Live energy model

Solar Battery Backup Time Calculator

Model battery bank capacity, inverter efficiency, depth of discharge, and load wattage to estimate real backup runtime — with an animated energy‑flow diagram.

Solar

Battery

Load

Battery & Load Inputs

Battery Voltage (V)

Nominal voltage — 12 V, 24 V, 48 V for lead‑acid / LiFePO₄ banks.

Battery Capacity (Ah)

Amp‑hour rating at the 20‑hour rate (C/20). Energy = V × Ah.

Theoretical: 2400 Wh

State of Charge (SoC) 80%

0%50%100%

Current available charge before discharge begins.

Depth of Discharge (DoD) 80%

10%50%100%

Maximum safe discharge. Lead‑acid ≈ 50 %, LiFePO₄ ≈ 80–90 %.

Inverter Efficiency 92%

50%75%100%

Typical pure sine wave inverters: 85–95 %. Include wiring losses.

Load Wattage (W)

Continuous AC load. Add 20–30 % headroom for surge if needed.

Optional Solar Input

Solar Array (W)

Peak Sun Hours

Solar charging reduces net discharge and extends runtime during daylight.

Energy Flow

Live diagram

Runtime Results

Estimated Backup Time

6.4 hours

≈ 6 h 24 min

Theoretical

2400

Usable Energy

1536

Current Charge

1920

Remaining

Wh after load

State of Charge 80%

0% DoD limit: 80% 100%

Formula: Runtime = (V × Ah × SoC × DoD × η_inv) ÷ Load_W. Solar input offsets load during daylight hours.

Real‑world note: Actual runtime is typically 10–20 % lower due to Peukert effect, temperature, inverter idle draw, and battery age. Use this estimate as an upper bound.

1 Ah vs Wh — Why both matter

Amp‑hours (Ah) measure charge capacity — how much current a battery can deliver over time. Watt‑hours (Wh) measure energy, which is what actually runs your appliances.

A 12 V, 200 Ah battery stores 12 × 200 = 2,400 Wh of theoretical energy. A 24 V, 100 Ah battery stores the same 2,400 Wh — same energy, different voltage/current trade‑off. Always convert to Wh before comparing batteries or calculating runtime.

Wh = V × Ah

Example: 48 V × 100 Ah = 4,800 Wh

2 Depth of Discharge (DoD) & battery chemistry

DoD is the percentage of total capacity you can safely use before recharging. Discharging a lead‑acid battery below 50 % regularly shortens its life dramatically. LiFePO₄ (lithium iron phosphate) tolerates 80–90 % DoD with minimal degradation.

Chemistry	Typical DoD	Cycle life
Flooded lead‑acid	50 %	300–500
AGM / Gel	50–60 %	500–800
LiFePO₄	80–90 %	3,000–7,000

3 Inverter efficiency — the hidden loss

Inverters convert DC battery power to AC household power. This conversion isn't 100 % efficient — 5–15 % is lost as heat. A 92 % efficient inverter means 8 % of your battery energy never reaches the load.

Why this matters: a 2,400 Wh battery with 92 % inverter efficiency delivers only 2,208 Wh of AC energy. Always include inverter efficiency in runtime calculations — it's one of the biggest real‑world derating factors.

Typical efficiency ranges

• Modified sine wave: 75–85 %
• Pure sine wave (budget): 85–90 %
• Pure sine wave (premium): 90–96 %

4 Load wattage & surge

Load wattage is the continuous power your appliances draw. But many loads — motors, compressors, pumps — have a surge (inrush) current 2–3× their running wattage for a few seconds.

For runtime calculations, use continuous wattage. For inverter sizing, use surge wattage. A refrigerator might run at 150 W but surge to 600 W on compressor start.

Common continuous loads

LED bulb: 10 W Laptop: 50–80 W Wi‑Fi router: 15 W TV (LED): 60–120 W Fridge: 100–200 W CPAP: 30–60 W

5 Solar charging — extending runtime

During daylight, solar panels offset your load and recharge the battery. A 600 W array in 4.5 peak sun hours produces roughly 2,700 Wh/day — but after charge controller losses (~95 % efficiency) and wiring losses, expect 2,300–2,500 Wh usable. If your load draws 400 W continuously, solar covers about 5–6 hours of that load per day, significantly extending battery runtime.

Step 1

Solar production

Array W × peak sun hours × 0.75 (derate)

Step 2

Net battery drain

Load − solar production (if positive)

Step 3

Extended runtime

Usable Wh ÷ net drain

6 Worked example — 12 V, 200 Ah LiFePO₄

Given

Battery voltage12 V
Capacity200 Ah
State of charge80 %
Depth of discharge80 %
Inverter efficiency92 %
Load400 W

Calculation

1. Theoretical energy = 12 × 200 = 2,400 Wh
2. Usable energy = 2,400 × 0.80 × 0.80 = 1,536 Wh
3. After inverter = 1,536 × 0.92 = 1,413 Wh
4. Runtime = 1,413 ÷ 400 = 3.53 h ≈ 3 h 32 min

With 600 W solar (4.5 sun hours) covering ~5 h of load, runtime extends to roughly 6–7 h net.

Frequently asked questions

How accurate is a solar battery backup time calculator?

It gives a theoretical upper bound. Real runtime is typically 10–20 % lower due to Peukert effect (lead‑acid), temperature derating, inverter idle draw, and battery age. Use it for planning and sizing, not as a guaranteed runtime.

What depth of discharge should I use for my battery?

Flooded lead‑acid: 50 %. AGM/Gel: 50–60 %. LiFePO₄: 80–90 %. Exceeding these regularly will shorten battery life. The calculator lets you adjust DoD so you can model both conservative and aggressive scenarios.

Does inverter efficiency really make that much difference?

Yes. Going from 85 % to 95 % efficiency recovers about 12 % more runtime. Over a 5‑hour discharge, that's an extra 30–40 minutes. Premium pure sine wave inverters are worth the investment for critical backup.

Can I use this calculator for a portable power station?

Yes. Enter the power station's battery voltage (often 12 V, 24 V, or 48 V), its Ah capacity, and its inverter efficiency (usually 85–90 %). Many power stations list Wh directly — divide Wh by voltage to get Ah.

How do I account for solar input in the runtime estimate?

The optional solar fields let you estimate daily solar production (array W × peak sun hours × 0.75 derate). If production exceeds load, the battery isn't draining during those hours — effectively extending runtime. The calculator uses a simplified net‑drain model for clarity.

Why does my calculated runtime differ from the manufacturer's spec?

Manufacturers often quote at 100 % DoD, 100 % inverter efficiency, and a specific discharge rate (C/20). Real‑world conditions — partial state of charge, lower DoD for longevity, inverter losses, and Peukert effect — reduce actual runtime. This calculator models those real‑world deratings.

What's the difference between usable capacity and theoretical capacity?

Theoretical capacity is the raw V × Ah energy. Usable capacity is what you can actually draw after applying state of charge, depth of discharge, and inverter efficiency. The gap between them is often 40–60 % — which is why knowing both is critical for realistic planning.

Should I size my battery for 1 day or 3 days of autonomy?

For off‑grid systems, 2–3 days of autonomy is common in cloudy climates. For backup power (grid‑tied with battery), 1 day is usually sufficient. Use this calculator to model different loads and see how many days your bank covers.

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