What is round-trip efficiency and why does it matter?

Round-trip efficiency measures how much energy you get back out compared to what you put in. If you charge a battery with 1,000Wh and only get 850Wh back during discharge, the round-trip efficiency is 85%. The lost 15% becomes heat. LiFePO4 batteries achieve 92-98% round-trip efficiency , while lead-acid manages 75-85%. Over thousands of cycles, that difference adds up to hundreds of kWh of wasted energy and heat over the battery's life.

Are my solar panels recharging the battery fast enough to keep up with daily use?

Compare total daily solar production (panel watts x peak sun hours x 0.8 derating) against total daily consumption (load watts x hours of use). If production exceeds consumption by at least 20%, you have enough margin. If they are close, you will slowly deplete the battery over cloudy days. Our solar battery charge time calculator helps you model this balance with your specific panel and battery setup. For a complete energy audit of all your loads, the off-grid load calculator tallies daily consumption across every appliance.

Should I cycle my battery to the same DoD every day?

Consistency helps with lead-acid batteries, which benefit from regular full charges to prevent sulfation. LiFePO4 batteries are more flexible — partial charges and partial discharges do not harm them. Avoid two extremes: repeatedly draining any battery to 100% DoD (shortens lifespan dramatically), and keeping a lead-acid battery at partial charge without periodic full charges (causes sulfation and capacity loss).

What happens if my battery never fully recharges?

A battery that stays partially charged develops problems over time. Lead-acid batteries sulfate — lead sulfate crystals harden on the plates and permanently reduce capacity. This starts within days of staying below 80% SOC. LiFePO4 batteries tolerate partial charge better, but running a daily deficit means your usable capacity shrinks each day until the system cannot support your loads at all. Monitor your battery's state with a multimeter and our voltage-to-percentage calculator to catch deficits early.

Battery Charge & Discharge Calc

4 min read

A complete battery cycle includes both a discharge phase (using power) and a charge phase (replenishing it). This calculator models both sides so you can see whether your system recharges fully before the next discharge cycle begins — a critical question for off-grid solar and backup power setups.

Four-stage battery cycle overview showing bulk charge, absorption, float, and discharge phases.

Example: A Day in the Life of an Off-Grid Battery

Here is a typical 24-hour cycle for a 200Ah 12V LiFePO4 battery bank on a cabin with solar panels:

Morning (7 AM - 10 AM): Battery is at 30% SOC after overnight loads. Solar panels start producing as the sun rises. The charge controller pushes 15A into the batteries. Energy goes both to charging and to running the fridge and lights.

Midday (10 AM - 3 PM): Peak solar production. The controller ramps up to 30A. Batteries reach full charge by 1 PM. Excess solar is wasted unless you have a dump load or second use (water heating, device charging).

Evening (5 PM - 11 PM): Solar production drops to zero. The battery powers lights (60W), fridge (50W avg), TV (80W), and phone charging (15W). Total load: 205W. At 80% DoD, the 200Ah battery gives you about 9.4 hours of runtime.

Overnight (11 PM - 7 AM): Only the fridge runs (50W avg). This draws about 400Wh over 8 hours. The battery still has capacity left, reaching roughly 30% SOC by morning — right where the cycle started.

Voltage rises through bulk and absorption charging, peaks during float, then declines steadily during discharge.

Battery Cycle Life by Chemistry and DoD

Chemistry	50% DoD Cycles	80% DoD Cycles	100% DoD Cycles
LiFePO4	4,000-6,000	2,000-3,500	1,500-2,000
AGM Lead-Acid	500-800	300-400	150-250
Flooded Lead-Acid	600-1,000	300-500	150-300
Gel Lead-Acid	700-1,200	400-600	200-350
Li-ion (NMC)	1,500-3,000	800-1,500	500-800

One "cycle" is a discharge to the specified DoD followed by a full recharge. Shallower cycles mean more total cycles. A LiFePO4 battery cycling daily at 50% DoD could last over 15 years.

Worked Examples

Daily Solar Cycle for an Off-Grid Cabin

Context

Your cabin uses 150W for 8 hours each evening (1,200 Wh). A 200Ah 12V LiFePO4 bank charges from a 30A solar charge controller during the day.

Calculation

Discharge: 1,200 Wh / (12V x 0.92 eff) = 108.7 Ah drawn

At 80% DoD, you use 108.7 of 160 Ah usable = 68% of usable capacity

Recharge: 108.7 Ah / (30A x 0.95 eff) = 3.8 hours of full sun

Interpretation

You need about 4 hours of strong sun to replenish the evening's usage. In summer with 5+ peak sun hours, you recharge with margin. In winter with 3 hours, you may fall short.

Takeaway

Size your solar array for the worst month, not the annual average. Our solar panel output calculator shows daily production by month for your location.

Marine Battery After a Full Day of Trolling

Context

Your trolling motor draws 40A at full speed from a 100Ah lead-acid battery. After 1.5 hours of fishing, you dock and plug into a 15A shore charger.

Calculation

Energy used: 40A x 1.5 hrs = 60 Ah (60% of capacity)

Recharge with absorption: 60 / (15A x 0.85) = 4.7 hours bulk + ~2 hours absorption = ~6.7 hours total

Interpretation

You need nearly 7 hours of shore charging to fully recover from 1.5 hours of full-speed trolling. An overnight charge handles it easily, but a quick afternoon top-up will not finish.

Takeaway

For detailed marine runtime estimates at different trolling speeds, use our marine battery runtime calculator which factors in thrust-to-amp-draw ratios.

Frequently Asked Questions

Glossary

Round-Trip Efficiency

The percentage of energy you get back from a battery relative to what you put in. LiFePO4 achieves 92-95% round-trip efficiency. Lead-acid is typically 80-85%. The lost energy becomes heat during charging and discharging.

Cycle Life

The number of complete charge-discharge cycles a battery can perform before its capacity drops below 80% of original. LiFePO4: 3,000-5,000 cycles. AGM: 500-800 cycles. Flooded lead-acid: 300-500 cycles at 50% DoD.

Daily Cycle

One complete discharge and recharge period, typically following the sun cycle in solar systems. Evening loads discharge the battery, and daytime solar recharges it. The depth of each cycle directly affects long-term battery life.

Need to size your solar array for full daily recharging? The <a href="/solar/solar-panel-and-battery-sizing-calculator">solar panel and battery sizing calculator</a> matches generation to consumption.

Modelling the complete charge-discharge cycle reveals whether your system is sustainable or slowly draining your batteries into an early grave. If the numbers above show your batteries never fully recharge, you need either more charging capacity (bigger panels, faster charger) or less load — there is no third option. For the chemistry-specific limits on how deep you can safely cycle, see our depth of discharge guide.

Last updated: March 16, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.