How Long Will My Solar Battery Last? (A Real-World Guide to Sizing)

Two questions come up in every solar conversation: “How long will my battery last on a single charge?” and “How long before I need to replace it?” They sound similar but they’re completely different questions — and confusing them is one of the most common reasons people end up disappointed with their solar setup. This guide answers both, clearly, with real numbers you can actually use.

Question 1: How Long Will My Battery Run on a Single Charge?

This is the runtime question — how many hours can your battery power your stuff before it needs recharging. The answer depends on four variables:

• Your battery’s rated capacity (in amp-hours or watt-hours)
• Your battery’s usable depth of discharge (DoD)
• The power draw of your appliances (in watts)
• Your inverter’s efficiency (typically 85–95%)

The Formula

Here’s the runtime calculation in plain terms:

Runtime (hours) = (Battery Voltage × Capacity in Ah × DoD × Inverter Efficiency) ÷ Load in Watts

Real example: You have a 200Ah 12V LiFePO4 battery (80% DoD), a 90% efficient inverter, and you’re running a 300W load (small fridge + lights):

(12 × 200 × 0.80 × 0.90) ÷ 300 = 5.76 hours of runtime

The same battery with the same load but only 50% DoD (which is what you’d use with AGM):

(12 × 200 × 0.50 × 0.90) ÷ 300 = 3.6 hours of runtime

That’s a 60% difference in runtime from the same rated capacity — purely because of battery chemistry and DoD.

Depth of Discharge: The Number That Changes Everything

DoD is the percentage of your battery’s total capacity that you can safely use without damaging it. Here’s what it looks like by battery type:

• LiFePO4: 80–90% DoD safe for daily cycling. Some high-quality cells can handle 95–100% short-term.
• AGM: 50% DoD recommended. Regularly draining past this shortens life significantly.
• Flooded Lead-Acid: 50% DoD, same as AGM. Some manufacturers claim more, but pushing it accelerates plate sulfation.

The practical impact: if you buy a 10 kWh AGM battery bank thinking you’re getting 10 kWh of storage, you’re actually getting 5 kWh. You paid for 10 kWh, you get 5. That’s a trap that catches a lot of budget shoppers.

Common Appliance Runtimes: Real-World Examples

Here’s what a 5 kWh usable LiFePO4 battery bank can actually power (assuming 90% inverter efficiency):

The combined essentials row is the most useful benchmark: a quality 5 kWh bank covers your fridge, lights, and device charging for most of a day and night — enough to bridge overnight until your panels start producing again.

How Much Solar Do You Need to Recharge It Each Day?

Your battery is only as good as your ability to refill it daily. Here’s the simple calculation for panel sizing:

Panel wattage needed = Daily energy use (Wh) ÷ Peak sun hours in your area

Most of the continental US gets 4–5 peak sun hours per day. If you’re using 3,000Wh per day and you get 4 peak sun hours:

3,000 ÷ 4 = 750W of solar panels (plus 25% buffer = ~940W, so round up to 1,000W)

Under-paneling is a common mistake. If your panels can’t refill your battery daily, you’ll run the bank deeper over time — shortening its life and leaving you short on cloudy days.

Question 2: How Long Before I Need to Replace the Battery?

This is the lifespan question — not hours per charge, but years of service life. The answer depends on battery chemistry, how deep you cycle it, temperature, and how well you maintain it.

Lifespan by Battery Type

• LiFePO4: 10–15+ years at normal daily cycling (1 cycle/day). Rated for 3,000–10,000 cycles to 80% remaining capacity.
• AGM: 3–5 years at daily cycling. 500–1,500 cycles.
• Flooded Lead-Acid: 3–7 years, though this is highly dependent on maintenance. Regular watering and proper charging extend life significantly; neglect kills them fast.

The Biggest Factors That Shorten Battery Life

• Cycling too deep too often. Running AGM below 50% DoD or LiFePO4 below 20% regularly accelerates degradation. Most quality lithium batteries have a BMS that prevents over-discharge, but check your settings.
• Heat. Batteries degrade faster in high temperatures. For every 8°C (14°F) above 25°C, battery degradation accelerates measurably. Keep your battery bank out of direct sun and in a ventilated space.
• Cold charging. Charging LiFePO4 at or below freezing damages the cells permanently. Many modern LiFePO4 batteries include a built-in low-temperature cutoff or heating element — worth paying for in cold climates.
• Leaving batteries fully discharged for extended periods. For lead-acid this causes sulfation. For lithium, sitting at very low state of charge accelerates capacity loss. Aim to recharge within 24–48 hours of full discharge.
• More than 1–2 cycles per day. A normal daily cycle is healthy. Running more than two full cycles per day measurably reduces lifespan.
• Overcharging. This is why a quality charge controller and BMS are non-negotiable — not optional add-ons.

How to Maximize Your Battery’s Lifespan

These habits make a real difference over a decade of use:

• Keep DoD conservative. Even though LiFePO4 can handle 90%+ DoD, regularly cycling to 80% DoD instead will meaningfully extend cycle life.
• Store at 20–80% state of charge if the system will sit unused for extended periods. Avoid long-term storage at 100% or near 0%.
• Keep temperatures moderate. Ideal battery operating temperature is 20–25°C (68–77°F).
• Install a battery monitor. Knowing your exact state of charge lets you manage cycles intelligently rather than guessing.
• Do a full charge cycle at least monthly even if you’re not actively using the system — a full top-up keeps the cells balanced.
• Check your BMS settings. Make sure your charge controller’s absorption voltage, float voltage, and low-voltage cutoff are set correctly for your specific battery’s specs.

Putting It All Together: A Sizing Example

Let’s say you want to power a weekend cabin with the following essentials:

• Fridge: 150W × 24hrs = 3,600Wh
• Lighting: 60W × 6hrs = 360Wh
• Phone/laptop charging: 50W × 3hrs = 150Wh
• Water pump: 200W × 0.5hrs = 100Wh
• Total daily use: 4,210Wh
• × 1.25 (system losses): 5,262Wh per day

For 2 days of autonomy: 5,262 × 2 = 10,524Wh usable needed. At 85% DoD for LiFePO4: 10,524 ÷ 0.85 = ~12.4 kWh rated battery capacity.

Panel sizing (4 peak sun hours): 5,262 ÷ 4 × 1.25 = ~1,650W of solar panels.

That’s a real, complete sizing calculation for a realistic off-grid cabin system.

Final Thoughts

Understanding battery runtime and lifespan isn’t just academic — it’s the difference between a system that meets your needs and one that leaves you frustrated in the dark. Size generously on both battery capacity and solar panels (under-sizing is always the more expensive mistake), choose LiFePO4 chemistry, keep temperatures moderate, and don’t cycle too deep. Do those things and your battery bank will outlast your expectations.

Need help running the numbers for your specific setup? Drop your appliance list in the comments and we’ll help you size it out.

Related posts you might like:
→ LiFePO4 vs AGM vs Lead-Acid: Which Battery Is Right for Your Solar Setup?
→ How to Build a DIY Solar Battery Bank from Scratch
→ MPPT vs PWM Charge Controllers: What’s the Difference?