Buying guide
How to size a battery bank
Turn your daily loads into a battery size in watt-hours.
Published · Updated
Add up the watt-hours you use per day (watts × hours for each device). Multiply by the days of autonomy you want, then divide by the usable fraction of the battery and by the inverter's efficiency. Example: 1,200 Wh/day × 2 days ÷ 0.92 (inverter, NREL PVWatts derate) ÷ 0.80 (usable depth of discharge, Victron LiFePO4 rating) ≈ 3,261 Wh of rated battery capacity. LiFePO4 batteries generally let you use more of their rated capacity than lead-acid. The power planner runs the math for you.
- Add up daily watt-hours: watts × hours for each device.
- Divide by usable depth of discharge and inverter efficiency.
- Multiply by the days of autonomy you want.
Step 1: build a daily load list
List every device you want to run and estimate watts × hours per day. The device's label or manual gives watts. For devices that cycle, like refrigerators, a plug-in energy meter gives a much better number than the label. The Virginia Cooperative Extension guide describes the same watts × hours method.
| Device (example) | Watts | Hours/day | Wh/day |
|---|---|---|---|
| LED lights (4 × 10 W) | 40 | 5 | 200 |
| Refrigerator (metered average) | — | 24 | 1,000–1,500 (typical modern full-size range; meter yours) |
| Laptop | 60 | 4 | 240 |
| Wi-Fi router + modem | 15 | 24 | 360 |
| Phone charging | 10 | 2 | 20 |
The numbers above are examples. Your devices will be different. Fridge use varies a lot by model, age and room temperature, so measure yours.
Watch for surge loads
Motors and compressors like fridges, pumps and power tools draw several times their running watts for a moment when they start. That affects inverter sizing more than battery capacity. See inverter sizing and surge.
Step 2: choose days of autonomy
Days of autonomy is how long the bank should run your loads with no charging.
| Use case | Common planning range |
|---|---|
| Short outage backup (no solar) | 0.5–1 day |
| Backup with solar recharge | 1–2 days |
| Off-grid cabin, sunny climate | 2–3 days |
| Off-grid, cloudy or winter use | 3–5 days, or add a generator |
These are rules of thumb, not standards. Where you live and how critical the loads are matter most.
Step 3: account for usable capacity and losses
Two corrections turn daily Wh into rated battery capacity.
Depth of discharge (DoD)
Not all of a battery's rated capacity is meant to be used on every cycle.
| Chemistry | Typical usable DoD for planning | Notes |
|---|---|---|
| LiFePO4 (LFP) | 80–100% | Many makers rate cycle life at 80% or more DoD |
| NMC lithium | 80–90% | Common in portable power stations |
| Flooded / AGM lead-acid | 50% | Deeper cycles shorten life considerably |
Use the figure in your battery's datasheet when you have it. See battery chemistry guide.
Inverter and wiring efficiency
When you run AC loads through an inverter, some energy is lost as heat. Many inverter datasheets list a peak efficiency in the high 80s to mid 90s percent. Real-world averages are often lower at light loads. This site uses 0.92, the inverter derate in the NREL PVWatts manual (Table 6). That is a grid-tie inverter figure; it is an estimate for a battery inverter. DC loads running straight from the battery skip this loss.
The formula
Rated capacity (Wh) = daily Wh × days ÷ inverter efficiency ÷ usable DoD
Worked examples
Example 1: weekend outage kit
Loads: lights, router, phones and laptop come to about 820 Wh/day. One day of autonomy, a LiFePO4 battery, 0.92 inverter efficiency (NREL PVWatts manual) and 0.80 usable depth (Victron LiFePO4 data, which rates cycle life at 80% DoD).
820 × 1 ÷ 0.92 ÷ 0.80 ≈ 1,114 Wh. A power station rated around 1,000–1,500 Wh fits, as long as its inverter handles the largest load. See the portable power station guide.
Example 2: keeping a fridge running
A fridge metered at 1,200 Wh/day, plus 300 Wh of other loads, is 1,500 Wh/day. Two days of autonomy, LFP at 0.80 DoD, 0.92 inverter efficiency:
1,500 × 2 ÷ 0.92 ÷ 0.80 ≈ 4,076 Wh. In 12 V LiFePO4 terms (12.8 V nominal), that's about 318 Ah, for example four 100 Ah batteries in parallel. At this size, many designers move to 24 V or 48 V. See system voltage.
Example 3: same loads with lead-acid
1,500 × 2 ÷ 0.92 ÷ 0.5 ≈ 6,522 Wh of rated lead-acid capacity (the 0.5 lead-acid depth is a common rule of thumb; check your battery's datasheet), about 1.6 times the lithium figure, and much heavier. That's why usable DoD matters as much as the rated number.
Step 4: check the recharge side
A battery bank only works if you can refill it. With solar, compare the daily Wh you need against the panels' expected production. NREL's PVWatts estimates monthly output for your location and array. Winter months often produce far less than summer. See peak sun hours and solar panel sizing.
| Check | Question |
|---|---|
| Daily recharge | Can solar or the grid put back at least your daily Wh, plus losses? |
| Worst month | Does December (Northern Hemisphere) still cover your loads, or do you need a generator? |
| Charge rate | Can the charger or controller refill the bank in the hours you have? |
Buyer scenarios
Apartment dweller, short outages
A portable power station sized to one day of essential loads is simple and needs no wiring. Focus on the inverter's continuous rating and the Wh capacity.
Homeowner wanting fridge and furnace fan backup
Furnace blowers and fridges have surge loads. Some homeowners use a larger battery system with a transfer switch or interlock. That's work for a licensed electrician, and local permits apply. See home backup battery basics.
Off-grid cabin
Plan the bank and the solar array together, size for the worst month, and consider a generator for long cloudy stretches. See off-grid cabin planning.
Common mistakes
- Sizing in amp-hours across different voltages. Convert to watt-hours first.
- Using the fridge's label wattage × 24 hours. Compressors cycle, so meter the real use.
- Forgetting inverter idle draw. Many inverters use some power just by being on. Check the datasheet's no-load figure.
- Ignoring cold-weather charging limits on lithium batteries.
- Leaving no margin. Batteries lose capacity as they age. Many planners add 10–25% headroom.
- Undersized wiring and fuses. Higher currents at 12 V need thick cable and proper overcurrent protection.
Safety notes
- Follow the battery maker's instructions for fusing, cabling, ventilation and temperature.
- Large stationary battery systems in homes fall under fire and electrical codes such as NFPA 855 and local requirements. Permits and inspection often apply.
- Anything connected to home wiring or the grid should be designed and installed by a licensed electrician. See solar and battery safety.
FAQ
Should I size in amp-hours or watt-hours?
Watt-hours. They let you compare batteries at different voltages. To convert, multiply amp-hours by nominal voltage. A 100 Ah battery at 12.8 V is 1,280 Wh, and a 100 Ah battery at 51.2 V is 5,120 Wh.
How many days of autonomy do I need?
It depends on why you have the battery. For short outages, many people plan for one day. For off-grid systems with solar, two to three days of cloudy-weather reserve is a common planning range. More days means a bigger and costlier bank.
Does cold weather reduce capacity?
Yes. Batteries deliver less capacity in the cold, and many lithium batteries shouldn't be charged below about 0 °C (32 °F) unless they have built-in heating. Check the manufacturer's charge and discharge temperature limits.
Can I add more batteries later?
Often, but follow the manufacturer's rules. Many makers recommend matching model and age, and some systems support only certain expansion units. Mixing chemistries or very different ages in one bank is generally discouraged.
Where do I go next?
Try the power planner to put numbers to your own situation, or see the product picks on the deals page.
Terms used here
- Watt (W)
- Rate of power use or production.
- Watt-hour (Wh)
- Energy: one watt for one hour.
- Amp-hour (Ah)
- Battery charge capacity at a given voltage.
- Depth of discharge (DoD)
- Share of a battery's capacity that is used.
- Cycle life
- Number of charge-discharge cycles a battery is rated for at a stated DoD.
- LiFePO4 (LFP)
- Lithium iron phosphate battery chemistry.
- NMC
- Nickel manganese cobalt lithium chemistry.
- AGM
- Absorbent glass mat sealed lead-acid battery.
- Inverter
- Converts DC battery power to AC household power.
- Peak sun hours
- Hours of equivalent 1,000 W/m² sunlight per day.
- Derate
- Factor for real-world losses in solar output.
- Days of autonomy
- Days a battery can run loads without recharging.
- Transfer switch
- Device that isolates home circuits from the grid during backup.
Sources
This guide is informational and based on published specifications and general references, not hands-on testing. Check the manufacturer's current specs before you buy.