inverter system.
⚠️ Key User Pain Points & How This Calculator Solves Them
Most people buying a battery for their home inverter, UPS, solar system, or emergency generator backup make at least one of these eight costly mistakes. Here is how the calculator addresses each one directly.
Too small = system shuts down during an outage. Too large = wasted money on unused capacity and storage weight.
Discharging a flooded lead-acid battery to 0% kills it in under 100 cycles. Users assume 100Ah means 100Ah usable.
A pure sine wave inverter loses 5–10% converting DC to AC. A modified sine wave unit loses 15–20%. Users expect 8 hours and get 6.
At 0°C, a lead-acid battery delivers only ~80% of its rated capacity. Cold-weather standby UPS systems massively underperform.
Discharge a 100Ah lead-acid at high current (1C rate) and you only get 60–70Ah. This is the "Peukert Mystery" most guides skip.
Mixing amps, watts, volts, milliamps, and watt-hours in the same calculation causes wrong results every time.
A 2-year-old lithium-ion pack may only hold 80% of its original capacity. Calculations based on original ratings give false confidence.
Users list appliances in their head but cannot calculate peak watt demand or daily Wh consumption accurately enough to size a battery.
🗂️ 4 Calculation Modes Explained
The calculator is divided into four modes, each solving a different energy planning problem. Select the tab that matches your goal before entering any values.
📖 Step-by-Step User Guide
Mode 1: Backup Time Calculator (How Long Will My Battery Last?)
Use this mode when you already own a battery and want to know its runtime at a specific load — for example, how long a 12V 100Ah AGM UPS battery will run your home inverter at 500 watts during a power outage.
Choose from LiFePO4, Lithium-Ion, AGM, Gel, Flooded Lead-Acid, Deep-Cycle Lead-Acid, or NiMH. The calculator automatically pre-fills the recommended Depth of Discharge (DoD) and efficiency for that chemistry. You can override these manually.
Choose 12V, 24V, 48V or enter a custom voltage. A single car or home UPS battery is typically 12V. Large solar systems and EV packs run at 24V, 48V, or higher. Voltage is critical — getting it wrong will produce completely incorrect watt-hour and current draw results.
Enter the capacity printed on your battery label. Switch the unit dropdown to match: Ah (amp-hours), Wh (watt-hours), kWh (kilowatt-hours), or mAh (milliamp-hours for small power banks). The calculator converts everything to Wh internally before calculating.
If you have a battery bank with multiple units: Series multiplies voltage (2 × 12V = 24V bank); Parallel multiplies Ah capacity (2 × 100Ah = 200Ah bank). Enter individual battery specs, then set the counts. The calculator handles bank totals automatically.
The DoD slider controls how much of the battery capacity is considered safe to use. Lead-acid: 50% default (never discharge below 50% for longevity). LiFePO4: 85%. Discharging deeper gives more runtime now but dramatically shortens the battery’s overall life and cycle count.
A brand-new battery = 100% SoH. After 2–3 years of use, lithium-ion packs typically degrade to 80% SoH. Lead-acid degrades faster — especially if frequently over-discharged. Adjust this slider to reflect your battery’s actual health and get realistic runtime estimates instead of over-optimistic ones.
Enter the total simultaneous wattage of all appliances running during the outage. If you are running a ceiling fan (75W), four LED lights (40W), and a WiFi router (15W), enter 130W. Switch the unit to VA if your UPS shows apparent power, or kW for large systems. The power factor field corrects VA to real Watts.
Select your inverter type. Pure sine wave inverters auto-fill 93% efficiency. Modified sine wave auto-fills 82%. DC systems (no inverter) use 99%. You can manually adjust the efficiency slider. Every percentage point here directly changes your runtime result — this is one of the biggest sources of real-world disappointment.
Slide the temperature to match your environment. Below 15°C, lead-acid batteries lose meaningful capacity. The Peukert exponent (1.05–1.3) applies only to lead-acid types — leave it at the default for lithium. A higher exponent means more capacity loss at high discharge rates.
Results appear instantly below the calculator: backup time in hours and minutes, total bank energy in kWh, usable Wh after all losses, current draw in amps, a runtime comparison table at different load levels, a discharge curve chart, and a chemistry comparison table. Use the Copy Results button to save and share your calculation.
Mode 2: Battery Sizing (How Much Battery Do I Need?)
Use this mode before purchasing a battery. Input your load in watts, desired backup duration, system voltage, and battery chemistry. The calculator outputs the minimum Ah capacity required and recommends the next standard battery size to buy.
Key inputs unique to Sizing Mode:
- Required Backup Duration — enter in hours, minutes, or days (days of autonomy for off-grid planning)
- Safety Buffer % — add 15–25% extra capacity to account for future load growth, cold weather, and battery aging
- Result shows both the mathematical minimum Ah and the recommended standard size (e.g., 95Ah minimum → buy 100Ah)
Mode 3: Solar Battery Bank Planner
Size a complete off-grid or hybrid solar energy storage system. This mode calculates required battery bank capacity factoring in daily consumption, renewable solar input, and autonomy days for cloudy weather periods.
Solar Mode unique inputs:
- Solar Panel Total Wattage — sum of all panel ratings in W
- Peak Sun Hours/Day — average hours of usable solar irradiance (Bangladesh average: 4.5–5.5 hrs)
- Daily Consumption — total household energy use in Wh/day
- Days of Autonomy — how many consecutive cloudy days your battery must cover without solar input (typically 2–3 days)
- Charge Controller Efficiency — MPPT controllers run at 93–97%; PWM at 75–85%
Mode 4: Multi-Appliance Load Builder & Battery Planner
This mode is designed for home and office energy planning. Add every appliance you want powered during a grid outage, assign its wattage and daily usage hours, and the calculator totals your peak demand and daily consumption before sizing your battery and inverter.
Quick-Add appliance reference wattages:
| Appliance | Typical Watts | Surge / Peak | Notes |
|---|---|---|---|
| Ceiling Fan | 50–80 W | — | Low startup surge; safe for modified sine |
| LED Lights (per bulb) | 8–15 W | — | Most efficient appliance per lux delivered |
| WiFi Router | 10–20 W | — | Runs continuously; add to standby load |
| Laptop / PC | 45–90 W | 120 W | Varies by model; gaming PCs draw 200–400 W |
| Refrigerator (100–250L) | 100–200 W | 400–600 W | High surge on compressor start; needs pure sine wave |
| CCTV Camera (per unit) | 5–15 W | — | DVR adds 10–30 W; total system ~40–80 W |
| Phone Charger | 5–20 W | — | USB-C fast chargers draw up to 45 W |
| Air Conditioner (1 ton) | 900–1200 W | 3000–4000 W | Very high surge rating; requires large inverter bank |
| Water Pump (0.5 HP) | 400–500 W | 1200–1500 W | High inductive surge; size inverter for peak, not continuous |
| Fire Alarm Panel | 20–80 W | — | Emergency standby load; battery must cover 24–72 hr operation |
| EV Charger (Level 1) | 1200–1800 W | — | Slow charge for EV; not practical on UPS battery |
| Nickel Strip Spot Welder | 500–2000 W | Very high | Short pulse; calculate average wattage, not peak |
🧮 All Formulas Used in the Battery Calculator
Every result the calculator produces is derived from the six core electrical engineering formulas below. These are the same equations used by professional power system engineers. Understanding them helps you interpret results and catch input errors.
Formula 1: Backup Time (Runtime) Calculation
This is the primary formula for answering “how long will my battery last?” It converts battery capacity to usable watt-hours and divides by load wattage, accounting for all real-world loss factors.
T = Backup time in hours |
V = System voltage (V) |
Ah = Single battery capacity (Ah) |
nₛ = Batteries in series |
nₚ = Batteries in parallel |
DoD = Depth of Discharge as decimal (e.g., 0.50) |
η𝕮𝕲𝕶 = Inverter efficiency (e.g., 0.85) |
η𝕥𝕮𝕫𝕥 = Temperature derating factor |
SoH = State of Health decimal (e.g., 0.90) |
L𝐰 = Total load in Watts
Worked Example — Formula 1
\[ T = \frac{12 \times 100 \times 1 \times 1 \times 0.50 \times 0.85 \times 1.0 \times 1.0}{500} = \frac{510}{500} = \mathbf{1.02 \text{ hours}} \approx \mathbf{1\, hr\; 1\, min} \]
Formula 2: Required Battery Capacity (Sizing)
This rearranges Formula 1 to find the minimum battery capacity needed to achieve a desired runtime. The safety buffer is added as a multiplier to ensure reliable long-term operation.
Buffer = Safety buffer as decimal (e.g., 0.20 for 20%) | All other variables same as Formula 1.
Worked Example — Formula 2
\[ Ah = \frac{800 \times 4 \times 1.20}{12 \times 0.50 \times 0.85} = \frac{3840}{5.1} = \mathbf{752.9\, Ah} \] → Buy: 800Ah bank at 12V (or e.g. 2×400Ah in parallel, or 8×100Ah in parallel)
Formula 3: Energy Unit Conversions (Ah, Wh, kWh, mAh)
These conversions run in the background every time you switch unit dropdowns. Understanding them removes the confusion between capacity units.
Formula 4: Temperature Derating Factor
Battery capacity drops in cold weather. The calculator applies this correction factor automatically when you move the temperature slider. Lead-acid batteries are far more sensitive to cold than lithium-ion chemistry.
k𝑇 ≈ 0.008 per °C for lead-acid | k𝑇 ≈ 0.004 per °C for LiFePO4 | Factor is capped: minimum 0.60, maximum 1.05.
| Temp (°C) | Lead-Acid Factor | LiFePO4 Factor | Effective Capacity (100Ah Lead-Acid) |
|---|---|---|---|
| -20°C | 0.64 | 0.82 | 64 Ah |
| -10°C | 0.72 | 0.86 | 72 Ah |
| 0°C | 0.80 | 0.90 | 80 Ah |
| 15°C | 0.92 | 0.96 | 92 Ah |
| 25°C (standard) | 1.00 | 1.00 | 100 Ah |
| 40°C | 1.04 | 1.02 | 104 Ah (but shorter life) |
Formula 5: Peukert Effect (Lead-Acid Batteries Only)
The Peukert effect explains why a 100Ah lead-acid battery does not deliver 100Ah at high discharge rates. The higher the current draw relative to the battery’s rated capacity, the less total energy it delivers. This does not apply significantly to lithium-ion or LiFePO4.
k = Peukert exponent (1.1–1.3 for lead-acid, ~1.05 for LiFePO4) |
I𝐰𝐚𝐭𝐞𝐡 = Actual discharge current |
I𝐫𝐚𝐭𝐞𝐡 = C/20 rated current | Applied only when discharge rate > 0.2C.
Formula 6: Battery Charging Time
Once you know the usable capacity that was discharged, you can estimate how long your charger or solar array needs to replenish it. This formula applies to both grid chargers and solar MPPT charge controllers.
Ah𝐡𝐢𝐬𝐜𝐗𝐚𝐫𝐠𝐞𝐡 = Amp-hours to be restored = Total Ah × DoD |
I𝐜𝐗𝐚𝐫𝐠𝐞𝐫 = Charger output in Amps |
η𝐜𝐗𝐚𝐫𝐠𝐞 = Charging efficiency (typically 0.85–0.95)
Formula 7: Solar Battery Bank Sizing
E𝐡𝐚𝐢𝐥𝐲 = Daily energy consumption in Wh |
D𝐚𝐞𝐭 = Days of autonomy (cloudy day buffer) |
P𝐩𝐚𝐧𝐞𝐥𝐬 = Total solar panel wattage |
H𝐬𝐞𝐧 = Peak sun hours per day
📋 Input Fields, Units & Validation Rules
Every field in the calculator has valid input ranges. Entering values outside these ranges will produce warnings or unrealistic results. Use this table as a quick validation reference.
| Input Field | Unit Options | Valid Range | Default | Common Mistake |
|---|---|---|---|---|
| Battery Capacity | Ah, Wh, kWh, mAh | 0.1 – 50,000 | 100 Ah | Entering mAh value in Ah field (1000x error) |
| System Voltage | V (preset or custom) | 1 – 1000 V | 12 V | Using nominal cell voltage (3.7V) for a 12V pack |
| Total Load | W, kW, VA | 1 – 100,000 | 500 W | Using VA rating without power factor correction |
| Depth of Discharge | % (slider) | 10% – 100% | 50% (lead-acid) | Using 100% DoD on lead-acid — kills battery in months |
| Inverter Efficiency | % (slider) | 60% – 99% | 85% | Forgetting this field entirely — gives 15% over-estimate |
| State of Health (SoH) | % (slider) | 50% – 100% | 100% | Leaving at 100% for a 3-year-old battery |
| Temperature | °C (slider) | -20 – 50 °C | 25°C | Leaving at 25°C for a battery in an outdoor cabinet |
| Safety Buffer | % (slider) | 0% – 50% | 20% | Setting 0% buffer for mission-critical systems |
| Peukert Exponent | Numeric | 1.00 – 1.50 | 1.20 (lead-acid) | Using lead-acid exponent for lithium batteries |
| Batteries in Series | Integer | 1 – 100 | 1 | Counting individual cells instead of full batteries |
| Batteries in Parallel | Integer | 1 – 100 | 1 | Assuming parallel doubles voltage (it does not) |
| Backup Duration (sizing) | hours, minutes, days | 0.1 – 720 hrs | 4 hours | Entering 8 hours when only 4 hours is actually needed |
Input Validation Indicators
The calculator displays colored warning badges after each calculation to signal whether your inputs produce a safe, efficient, or problematic system design:
🧪 Battery Chemistry Comparison: Lead-Acid vs Lithium vs AGM vs Gel
Choosing the right battery chemistry is as important as choosing the right capacity. The same 100Ah battery delivers completely different usable energy, cycle life, weight, and cost depending on chemistry. The calculator’s chemistry comparison table shows this side-by-side for your specific load.
| Chemistry | Safe DoD | Efficiency | Cycle Life | Weight (100Ah) | Ideal Use Case | Cost Level |
|---|---|---|---|---|---|---|
| LiFePO4 Best Overall | 80–90% | 95–98% | 2,000–6,000 | ~13 kg | Solar, EV planner, off-grid, long-term home backup | High |
| Lithium-Ion Portable | 80% | 90–95% | 500–1,000 | ~8 kg | Power banks, laptop packs, mAh-rated consumer devices | Medium–High |
| AGM Reliable | 50–60% | 85–92% | 400–800 | ~28 kg | UPS, telecom, fire alarm standby, marine | Medium |
| Gel | 50–70% | 85–90% | 500–1,000 | ~27 kg | Deep cycle applications, telecom reserve, solar | Medium |
| Flooded Lead-Acid Budget | 50% | 80–85% | 200–400 | ~30 kg | Home inverter, car battery, generator start | Low |
| Deep-Cycle Lead-Acid | 50–60% | 80–88% | 300–600 | ~32 kg | RV, boat, solar with budget constraints | Low–Medium |
| NiMH (Nickel-Metal Hydride) | 70% | 65–75% | 300–500 | ~22 kg | Small electronics, hybrid cars, nickel strip applications | Medium |
🏠 Worked Examples for Real-World Scenarios
Example 1: Home UPS Backup (Bangladesh Power Outage)
Goal: 4-hour backup for basic home appliances during load shedding
Appliances: 2 ceiling fans (75W each), 6 LED lights (10W each), 1 WiFi router (15W) = Total: 225W
System: 12V flooded lead-acid, 50% DoD, 85% efficiency, 25°C
\[ Ah = \frac{225 \times 4 \times 1.20}{12 \times 0.50 \times 0.85} = \frac{1080}{5.1} = \mathbf{211.8\, Ah} \]Result: Buy 2× 12V 120Ah batteries in parallel (240Ah total) — or a single 200Ah deep-cycle battery. Select Battery Sizing Mode, set load to 225W, duration to 4 hours.
Example 2: Solar Off-Grid Energy Storage System
Goal: 3 days of autonomy for a rural home with 400W solar panels
Daily consumption: 1,200 Wh/day | Peak sun hours: 5 hrs | System voltage: 24V | DoD: 80% (LiFePO4) | Charge efficiency: 95%
\[ E_{\text{solar}} = 400 \times 5 \times 0.95 = \mathbf{1,900\, Wh/day} \] \[ Ah_{\text{bank}} = \frac{1200 \times 3}{24 \times 0.80} = \frac{3600}{19.2} = \mathbf{187.5\, Ah} \]Result: Solar surplus of 700 Wh/day. Battery bank: 200Ah at 24V LiFePO4 (or 2× 12V/200Ah in series). Select Solar Mode to get this result automatically.
Example 3: CCTV Backup Battery Sizing
Goal: 8-hour standby backup for a 4-camera CCTV system with DVR
Load: 4 cameras × 8W + DVR 30W = 62W total | System: 12V AGM, 55% DoD, 88% efficiency
\[ Ah = \frac{62 \times 8 \times 1.20}{12 \times 0.55 \times 0.88} = \frac{595.2}{5.808} = \mathbf{102.5\, Ah} \]Result: Buy a 12V 100Ah AGM battery. Runtime check: ≈ 7.8 hours, which meets the 8-hour target within margin.
Example 4: Phone / Laptop Power Bank Sizing (mAh → Wh)
Goal: How many phone charges from a 20,000mAh power bank?
20,000 mAh at 3.7V (Li-ion cell voltage) = 74 Wh. Phone battery: 4,000mAh at 3.7V = 14.8 Wh. With 80% charge efficiency: 74 × 0.80 = 59.2 Wh usable. 59.2 / 14.8 = ~4 full charges.
Enter 20,000 mAh, 3.7V, load = phone charging wattage (5–15W) in Backup Time Mode to get exact runtime in hours.
🚨 Common Mistakes & Microcopy Fixes
These are the most frequent errors users make when calculating battery size or backup time. Each one is flagged inside the calculator as a warning when detected.
📏 Accuracy Note & Disclaimer
The Battery Size & Backup Time Calculator uses peer-reviewed electrical engineering formulas (backup time, Peukert correction, temperature derating, and DoD-adjusted capacity) that are accepted industry-standard methods for power system design. However, several real-world variables cannot be captured in any formula-based calculator:
► Wiring resistance losses — long or undersized cables reduce available voltage and power.
► Load variability — most appliances cycle on and off; instantaneous peaks can differ from average consumption.
► Battery brand quality — a budget 100Ah battery may actually hold only 80Ah at C/20 rate.
► Actual charge state — a battery advertised as 100% charged may only be at 90–95%.
► Self-discharge — batteries left in standby for weeks lose charge, especially lead-acid.
Expected real-world accuracy: 널–15% of calculated result. For critical systems (hospital equipment, fire alarm backup, emergency power packs), always verify with a licensed electrician and add a mandatory safety buffer of at least 25%.
❓ Frequently Asked Questions (FAQ)
Use Formula 1: T = (12 × 100 × 0.50 × 0.85) / 500 = 510/500 = 1.02 hours (at 50% DoD, 85% inverter efficiency). To get 4 hours at 500W from 12V, you would need approximately 400Ah total capacity. Enter all your values in the Backup Time Mode for an instant result with all correction factors applied.
Ah (Amp-hours) tells you how much current a battery can deliver for one hour. It does not tell you the actual energy stored without knowing the voltage. Wh (Watt-hours) = Ah × Voltage — this is the true energy unit. kWh = Wh ÷ 1000, used for larger systems. Example: 100Ah at 12V = 1,200 Wh = 1.2 kWh of stored energy. The calculator auto-converts between all units.
The most common reasons are: (1) Inverter efficiency not set correctly — if you left efficiency at 100% instead of 85%, results are ~18% too optimistic. (2) Battery SoH — an aged battery has less actual capacity than its label says. (3) Temperature — batteries in hot or cold environments lose capacity. (4) Surge loads — appliances like refrigerators draw more during startup. (5) Peukert effect — lead-acid batteries deliver less Ah at high discharge rates. Adjust all sliders to match your real conditions.
Use Solar Mode. Enter your daily energy consumption in Wh/day, solar panel wattage, peak sun hours, and the number of cloudy days you need to cover (autonomy days). A typical home using 1,200 Wh/day with 3 days of autonomy at 24V with 80% DoD needs approximately 188Ah — so a 200Ah LiFePO4 at 24V is the practical choice. The number of physical batteries depends on the Ah of each unit you purchase.
Yes — the calculator works for any DC battery system. For a car battery (starting), note that cranking batteries are not designed for deep discharge; use a deep-cycle type for accessory backup. For fire alarm standby batteries, input the panel’s standby current draw (in Watts) and set the duration to the required standby period (24–72 hours per code). For UPS systems, use the battery’s VA rating as your load (or apply the power factor: VA × 0.8 = Watts).
DoD is the maximum percentage of a battery’s capacity you should use before recharging. Discharging a 100Ah flooded lead-acid battery to 100% DoD gives maximum energy per cycle but destroys it within 50–100 cycles (often under a year). At 50% DoD, the same battery lasts 300–600 cycles (3–5 years). LiFePO4 tolerates 80–90% DoD with 2,000+ cycles. The calculator’s DoD slider directly reduces the usable Wh in all formulas. Respecting DoD limits is the single biggest factor in battery longevity.
A pure sine wave inverter produces clean AC power identical to grid electricity — safe for all appliances including refrigerators, AC units, medical devices, and sensitive electronics. Efficiency: 90–95%. A modified sine wave inverter produces a stepped approximation — cheaper, but causes extra heat in some motors and can damage sensitive electronics. Efficiency: 78–85%. Lower efficiency means your battery empties faster for the same load. Always use pure sine wave for refrigerators, compressors, and medical equipment.
Click the Multi-Appliance tab at the top of the calculator. Use the Quick Add buttons to instantly add common appliances — fan, lights, refrigerator, WiFi router, PC, CCTV — or click "Add Custom Appliance" to enter any device. For each item, set the wattage, quantity, and hours per day. The calculator auto-sums your total simultaneous load in watts and daily energy in Wh/day. Then either enter an existing battery Ah to calculate runtime, or set a desired backup duration to get the required battery size.
The Peukert exponent (k) describes how quickly a lead-acid battery loses effective capacity at high discharge rates. A value of 1.0 means no effect (ideal). Lead-acid typical range: 1.1–1.3. The default of 1.2 suits most flooded batteries. If you know your specific battery’s Peukert constant from the datasheet, enter it. For AGM use 1.1–1.15. For LiFePO4 and lithium-ion, leave it at 1.05 (near-ideal). Only change this field if you have reliable data — incorrect values worsen accuracy.
After running any calculation, scroll down to the Charging Time Estimator in the results section. Enter your charger’s output current in amps (printed on the charger) and the charging efficiency (90% for most smart chargers). The calculator uses: Charge Time = Ah discharged ÷ (Charger amps × efficiency). Example: 50Ah to restore, 10A charger, 90% efficiency = 50 ÷ 9 = 5.6 hours.
🧰 Try the Battery Calculator & Explore Related Tools
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