Solar Battery Discharge Calculator

The baseline formula is battery capacity divided by continuous load — but real runtime is always lower after applying DoD limits, inverter efficiency, and temperature penalties:

A 2,000 Wh LiFePO4 battery running a 200 W load with 90% DoD and 92% inverter efficiency:
(2,000 × 0.90 × 0.92) ÷ 200 = 8.3 hours of real runtime, not the 10 hours simple math would give.

Depth of Discharge is the percentage of a battery’s rated capacity that’s safely usable before you hit a chemistry-specific cutoff. Going past it damages cells and shortens lifespan dramatically.

A 5 kWh AGM battery only delivers 2,500 Wh before you hit the 50% DoD cutoff. A 5 kWh LiFePO4 delivers 4,500 Wh — 80% more usable energy from the same nominal capacity.

Rated capacity is the theoretical maximum a manufacturer prints on the label (e.g. “100 Ah @ 12 V = 1,200 Wh”). Usable energy is what actually reaches your loads after four stacking losses:

• DoD cutoff — you can’t touch the bottom slice (50% for lead-acid, 10% for lithium).
• Inverter efficiency — DC→AC conversion costs 8–15% depending on inverter quality and load.
• Cable/busbar losses — typically 2–4% at full load.
• Temperature derating — cold reduces delivered capacity; lead-acid is worst.

Yes — that’s their primary job in an off-grid or hybrid system. Panels produce from roughly sunrise to sunset; the battery carries everything through the night plus any daytime consumption above what panels deliver.

A well-sized system follows this daily rhythm:

• Sunrise → late morning: panels recharge battery from overnight draw while also powering morning loads.
• Midday → late afternoon: battery sits at or near 100%; excess solar is curtailed or fed to grid.
• Sunset → next sunrise: battery discharges through evening loads, fridge cycles, parasitic standby, then any early-morning appliances.

Anything that turns electricity into heat — resistive heating loads have the worst battery-runtime economics:

Fridges cycle on and off, so use their daily energy use (not peak wattage) for accurate estimates. Look at the yellow EnergyGuide label or read it from a Kill-A-Watt meter.

A 5 kWh usable battery runs a typical full-size fridge for roughly 3 days with no solar input. A 12 V DC compressor fridge (Dometic, SunDanzer) uses 30–60% less and can run 5–7 days on the same battery.

Yes, but AC units are one of the most demanding loads and need careful battery sizing. Real-world runtime estimates:

Tips for running AC on battery:

• Choose inverter-compressor AC — variable-speed units use 30–40% less than on/off compressors.
• Use a soft-start device to cut startup surge from 3–5× to ~1.5× running watts.
• Pre-cool during the day while panels are producing, so the battery only handles overnight maintenance cooling.

Three reasons — all of which stack on top of each other:

• Startup surge (locked rotor amps). Motors draw 3–7× running watts for 1–3 seconds every time they start. A fridge that cycles on 20 times a day has 20 surge events.
• Power factor. Motors draw reactive power that doesn’t do useful work but still loads the inverter. Cheap inverters can lose 15–25% efficiency driving motors vs. pure resistive loads.
• Duty-cycle penalty. A rated 150 W compressor might pull 250 W while actually running and 0 W while off — the instantaneous load bounces around your inverter’s efficiency curve.

More than most people realize. “Standby” draws add up over 24 hours even when nothing appears to be in use:

A typical home runs 40–60 W of parasitic draw 24/7, which eats 1–1.5 kWh per day — often more than the fridge. Use a master kill switch for unused electronics and put the inverter into eco/standby mode overnight if your loads allow.

Yes — and the effect compounds. A 92% inverter delivers 92 Wh of AC for every 100 Wh of DC pulled from the battery. The missing 8 Wh becomes heat in the inverter. Over a full battery discharge, you lose:

Inverter efficiency is not flat. Most inverters hit peak efficiency at 30–70% of rated output and drop sharply at very light loads:

• 2% of rated load (e.g. 40 W on a 2,000 W inverter): efficiency 55–70%.
• 30–70% of rated load: peak efficiency 90–95%.
• Over 90% of rated load: efficiency drops 2–3 points from peak.

Peukert’s law says that lead-acid batteries deliver less usable capacity at higher discharge currents — the faster you drain them, the less total energy you get out. Lithium batteries are not affected.

If you routinely pull more than 0.2C from flooded or more than 0.3C from AGM, Peukert will cost you 15–30% of rated capacity in real runtime. The fix is either (a) bigger battery bank to keep C-rate low, or (b) switch to lithium where Peukert doesn’t apply.

Cold reduces both delivered capacity and maximum sustainable current across all chemistries, but the penalty varies:

Most lithium batteries discharge fine in cold — the damage risk is during charging below freezing, not discharging. Lead-acid gets hit on both sides: reduced discharge capacity and slower absorb/float during charge.

Simple capacity-÷-load math ignores at least six real-world deductions. Walk through this stack for a 5 kWh AGM battery running a 500 W load:

The honest answer is that a battery label tells you less than 40% of the story. Use the discharge calculator above to get an estimate that includes all these factors.

Going past safe DoD causes permanent damage — the severity depends on chemistry:

• LiFePO4: the BMS usually disconnects at about 10% SoC to protect cells. Repeated near-zero cycles still shorten life but the cells survive.
• NMC Lithium: over-discharge below 2.5 V per cell can cause copper shunt formation — cells become unsafe to re-charge and must be retired.
• AGM / Flooded / Gel: discharging below 50% SoC causes plate sulfation, where sulfate crystals grow and don’t re-dissolve. Each over-discharge cycle permanently reduces capacity. A single 100% discharge can cost 20–30% of rated capacity.

LiFePO4 has 4 major discharge advantages over lead-acid:

The combined effect: a 5 kWh LiFePO4 delivers roughly the same real-world runtime as a 10 kWh flooded lead-acid bank, at a quarter of the weight and 4× the cycle life. The higher upfront cost pays off within 3–5 years of daily cycling.

Yes — and the damage is cumulative and mostly irreversible. Each over-discharge cycle:

• Lead-acid: grows sulfate crystals that don’t fully dissolve during normal charging. Three or four deep discharges can cut capacity in half permanently.
• Lithium: copper corrosion at the anode creates internal micro-shorts. The cell eventually fails open-circuit or, rarely, thermally.

Modern batteries include a BMS (Battery Management System) that disconnects the pack before damage occurs. Older lead-acid systems rely on the inverter’s low-voltage cutoff — always verify it’s set correctly. Default 10.5 V cutoffs are dangerous for 12 V lead-acid banks; raise to 11.8 V.

Lead-acid batteries almost never deliver their published cycle life in real off-grid use. Common reasons:

• Chronic undercharge. Solar systems that don’t reach 100% SoC daily build sulfate crystals on the plates. Need proper 3-stage charging with regular absorb completion.
• Heat. Every 15°F above 77°F roughly halves battery life. A battery at 95°F ambient may last 40% as long as one at 77°F.
• High C-rate discharging. Regularly pulling more than 0.2C dramatically accelerates plate wear via Peukert stress.
• Mixed-age banks. Adding a new battery to an old bank drags the new one down — all cells average to the weakest.
• Not equalizing flooded cells. Monthly equalization charge is required to prevent stratification; skipping it costs 30–50% of cycle life.

Cycle-life ratings are measured in lab conditions at 77°F, C/20 discharge, and perfect charging. Real systems usually deliver 50–70% of lab-rated cycle life.

Yes — and in a properly configured hybrid system, this is how daytime loads get priority over battery draw. The charge controller and inverter negotiate where power flows:

• Load less than solar input: panels power the load directly, excess charges the battery.
• Load equal to solar input: battery sees zero charge/discharge current — “neutral zone.”
• Load greater than solar input: panels supply what they can, battery fills the gap.

In order of cost-per-hour-gained:

• Eliminate parasitic draws — kill switches on unused electronics can save 500–1,500 Wh/day for free.
• Swap resistive loads for alternatives — propane cooking, solar water heating, LED lights. Highest ROI single change.
• Move heavy loads to daytime — run off direct solar instead of battery. No hardware cost.
• Upgrade inverter — a 95% pure-sine vs 85% modified-sine gains ~10% on every Wh. Usually $200–500.
• Switch lead-acid → LiFePO4 — 80% more usable capacity from the same nameplate. Expensive upfront but permanent fix.
• Add battery capacity — last resort. Most expensive per additional hour. Consider all the above first.

For the same total capacity, both give similar runtime — but the tradeoffs differ:

Key rules for parallel banks:

• Use identical make, model, age, and state of charge. Mixing old with new kills the new ones within months.
• Wire with equal-length cables from each battery to a common busbar, not daisy-chained — this balances current draw across the pack.
• Add fuses at each battery positive terminal, sized for the battery’s individual C-rate.
• For lithium, many brands support parallel communication between BMS units — use it where available.