How many batteries does it take to run your fridge and lights through a long outage? The Battery Backup Calculator answers that with a quick sizing estimate. You enter your load, runtime, and battery type, and it returns amp-hours, kilowatt-hours, and a count of 100Ah units.
It suits homeowners planning a small backup system, cabin owners, and RV or off-grid builders. It sizes only the battery bank, not the inverter, charger, or solar panels.
Table of Contents
- What the Calculator Does
- How to Use It, Step by Step
- The Battery Backup Formula
- Worked Example
- How to Read the Results
- Choosing Voltage and Battery Type
- Limitations and Assumptions
- FAQs About the Battery Backup Calculator
What the Calculator Does
The tool takes four inputs and produces three results. It finds the battery energy needed after inverter losses and your chosen depth of discharge.
- Required Capacity (Amp-Hours @ Voltage): total amp-hours at the system voltage, rounded up to a whole number.
- Battery Bank Size Needed (Gross Nameplate Energy): the same requirement in kilowatt-hours, with two decimals.
- 100Ah Battery Units Needed: how many 100Ah-equivalent batteries that capacity needs, rounded up.
There is one button, Calculate Battery Capacity. The results show the sample values when the page loads. They then update as you type or change a menu, and the button recalculates too.
How to Use It, Step by Step
- Enter Total Power Draw Load (Watts). Add up everything you want running at the same time.
- Enter Required Backup Runtime (Hours). The sample value is 12 hours.
- Pick Battery System Voltage: 12, 24, or 48 volts DC. The sample uses 48 volts.
- Pick Battery Depth of Discharge DoD (%). The choices are 80% lithium, 50% lead-acid or AGM, and 90% high-grade LFP.
- Read the three result cards, or press Calculate Battery Capacity to refresh them.
The Battery Backup Formula
The tool first finds the energy your load uses. It then divides by an inverter efficiency of 0.88 and by the depth of discharge. That gives the battery energy to buy.
Load W × Hours÷0.88 × DoD÷System Volts=Amp-Hours
- Net watt-hours = load in watts × hours.
- Gross watt-hours = net watt-hours ÷ (0.88 × DoD ÷ 100).
- Amp-hours = gross watt-hours ÷ system voltage, rounded up.
- Kilowatt-hours = gross watt-hours ÷ 1,000.
- Units = the rounded-up amp-hours ÷ 100, rounded up again.
Worked Example
Omar wants to run a 400 watt load for 8 hours from a 24 volt lead-acid bank. He picks the 50% depth of discharge option.
| Step | Calculation | Result |
|---|
| Net Energy | 400 × 8 | 3,200 Wh |
| Gross Energy | 3,200 ÷ (0.88 × 0.50) | 7,272.7 Wh |
| Amp-Hours | 7,272.7 ÷ 24 = 303.03, rounded up | 304 Ah @ 24V |
| Energy Card | 7,272.7 ÷ 1,000 | 7.27 kWh |
| Units | 304 ÷ 100 = 3.04, rounded up | 4 x 100Ah-equivalent at 24V |
The sample inputs are 600 W, 12 hours, 48 V, and 80%. They give 214 Ah, 10.23 kWh, and 3 units.
How to Read the Results
Required Capacity is the amp-hour rating the whole bank needs at the voltage you chose. Compare it with the rated capacity on the battery label.
Battery Bank Size Needed is the nameplate energy to buy, not the energy your load uses. It is larger because it includes inverter loss and the unused battery reserve.
100Ah Battery Units Needed counts 100Ah-equivalent units at the system voltage you chose. Treat the count as a starting point, since the tool does not design series or parallel wiring.
Choosing Voltage and Battery Type
Higher voltage means fewer amp-hours for the same energy. The 600 W, 12 hour job needs 214 Ah at 48 volts and 853 Ah at 12 volts. Larger loads usually suit 24 or 48 volt banks.
Depth of discharge is how much of the battery you plan to use. Lead-acid and AGM batteries last longer when you use only about half. Lithium iron phosphate batteries are often used deeper. Check your battery maker's data first.
The load number matters most. To turn amps and volts into watts, use the watt calculator. To size the panels that recharge the bank, try the solar panel calculator.
If you prefer an engine for backup, the generator size calculator covers that path. For cable runs from bank to inverter, see the wire size calculator and the voltage drop calculator. More tools are in the math and science calculators hub.
Limitations and Assumptions
- Inverter efficiency is fixed at 88%. You cannot change it, and it applies even to DC loads. A note on the page says so.
- The tool does not add temperature loss, battery aging, or charging time.
- It assumes a steady load. Real loads rise and fall, and motors surge when they start.
- Load must be 10 to 1,000,000 watts and runtime 1 to 720 hours. Blank or out-of-range entries show a message and N/A in all three cards.
- Local electrical codes apply, and a licensed electrician should do or approve any wiring.
FAQs About the Battery Backup Calculator
How do I find my total power draw in watts?
Check the label on each device and add the watts of everything that runs together. If a label shows only volts and amps, multiply them to get watts.
Why does the tool ask for depth of discharge?
You should not drain most batteries to zero. A lower percentage means you need a bigger bank for the same runtime.
Why is the energy result bigger than my load times hours?
The tool adds an 88% inverter efficiency and your depth of discharge. Both increase the battery energy you must buy.
Does the result include solar panels or a charger?
No. It sizes the battery bank only. Use a separate tool for panels, and ask the charger maker about charging rates.
Can I use 200Ah batteries instead of 100Ah units?
Yes. Use the amp-hour card as your target and divide it by the size you choose. The unit card always counts 100Ah batteries.
Is the result safe to buy from?
Treat it as a planning estimate. Get a system design from a qualified installer before you buy batteries.
This calculator gives general estimates only and is not professional advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.