Free engineering tool
Solar Battery Calculator
OriginalSize your solar or backup battery bank from daily energy use, backup time, system voltage, and battery chemistry. Get amp-hours, watt-hours, kilowatt-hours, and a ready-to-build series-parallel configuration — instantly, with no page reloads.
Energy consumption
Backup requirement
System configuration
Auto-filled for Lithium LiFePO4. Adjust if your manufacturer specifies otherwise.
Battery specification
Leave blank to auto-calculate
Sun hours & safety margin
Used for cross-checking solar array sizing, 1–10 hours
Extra derate for cold-climate capacity loss
Derate for capacity fade over years of cycling
Recommended battery bank
Battery configuration
3 × 48V 100Ah
3 in series × 1 in parallel
Energy flow
Battery bank layout
Full calculation breakdown
Guide
Understanding solar battery storage
Sizing a solar battery bank is one of the most consequential decisions in any off-grid or backup power system, and it is also one of the easiest to get wrong. Undersize the bank and you will run out of power halfway through a cloudy stretch or an overnight outage. Oversize it and you have paid for capacity you will rarely use, while adding unnecessary weight, cost, and charging time to the system. The solar battery calculator on this page walks through the same methodology a system designer would use by hand: it starts from how much energy your home or equipment actually consumes in a day, applies the backup duration you need, and then works through efficiency losses, depth of discharge limits, and a safety margin to land on a realistic, buildable battery bank specification.
What makes battery sizing tricky is that the number printed on a battery's label rarely tells the whole story. A 200Ah lead-acid battery does not give you 200 usable amp-hours in practice, because pulling a flooded lead-acid battery below roughly 50% state of charge on a regular basis shortens its life dramatically. A lithium LiFePO4 battery of the same rated capacity, by contrast, can be discharged to 10% or lower hundreds of times without meaningful degradation. That single difference in usable depth of discharge is often the biggest factor separating an inexpensive lead-acid bank from a smaller, lighter, and more expensive lithium bank that delivers the same real-world backup time. This calculator bakes that distinction in directly: choosing a battery chemistry automatically loads in a sensible depth of discharge and round-trip efficiency, both of which you can still fine-tune if your manufacturer's datasheet specifies different numbers.
The second major factor is backup time. A whole-home backup system sized for three days of autonomy needs roughly three times the storage of a system sized for a single overnight outage, all else being equal. Most residential backup systems are sized for somewhere between eight hours and two days of autonomy, while true off-grid cabins and remote installations often plan for two to five days to ride out a stretch of poor weather without a generator. Entering your own daily consumption figure, whether that comes from a utility bill, an energy monitor, or an appliance-by-appliance tally using our energy consumption calculator, together with your desired backup window, is what drives every downstream result on this page.
System voltage and wiring configuration matter too, though they are often treated as an afterthought. Moving from a 12V to a 24V or 48V battery bank reduces the current draw for the same amount of power, which lowers resistive losses, allows thinner and cheaper cabling, and reduces the load on connectors and breakers. Larger systems, generally anything above a couple of kilowatts of continuous draw, benefit from the higher voltage architecture even though it means wiring more batteries in series to reach the target voltage. This calculator shows you both the series count needed to hit your chosen system voltage and the parallel count needed to reach your target amp-hour capacity, along with a simple diagram of how the bank connects together.
Finally, no battery sizing exercise is complete without a safety margin. Real-world loads fluctuate, batteries lose a little capacity as they age, cold weather temporarily reduces the amount of energy a battery can deliver, and inverters are never perfectly efficient. Building in a margin, typically somewhere around 20%, absorbs these variables so the system you build today still meets your needs a few years from now. Use the advanced options on this page to add extra headroom for temperature, aging, and future load growth if you want an even more conservative design. The result is a battery bank specification you can take directly to a supplier: a recommended number of batteries, their series-parallel wiring configuration, total usable storage in both watt-hours and kilowatt-hours, and an estimate of how many hours of backup that translates to under your stated daily load.
Guide
How solar battery storage works
A solar battery bank stores DC energy produced by your solar array (or delivered by a charger or generator) so that it can be drawn down later, typically through an inverter, to run AC loads in your home or equipment. During the day, a charge controller regulates the flow of current from the panels into the batteries, tapering the charge rate as the batteries approach full capacity to avoid overcharging. At night or during an outage, the inverter pulls stored energy back out of the battery bank and converts it from DC to the AC voltage your appliances expect.
Every stage of that journey loses a small amount of energy: charge controllers, battery chemistry itself, and inverters are all less than 100% efficient. That is why the total storage capacity you need is always somewhat larger than your raw daily consumption figure, and why this calculator explicitly tracks efficiency loss and depth of discharge as separate line items rather than folding them into a single vague multiplier.
Guide
How to calculate battery capacity
- 1
Normalize your energy and time units
Convert daily consumption to watt-hours per day and backup time to hours, so every later step uses consistent units.
- 2
Find the required energy
Multiply your daily Wh consumption by the fraction of a day your backup window represents to get the raw energy needed.
- 3
Add a safety margin
Increase that figure by your chosen safety margin, typically 20%, to cover real-world variability.
- 4
Account for round-trip efficiency
Divide by your battery's round-trip efficiency, since some energy is always lost between charging and discharging.
- 5
Convert to amp-hours
Divide the resulting watt-hour figure by your system voltage to get the required amp-hour capacity.
- 6
Correct for depth of discharge
Divide again by your battery's usable depth of discharge, since only a portion of rated capacity is safe to use.
- 7
Round up to a whole number of batteries
Divide the resulting amp-hour figure by a single battery's rated Ah capacity and round up to the next whole battery.
Comparison
Battery types compared
| Battery type | Usable DoD | Typical cycle life | Round-trip efficiency | Upfront cost |
|---|---|---|---|---|
| Lead Acid (Flooded) | 50% | ~300–500 | ~80–85% | Low |
| AGM | 50% | ~400–600 | ~85% | Low–Medium |
| Gel | 60% | ~500–900 | ~85% | Medium |
| Lithium LiFePO4 | 90% | ~3,000–6,000 | ~95% | High |
Comparison
Lead acid vs. lithium
Lead acid (AGM / Gel / Flooded)
- Lower upfront cost per battery
- Must be oversized because only ~50–60% of capacity is usable
- Shorter cycle life, typically a few hundred cycles
- Heavier per kWh of usable storage
- Well understood, widely available, easy to recycle
Lithium LiFePO4
- Higher upfront cost per battery
- Up to 90% usable depth of discharge
- Much longer cycle life, often 3,000+ cycles
- Lighter and more compact per kWh of usable storage
- Higher round-trip efficiency reduces charging losses
Worked examples
Example calculations
Guide
Common mistakes to avoid
- Sizing a lead-acid or AGM bank using full rated capacity instead of usable depth of discharge.
- Ignoring inverter and round-trip efficiency losses, which understates the bank size needed by 10-20%.
- Skipping a safety margin entirely, leaving no room for aging batteries or unexpected loads.
- Mixing battery ages, brands, or chemistries in the same bank, which causes uneven charge and discharge.
- Choosing a system voltage too low for the load, resulting in excessive current and cable losses.
- Forgetting to account for cold-weather capacity loss in unconditioned spaces.
FAQ
Frequently asked questions
Summary
Conclusion
Getting battery bank sizing right is the foundation of a solar backup or off-grid system that actually performs the way you expect it to. By working through daily consumption, backup duration, depth of discharge, efficiency, and a sensible safety margin, this calculator gives you a specification, not just a rough guess, that you can hand directly to a supplier or use to plan your own install. Adjust the inputs as your load estimates firm up, and revisit the advanced options if you are designing for a harsh climate or planning to expand the system later.