Sizing a battery comes down to one core calculation: figure out how much energy you use in a day, then build in enough extra capacity to account for efficiency losses, safe discharge limits, and the number of days you need your battery to last without recharging. The math itself is straightforward, but skipping any one of these factors can leave you with a battery bank that’s too small, or one that wears out years before it should.
Step 1: Calculate Your Daily Energy Use
Start by listing every device your battery will power. For each one, multiply its wattage by the number of hours you’ll run it per day. The result is watt-hours (Wh), which is the standard unit for measuring battery capacity.
A CPAP machine that draws 40 watts and runs 8 hours per night uses 320 watt-hours. A refrigerator rated at 150 watts that cycles on and off for roughly 8 hours of actual compressor time uses 1,200 watt-hours. A few LED lights at 10 watts each, running 5 hours, add another 150 watt-hours. Add every device together and you have your total daily energy requirement. For this example, that’s 1,670 Wh per day.
If you’re not sure of a device’s wattage, check the label on the back or bottom. You can also use a plug-in power meter (around $20) to measure actual consumption, which is especially helpful for devices like refrigerators that don’t run continuously.
Step 2: Factor In Inverter Efficiency
If your battery stores power as DC (direct current) but your devices run on standard AC (alternating current), you’ll need an inverter. Inverters aren’t perfectly efficient. A good modern inverter converts power at roughly 90 to 95 percent efficiency, meaning 5 to 10 percent of your stored energy is lost as heat during conversion.
To account for this, divide your daily watt-hours by the inverter’s efficiency rating. Using a typical 92.5 percent efficiency: 1,670 Wh ÷ 0.925 = 1,805 Wh. That’s the amount of energy you actually need to pull from the battery each day. If all your devices run directly on DC (common in RVs and small off-grid setups), you can skip this step.
Step 3: Choose Your Battery Chemistry
The type of battery you pick determines how much of its rated capacity you can safely use on a regular basis. This is called depth of discharge (DoD), and it’s one of the biggest factors in sizing.
Lithium iron phosphate (LiFePO4) batteries can technically discharge to 100 percent, but manufacturers recommend stopping at 80 percent DoD to extend their lifespan. They’re lighter, last longer (often 3,000 to 5,000 cycles), and deliver consistent power regardless of how fast you drain them. They cost more upfront.
Lead-acid batteries (including AGM and flooded types) experience significantly reduced cycle life if discharged below 50 percent. That means you can only use half the battery’s rated capacity on a regular basis before you start shortening its life dramatically. They’re cheaper per unit but heavier, bulkier, and typically last 500 to 1,200 cycles.
This difference is critical for sizing. A lead-acid battery bank needs to be roughly twice the size of a lithium bank to deliver the same usable energy every day.
Step 4: Account for Days of Autonomy
Days of autonomy is the number of days your battery bank can power your loads without any recharging. If you’re building a solar system, cloudy stretches mean your panels may produce little to no power for days at a time. Three to five days of autonomy is the generally accepted standard for off-grid solar systems.
For backup power during grid outages, you might size for one or two days if outages in your area are short, or up to five days if you live somewhere with frequent extended outages. An RV or boat setup where you’re plugging in regularly might only need one day.
Multiply your adjusted daily energy need by your chosen days of autonomy. Using two days for a home backup system: 1,805 Wh × 2 = 3,610 Wh.
Step 5: Adjust for Depth of Discharge
Now divide by the safe depth of discharge for your battery type to get the total battery capacity you need to purchase.
For LiFePO4 at 80 percent DoD: 3,610 Wh ÷ 0.80 = 4,513 Wh. For lead-acid at 50 percent DoD: 3,610 Wh ÷ 0.50 = 7,220 Wh. The difference is stark. The lead-acid system needs nearly 60 percent more rated capacity to safely deliver the same usable energy.
Step 6: Convert to Amp-Hours
Batteries are often sold with amp-hour (Ah) ratings rather than watt-hour ratings. To convert, divide your total watt-hours by the battery bank’s voltage.
For a 12V lithium system: 4,513 Wh ÷ 12V = 376 Ah. For a 24V system: 4,513 Wh ÷ 24V = 188 Ah. For a 48V system: 4,513 Wh ÷ 48V = 94 Ah. Higher voltage systems need fewer amp-hours because each amp carries more power. They also use thinner wiring and lose less energy to resistance, which is why larger systems (generally above 3,000 watts) typically use 24V or 48V architectures.
The Lead-Acid Discharge Rate Problem
Lead-acid batteries have a quirk that lithium batteries mostly avoid. Their rated amp-hour capacity is based on a slow, 20-hour discharge. If you drain them faster than that, you get less usable energy than the label suggests. This relationship is described by Peukert’s Law.
A practical example: a battery rated at 100 Ah (discharged at 5 amps over 20 hours) will not actually last 20 hours at a 5-amp draw. Real-world discharge time drops to roughly 12 hours because of internal chemical inefficiencies that worsen at higher loads. If you’re running high-draw appliances like a microwave or power tools from lead-acid batteries, you may need 10 to 20 percent more capacity than the formula alone suggests. LiFePO4 batteries deliver nearly their full rated capacity regardless of discharge rate, which is one reason they’ve become the preferred choice for most new installations.
Putting It All Together
Here’s the full sizing formula in sequence:
- Daily watt-hours: Add up (watts × hours per day) for every device
- Inverter adjustment: Divide by inverter efficiency (typically 0.90 to 0.95)
- Autonomy: Multiply by days of autonomy (1 to 5 depending on your situation)
- Depth of discharge: Divide by safe DoD (0.80 for lithium, 0.50 for lead-acid)
- Convert to amp-hours: Divide by system voltage (12V, 24V, or 48V)
Using the running example with lithium batteries, a 12V system, two days of autonomy, and a 92.5 percent inverter: a household drawing 1,670 Wh per day needs a battery bank rated at roughly 376 Ah at 12V. In practice, you’d round up to the nearest available battery size. Two 200 Ah lithium batteries wired in parallel would give you 400 Ah, providing a small margin above the minimum.
Common Sizing Mistakes
The most frequent error is sizing based only on daily watt-hours without accounting for depth of discharge. A 3,600 Wh lead-acid battery bank does not give you 3,600 Wh of usable energy. It gives you 1,800 Wh if you want reasonable battery life. People who skip this step end up draining batteries too deeply, replacing them in two or three years instead of five to eight.
Another common mistake is underestimating surge loads. Devices with motors (refrigerators, pumps, air conditioners) draw two to seven times their running wattage for a few seconds at startup. This doesn’t significantly affect your Wh calculation, but it does affect your inverter sizing and can cause problems if your battery can’t deliver the peak current. Check startup wattage for any motor-driven device and make sure your battery’s maximum continuous discharge rate can handle it.
Finally, temperature matters. Lead-acid batteries lose roughly 1 percent of capacity for every degree Fahrenheit below 77°F. A battery bank in an unheated garage during winter may deliver 20 to 30 percent less than its rated capacity. Lithium batteries are less affected but still perform best between 32°F and 95°F. If your batteries will live in extreme temperatures, add 10 to 25 percent extra capacity as a buffer.