"How long will the battery last?" is the first question every backup-power customer asks and the one most online calculators answer badly — usually by ignoring depth of discharge, inverter losses, or both. This calculator does the full chain: battery capacity in amp-hours or watt-hours, derated by the chemistry-appropriate depth of discharge, derated again by inverter efficiency, divided by your actual load in watts. The result is the runtime number you can plan around, not the marketing number on the box.
We already publish a Battery Backup Runtime guide with quick-reference charts — think of this page as the deeper interactive version: full control over DoD, efficiency, and load, plus the engineering math to verify any answer by hand.
Battery Runtime Calculator
Ah/Wh × depth of discharge × inverter efficiency ÷ load · LFP, AGM, gel & flooded presets
Method: Runtime (h) = (V × Ah × DoD × efficiency) ÷ load watts. Sizing a new bank? Use the home battery bank sizing guide to work backward from the hours you need.
The Runtime Formula and Why Every Term Matters
Battery runtime is a four-step derating problem, and skipping any step produces a fantasy number:
Runtime (hours) = (Voltage × Amp-hours × DoD × Inverter efficiency) ÷ Load watts
Step 1 — nameplate energy. Voltage × amp-hours gives watt-hours, the only honest unit of stored energy. A 48 V × 100 Ah bank holds 4,800 Wh; a 12 V × 100 Ah battery holds 1,200 Wh. Same amp-hours, four times the energy. This is why comparing batteries on Ah alone is meaningless.
Step 2 — depth of discharge. Lead-acid chemistries (flooded, AGM, gel) live short, unhappy lives below 50% DoD, so only half the nameplate is practically usable. LiFePO4 comfortably delivers 80–100% — we use 90% as the planning figure — which is a large part of why LFP beats AGM on cost per delivered kilowatt-hour over the battery's life.
Step 3 — inverter efficiency. Converting DC to AC costs 5–15% of the energy. A quality hybrid like the EG4 18kPV runs in the low-to-mid 90s; budget modified-sine units can waste 15% and damage sensitive loads while they're at it. DC-only loads (lights, USB, RV circuits) skip this loss entirely.
Step 4 — divide by load. Watts, measured at the wall, not guessed from nameplates. A "1,500 W" space heater on medium may draw 900 W; a fridge rated 700 W averages 100–150 W across a day because the compressor cycles.
Worked Example 1: 48 V 100 Ah LFP Server Rack Battery
The most common battery we sell — a 48 V, 100 Ah LiFePO4 module like the EG4 LifePower4 — feeding a 500 W critical-loads panel through a 92%-efficient hybrid inverter:
Runtime = (48 × 100 × 0.90 × 0.92) ÷ 500 = 3,974 ÷ 500 = 7.9 hours
Two rack modules in parallel double it to ~15.9 hours. Three modules cover a full night for a typical critical-loads panel — which is why the 15 kWh class in our 15–30 kWh battery collection is the sweet spot for overnight backup.
Worked Example 2: 12 V 200 Ah AGM Bank
A pair of 12 V 100 Ah AGM batteries in parallel (12 V, 200 Ah total) running a 300 W RV load through an 85%-efficient inverter:
Runtime = (12 × 200 × 0.50 × 0.85) ÷ 300 = 1,020 ÷ 300 = 3.4 hours
The same money in a single 12 V 200 Ah LFP at 90% DoD and 92% efficiency delivers 6.6 hours — nearly double the runtime — before you even count cycle life. For charging-side math on small systems like this, see how long a 200 W panel takes to charge a 12 V 100 Ah battery.
Worked Example 3: Whole-Home 51.2 V 280 Ah LFP
A 14.3 kWh wall-mount bank (51.2 V × 280 Ah) carrying a 1,000 W average home load at 92% efficiency:
Runtime = (51.2 × 280 × 0.90 × 0.92) ÷ 1,000 = 11,870 ÷ 1,000 = 11.9 hours
That is an evening through sunrise for a disciplined home. Pair it with the Solar Panel Sizing Calculator to confirm the array can refill 14 kWh the next day.
Depth of Discharge Reference by Chemistry
| Chemistry | Planning DoD | Typical cycle life @ that DoD | Notes |
|---|---|---|---|
| LiFePO4 (LFP) | 90% | 4,000–6,000 cycles | BMS-limited; 100% possible but trims lifespan |
| AGM | 50% | 400–600 cycles | 80% DoD drops life to ~250 cycles |
| Gel | 50% | 500–800 cycles | Slow-charge only; heat sensitive |
| Flooded (FLA) | 50% | 300–500 cycles | Needs watering and venting |
Runtime Quick Table: 48 V 100 Ah LFP (90% DoD, 92% inverter)
| Load | Runtime | What that load looks like |
|---|---|---|
| 200 W | 19.9 h | Fridge cycling + lights + router |
| 500 W | 7.9 h | Critical-loads panel, disciplined use |
| 1,000 W | 4.0 h | Fridge + window AC fan + TV |
| 2,000 W | 2.0 h | Microwave use + kitchen circuits |
| 3,500 W | 1.1 h | Window AC running continuously |
| 5,000 W | 0.8 h | Near the limit of one 100 Ah module's BMS |
Battery Bank Sizing Quick Reference (Overnight, 10 h autonomy)
Working backward from a target: required bank Wh = load W × 10 h ÷ DoD ÷ 0.92 inverter efficiency. Common scenarios on LFP at 90% DoD:
| Overnight load | Required bank | Practical build |
|---|---|---|
| 200 W | 2,415 Wh | 1 × 12 V 200 Ah LFP or 1 × 24 V 100 Ah |
| 400 W | 4,831 Wh | 1 × 48 V 100 Ah rack module |
| 800 W | 9,662 Wh | 2 × 48 V 100 Ah modules (~10 kWh) |
| 1,200 W | 14,493 Wh | 1 × 14.3 kWh wall-mount or 3 rack modules |
| 2,000 W | 24,155 Wh | 2 × 14.3 kWh wall-mounts or 5 rack modules |
On AGM at 50% DoD every row nearly doubles — 400 W overnight needs about 8.7 kWh of lead-acid nameplate, which is four 12 V 200 Ah AGMs weighing ~520 lb. The same job in LFP is one rack module at ~100 lb. Weight, floor space, and cycle life all tilt the same direction.
Temperature, Age, and Other Real-World Derates
The formula assumes a healthy battery at room temperature. Cold hits lead-acid hard: at freezing, an AGM delivers roughly 80% of its rated capacity, and charging below freezing risks damage. LFP holds capacity better in the cold but most BMS units block charging below 0 °C entirely — a runtime-only concern in winter outages if the battery can't also recharge. Age is the quieter derate: a five-year-old AGM bank may hold 70–80% of nameplate even when well maintained. For outage planning on older banks, measure with a controlled discharge or shave 20% off the math. Finally, inverter standby draw — typically 20–60 W with no load — runs 24/7 whenever the inverter is on, quietly consuming 0.5–1.4 kWh per day that never reaches an appliance. On small banks, idle draw alone can eat 10–25% of stored energy overnight.
Inverter Efficiency Reference
| Inverter class | Typical efficiency | Impact on 4,800 Wh bank |
|---|---|---|
| Premium hybrid (48 V) | 94–96% | 4,512–4,608 Wh delivered |
| Typical hybrid/AIO | 90–93% | 4,320–4,464 Wh delivered |
| Budget pure sine | 85–90% | 4,080–4,320 Wh delivered |
| Modified sine | 80–85% | 3,840–4,080 Wh — plus waveform risk |
| DC loads only | ~100% | 4,800 Wh delivered (no conversion) |
Common Battery Runtime Mistakes
- Using nameplate watt-hours as usable energy. On lead-acid that overstates runtime by 2×. On LFP it still overstates by 10%.
- Forgetting inverter losses. A 90%-efficient inverter turns a calculated 10 hours into 9. On multi-day autonomy plans, that lost hour compounds.
- Ignoring the Peukert effect on lead-acid. Lead-acid capacity is rated at a 20-hour discharge. Pull at 0.5C or faster and usable capacity drops 10–25% below the math. LFP's flat curve makes it nearly immune at normal loads.
- Sizing to surge instead of average. Runtime cares about average watts; the inverter's surge rating handles motor starts. Conflating the two oversizes the battery or undersizes the inverter.
- Believing "10 kWh" labels blindly. Some brands label by cell energy before BMS reserves. Reputable lines like EG4, Fortress, and SimpliPhi publish usable figures — check the datasheet, not the sticker.
- Deep-cycling lead-acid daily. The 20/80 battery rule exists for a reason: habitual 80%+ discharges on AGM cut calendar life to a fraction of spec.
Measuring Your Real Load (The Step Everyone Skips)
Every runtime answer is only as good as the load figure feeding it. Nameplate watts overstate steady draw on anything with a compressor or thermostat and understate it on resistive heaters at full tilt. The reliable approach: plug suspect loads through an inexpensive plug-in watt-meter for 24 hours and use the average, not the peak. For whole-panel figures, clamp meters and energy monitors on the critical-loads panel give you the honest hourly profile — and usually reveal that your "essential" loads are 30–40% smaller than feared, which translates directly into either longer runtime or a smaller, cheaper battery bank.
Working Backward: From Hours Needed to Bank Size
Flip the formula to design a bank: Required Wh = Load W × hours needed ÷ DoD ÷ efficiency. Need 500 W for 10 hours on LFP at 92%? That's 500 × 10 ÷ 0.90 ÷ 0.92 = 6,039 Wh ≈ a 48 V 130 Ah bank — in practice, two 100 Ah modules for margin. The home battery bank sizing guide covers multi-day autonomy, and our off-grid storage sizing guide layers solar recharge on top. When the outage risk is measured in days, not hours, run the Generator Sizing Calculator alongside this one — batteries and generators solve different outage shapes.
Ready to price the bank? Start with our 10 kWh batteries for critical loads or the 15–30 kWh range for overnight whole-home coverage, or hit "Get this quoted" above and we'll build the bill of materials from your numbers.
Frequently Asked Questions
How do I convert amp-hours to watt-hours?
Multiply amp-hours by the battery's nominal voltage: 100 Ah × 48 V = 4,800 Wh (4.8 kWh). Never compare Ah across different voltages — a 100 Ah 12 V battery stores a quarter of the energy of a 100 Ah 48 V battery.
What depth of discharge should I use for LiFePO4?
90% is the standard planning figure. LFP can technically discharge to near 100%, but holding 10% in reserve extends cycle life and keeps the BMS from cutting power at the worst moment. Most manufacturers warranty 80–100% DoD; check your datasheet.
Why is my real runtime shorter than the calculation?
The usual suspects: inverter standby draw (20–60 W even at idle), loads larger than measured, battery capacity faded with age, cold ambient temperatures (lead-acid loses ~20% at freezing), and the Peukert effect on lead-acid at high discharge rates. Measure loads at the wall and re-run the numbers.
Does a bigger inverter reduce runtime?
Indirectly, yes. Larger inverters have higher idle draw and often worse efficiency at light loads. An 18 kW hybrid running a 300 W load may operate well below its peak-efficiency band. Match inverter size to realistic loads, not worst-case fantasies.
Can I mix old and new batteries in one bank?
Avoid it. Mixed-age lead-acid banks charge and discharge unevenly, dragging the new batteries down to the old ones' level. Even in LFP, mixing capacities or brands complicates BMS coordination. Expand with identical modules within the manufacturer's supported window.
How many batteries do I need for overnight backup?
Add your overnight loads in watts, multiply by hours (typically 10–12), then divide by DoD and inverter efficiency. A 400 W overnight load × 12 h ÷ 0.90 ÷ 0.92 ≈ 5,800 Wh — one 48 V 100 Ah LFP plus margin, or two for comfort. Run your exact numbers in the calculator above.
Shop the products mentioned in this guide and get everything you need for your project.
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