Free Battery Run Time Calculator
Calculate how long a battery will power your device. UPS, camera, phone, and solar.
What is a battery run time calculator?
A battery run time calculator is a free online tool that helps you estimate how long a battery will last based on capacity and load current. It runs entirely in your browser — no signup, no app, and no data is ever sent to a server.
A battery run time calculator estimates how long a battery can power a device based on battery capacity (Ah, mAh, or Wh) and device power draw in watts. Whether you're sizing a UPS for your home office, estimating camera battery life on location, or planning a solar power system, knowing the real run time helps you choose the right battery and avoid unexpected power loss. The calculator accounts for inverter efficiency losses and depth-of-discharge limits.
How it works
Capacity in Wh = Ah × voltage, or mAh ÷ 1000 × voltage, or directly in Wh. Effective load = device watts ÷ inverter efficiency. Run time = Wh ÷ effective load. Depth of discharge (DoD) shows run time at 80% capacity (recommended maximum for lead-acid battery health; lithium can go to 90-100%).
When to use a battery run time calculator
Sizing a UPS for electronics and networking equipment. Planning off-grid solar battery banks. Estimating camera battery needs for event coverage. Choosing a power bank for travel. Calculating emergency backup needs for sump pumps, medical devices, or refrigerators. The calculator is essential for any battery-powered setup where you need to know the real runtime.
Common mistakes in battery run time estimation
The biggest mistake people make is ignoring inverter efficiency losses. A 100Ah 12V battery stores 1200 Wh, but if you are powering an AC device through an inverter, you lose 10–15% right away. That means your 1200 Wh battery effectively provides 1020–1080 Wh. The calculator includes an inverter efficiency input, but most online tools do not, which is why they consistently overestimate run time.
Another common error is using the full rated capacity of a lead-acid battery. Deep-cycle lead-acid batteries should not be discharged below 50% for regular use or 80% for occasional use. Draining them to zero regularly destroys the battery in months. Lithium batteries handle deeper discharges better — 90–100% is safe with LiFePO4 chemistry. The depth-of-discharge reading on this calculator helps you see the real usable capacity.
Temperature effects are also frequently overlooked. Lead-acid batteries lose 20–40% capacity below 50°F (10°C). If you are sizing a battery for a cold garage or an off-grid cabin in winter, you need to oversize by 30–50% to compensate. The calculator includes a temperature adjustment for this reason.
How to use this calculator
- Enter battery capacity — Input your battery's capacity in Ah, mAh, or Wh.
- Enter device power — Input what your device draws in watts.
- Read the result — See estimated run time in hours, plus capacity in multiple units.
Frequently asked questions
How do I calculate battery run time?
Run time (hours) = battery capacity (Wh) ÷ device power draw (W). For a 100Ah 12V battery powering a 50W device: capacity = 100 × 12 = 1200 Wh. Run time = 1200 ÷ 50 = 24 hours. For mAh batteries (phones, cameras): capacity (Wh) = mAh ÷ 1000 × voltage. The calculator works with all unit types.
How long will a 100Ah battery run a TV?
A typical 55-inch LED TV draws about 100W. A 100Ah 12V battery = 1200 Wh. 1200 ÷ 100 = 12 hours. For a 32-inch TV (50W): 24 hours. For a large OLED TV (200W): 6 hours. These are theoretical maximums — inverter efficiency (85-90%) reduces run time by 10-15%.
How long does a 5000mAh power bank charge a phone?
Assuming 90% efficiency: usable capacity = 5000 × 0.9 = 4500 mAh at 3.7V = 16.65 Wh. An iPhone 15 (3349 mAh battery at 3.7V = 12.4 Wh) gets about 16.65 ÷ 12.4 = 1.3 full charges. An Android phone with 5000mAh battery (18.5 Wh) gets about 0.9 charges.
How do I size a battery for my UPS?
For a home UPS powering a PC (300W) + monitor (50W) + modem (10W) for 2 hours: total load = 360W. Energy needed = 360 × 2 = 720 Wh. With 90% inverter efficiency: 720 ÷ 0.9 = 800 Wh. At 12V: 800 ÷ 12 = 67 Ah minimum. Always buy 20-30% extra capacity for battery degradation over time.
How does temperature affect battery run time?
Cold temperatures (below 50°F/10°C) reduce effective capacity by 20-40% for lead-acid and 10-30% for lithium-ion. Hot temperatures (above 95°F/35°C) increase short-term capacity but permanently degrade the battery faster. The calculator includes a temperature adjustment factor for realistic estimates.
What is the Peukert effect and does this calculator account for it?
The Peukert effect describes how lead-acid batteries deliver less usable capacity at higher discharge rates. A battery rated at 100 Ah over 20 hours may only deliver 70 Ah over 1 hour. The calculator uses simple capacity times voltage for its estimate, so for high-drain applications like inverters, add a 15–25% safety margin to the runtime result.