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.
Charging an electric vehicle from rooftop solar is the closest thing to genuinely free fuel — once the panels are paid for, every kilometer you drive costs you almost nothing. The engineering question is simple but specific: how much array do you need to refill the miles you drive every single day? This calculator answers it in three steps: your daily energy consumption, your local sun hours, and the losses every real system takes along the way.
The three numbers that decide everything
- Your daily distance — commute plus errands. Most drivers sit between 30 and 60 km per day; the calculator defaults to 40.
- Car efficiency (kWh/100 km) — how much electricity your specific EV drinks per 100 km. Compact EVs do 14–16; larger SUVs need 20–24. Check your car's rated figure for a precise result.
- Local sun hours — the equivalent full-sun hours your location receives per day, averaged across the year. Desert climates hit 6+; northern Europe sits around 3.
Multiply distance by efficiency to get daily kWh, then divide by sun hours and by 0.8 (the standard derate for inverter conversion, wiring losses, dust and panel heat). That final number is the array size in watts.
The detail that trips most EV owners up isn't the math — it's the charging schedule. Charging straight off solar only works if the car actually sits plugged in during daylight hours. In the systems I've designed, a cheap smart plug or the car's scheduled-charging setting makes the difference between "my solar feeds my car" and "my solar feeds the grid while my car still pays grid rates". Set the charge window to 10:00–15:00 and the system does the rest.
What the array size really means
If the calculator says you need 2.5 kW of panels, that's roughly six 450 W panels — typically 10–12 m² of roof. Most EV owners in sunny regions can cover their driving needs with an array that fits on a single roof section. Two practical notes from real installations:
- Oversize slightly in winter. The yearly average works out, but in cloudy months the same array produces 30–50% less. If winter driving matters, add 20% to the result.
- Check your home's spare roof before adding panels. If you already have a grid-tied system, the spare capacity on your roof (and your inverter's headroom) determines whether an EV-charging extension is cheap or needs new hardware.
| Scenario | Daily energy | Array @ 5 sun hrs | 450 W panels |
|---|---|---|---|
| Light commuter (25 km/day, 15 kWh/100km) | 3.75 kWh | ~0.9 kW | 2 panels |
| Typical driver (40 km/day, 16 kWh/100km) | 6.4 kWh | ~1.6 kW | 4 panels |
| Long commute (70 km/day, 18 kWh/100km) | 12.6 kWh | ~3.2 kW | 7 panels |
| Electric SUV (60 km/day, 23 kWh/100km) | 13.8 kWh | ~3.5 kW | 8 panels |
Is it worth it? The economics
At typical grid rates of $0.15/kWh, a driver covering 40 km/day spends about $290 a year on EV charging. A dedicated 1.6 kW solar extension costs roughly $1,200–$1,800 in panels and mounting, paying for itself in 4–6 years — and panels last 25+. If your existing rooftop system has spare capacity, the payback is even faster because the marginal cost is close to zero. Use our ROI calculator with your local electricity rate for an exact figure.
Related tools: size your whole home system with the off-grid system wizard, check the panel count for your roof with the panel calculator, or find your local sun hours with the peak sun hours tool.
Frequently asked questions
Do I need a battery to charge my EV with solar?
No — a battery isn't required if your car is plugged in during daylight. The panels charge the car directly through your inverter. A battery only helps if you want to charge overnight using solar stored during the day, which adds cost without much benefit for most drivers.
Can I use my existing rooftop solar for EV charging?
If your system produces more than your home uses during the day, the excess already flows to the grid (or to the car if you set scheduled charging while it's producing). Check your midday export numbers first — a 5 kW system with low household daytime load can often cover an EV with no new panels at all.
What if my location has few sun hours?
The array size simply scales up — 3 sun hours needs about 67% more panels than 5. In low-sun regions, the economics still work if your grid electricity is expensive (common in Europe), but the payback period extends. Run the numbers with your local figure and decide from there.