Battery sizing trips up more people than any other part of a solar or backup system, mostly because of one detail that's easy to miss: the capacity printed on a battery isn't the capacity you can actually use. This calculator handles that automatically by comparing two of the most common battery chemistries side by side, so you can see how the choice of battery type changes the size — and cost — of what you need to buy.
Why nominal capacity isn't usable capacity
Every rechargeable battery has a depth of discharge (DoD) limit — the percentage of its total capacity that can be safely used before recharging, without significantly shortening its lifespan. Draining a battery further than its recommended DoD, especially repeatedly, degrades it much faster than the manufacturer's rated cycle life would suggest.
- Lithium (LiFePO4) — can typically handle 90–100% DoD without meaningful damage, thanks to its chemistry and built-in battery management system (BMS).
- Lead-acid (flooded or AGM) — is usually limited to 50% DoD for reasonable lifespan. Some manufacturers rate their batteries for deeper discharge, but 50% is the widely recommended figure for maximizing usable cycles over time.
This is why the same daily backup target requires very different battery sizes depending on chemistry — the calculator above divides your target usable capacity by 0.9 for lithium and by 0.5 for lead-acid to arrive at the nominal (rated) capacity you'd need to purchase.
If the charge controller isn't programmed correctly for the battery type in use, it leads to fast, uncontrolled charge and discharge cycles instead of the gradual, protected cycling batteries are designed for — and that shortens their lifespan considerably. This is especially damaging for lead-acid batteries, which I've seen fail well before their rated cycle life simply because the controller settings were left on generic defaults instead of being matched to the actual battery bank.
Lithium vs. lead-acid: the full comparison
| Factor | Lithium (LiFePO4) | Lead-acid |
|---|---|---|
| Recommended DoD | 90–100% | 50% |
| Cycle life | 3,000–6,000 cycles | 300–800 cycles |
| Upfront cost | Higher per kWh | Lower per kWh |
| Cost per usable cycle | Often lower long-term | Higher long-term due to shorter life |
| Weight | Roughly half of lead-acid for the same usable capacity | Heavier, bulkier for the same usable capacity |
| Maintenance | Minimal to none | Flooded types need regular water top-ups; AGM is maintenance-free |
| Cold-weather performance | Charging below freezing needs a BMS with low-temp cutoff | More tolerant of cold, but capacity drops in low temperatures too |
Lead-acid remains attractive for tight budgets or occasional backup use where the battery won't be cycled often. Lithium tends to win out for daily-cycling systems, off-grid homes, or anywhere weight and space are limited — the higher upfront cost is usually offset by a much longer service life.
Choosing how much backup you actually need
The backup percentage you pick has a bigger effect on cost than almost any other decision in the system. It helps to think in terms of what you're actually protecting against:
- 25% (a few essential hours) — enough to keep a refrigerator, some lighting, and a router running through a short outage. The most budget-friendly option.
- 50% (half a day) — covers a typical evening-to-morning outage without needing to ration power.
- 100% (a full day) — lets you ride out a full day with no solar input at all, useful in areas with frequent extended outages.
- 200% (two days) — a genuine multi-day autonomy buffer, typically chosen for remote off-grid sites or regions with unreliable grid infrastructure and long stretches of low sun.
There's no universally "correct" answer here — it's a direct trade-off between upfront cost and peace of mind during outages.
A worked example
Say your daily usage is 20 kWh and you want 50% of that backed up on a 24V system. Your target usable capacity is 10 kWh. For lithium, that means a nominal capacity of about 11.1 kWh (10 ÷ 0.9), which works out to roughly 463 Ah at 24V. For lead-acid, the same 10 kWh target needs 20 kWh nominal (10 ÷ 0.5) — nearly double — which comes to about 833 Ah at 24V. This is exactly what the calculator returns with its default values, and it illustrates why lead-acid banks end up so much larger and heavier for the same real-world backup.
Related tools: check how long a given bank will last on a specific load with the battery runtime calculator, size the charge controller that will charge it, or plan a full system with the off-grid system wizard.
Frequently asked questions
Can I mix lithium and lead-acid batteries in the same bank?
This is generally not recommended. The two chemistries have different charging voltage profiles and charge/discharge characteristics, which can cause uneven charging, reduced lifespan, or damage to one or both battery types. Keep battery banks to a single chemistry.
Does battery capacity need to match my inverter size?
Not directly — they answer different questions. Inverter size is about how much power you can draw at once (see our inverter calculator), while battery capacity is about how much energy you can store and use over time. Both need to be sized for your system, but independently of each other.
Why does the calculator use kWh instead of just Ah?
Ah (amp-hours) only tells you the full story once you know the voltage, since energy (kWh) is voltage × amp-hours ÷ 1000. The calculator shows both: kWh as the voltage-independent figure for comparing battery types, and Ah at your chosen system voltage for matching against real battery specs.
When the grid goes down, most people realize too late that "having solar panels" doesn't mean the lights stay on. A standard grid-tied solar system shuts off automatically during an outage for safety. What actually keeps your home running is a backup loop: a battery that stores energy and an inverter that can run independently of the grid. This calculator works out both halves of that loop — how much battery you need, and how big the solar array must be to refill it every day.
How the backup loop works
During normal daylight hours, your panels charge the battery and power your home. When the grid fails, the system switches to island mode: the battery becomes your power source, and the panels top it up the next morning. The three numbers that matter are your essential load (the watts you actually need), your backup hours (how long the battery must hold), and your local sun hours (how fast panels can recharge).
- Essential load (W) — the sum of everything you refuse to live without: fridge, lights, router, a fan. Leave out the washing machine and iron for now.
- Backup hours — how many hours per day the battery must deliver that load. Eight hours covers evening-to-morning for most families.
- Sun hours — the equivalent full-sun hours your location gets per day. Desert regions often hit 6+; cloudy climates sit around 3–4.
In Ma'an, southern Jordan, where outages and weak grid supply are common, I've installed dozens of small backup loops. The most common mistake isn't undersizing the battery — it's overloading the inverter at startup. A fridge's running wattage is low, but its compressor pulls 3–5 times more for a second or two when it kicks in. Always size the inverter for 3× the largest motor load, not just the sum of running watts.
Why we size for lithium at 90% depth of discharge
Lithium iron phosphate (LiFePO4) batteries safely deliver about 90% of their rated capacity, while lead-acid is limited to roughly 50%. That means a 5 kWh lithium battery gives you ~4.5 kWh of usable energy — nearly double what the same rated lead-acid bank delivers. The calculator uses the lithium figure because it's the realistic choice for daily-cycling backup duty.
The recharge math, step by step
Once you know your daily backup energy need, the array size follows: daily energy ÷ sun hours ÷ 0.8 (the 0.8 accounts for charging losses, dust, and heat). If your battery needs 3.6 kWh per day and you get 5 sun hours, you need roughly 900 W of panels — two 450 W panels. This is the number the calculator returns, and it's the same engineering rule we apply on real installations.
| Household scenario | Typical load | Battery (8h backup) | Array (5 sun hrs) |
|---|---|---|---|
| Fridge + lights + router | ~200 W | ~2 kWh | ~500 W |
| Above + fan + TV | ~350 W | ~3.4 kWh | ~850 W |
| Above + small AC | ~1,200 W | ~11.5 kWh | ~3 kW |
| Full essential home | ~1,800 W | ~17 kWh | ~4.5 kW |
Practical tips before you buy
Start with the hybrid inverter, not the battery — it's the brain of the system and determines which batteries you can add later. Buy a battery with a matching BMS communication port so the inverter can read its real state of charge. And don't oversize for rare worst-case scenarios: sizing for 8 hours instead of 24 typically cuts your cost in half, and you can add a second battery later if outages get worse.
Related tools: plan your full off-grid system with the off-grid system wizard, compare battery chemistry options with the battery size calculator, or check how long a specific load will drain a given bank with the battery runtime calculator.
Frequently asked questions
Can I use my existing grid-tied panels for backup?
Not directly — grid-tied inverters shut down during outages by law. You need a hybrid inverter (or an AC-coupled backup box) that can disconnect from the grid and run your home on battery power. Your existing panels can usually be reused with the right inverter, which is a big cost saving.
How much does a whole-home backup loop cost?
For the essentials (fridge, lights, router, fan), a realistic build is a 5 kWh lithium battery plus ~1.5 kW of panels: roughly $2,000–$3,500 in parts depending on your market. Whole-home coverage (including AC) runs 3–4× that. Our ROI calculator can compare this against generator fuel costs over time.
Why do I need more panels than my load requires?
Because the panels must refill the entire battery every day, not just match your momentary consumption. A 300 W load running 8 hours needs 2.4 kWh — but recharging that in 5 sun hours needs ~600 W of panels after accounting for losses.