Solar Battery Runtime Calculator

Runtime is usable watt-hours divided by load in watts. The trick is that "usable" is much less than the battery's nameplate capacity — you have to shave off depth of discharge and inverter efficiency first.

This is why advertised capacity can be misleading when planning backup power. Compare your numbers with the Solar Battery Size Calculator to see how much nameplate capacity you actually need.

Three reasons stack up between the sticker and real runtime:

• Depth of discharge (DoD) — you should not drain most batteries to 0%. LiFePO4 is rated to 90%, AGM and flooded lead-acid to ~50% for reasonable cycle life.
• Inverter efficiency — converting DC battery power to AC for your appliances loses 5–15% as heat. Typical quality inverters are 90–95% efficient.
• Cold/high-load losses — lead-acid batteries lose additional capacity at low temperatures or high discharge rates (Peukert's law).

Together, these can turn a 2,000Wh nameplate into as little as 800–900Wh of real usable AC energy on older lead-acid chemistries.

Depth of discharge (DoD) is the percentage of a battery's capacity you can safely use without damaging it or shortening its life.

Pushing lead-acid past 50% DoD is possible but cycle life drops fast — a bank rated for 1,200 cycles at 50% DoD may only deliver 400 cycles at 80% DoD.

A 2,000Wh battery with 80% DoD and 90% inverter efficiency gives 2000 × 0.80 × 0.90 = 1,440 Wh of usable AC energy.

• Powering a 100W load → 1440 ÷ 100 = 14.4 hours
• Powering a 300W load → 1440 ÷ 300 = 4.8 hours
• Powering a 600W load → 1440 ÷ 600 = 2.4 hours

This is why runtime planning should always start with realistic appliance loads. After estimating battery hours, use the Solar Panel Output Calculator to see how quickly panels can recharge the system.

A fridge is a duty-cycle load — the compressor runs maybe 30–50% of the hour, averaging 80–150W for a typical full-size efficient unit. That means a 2,000Wh battery with ~1,440Wh usable can run a fridge for roughly 10–18 hours depending on ambient temperature, door openings, and compressor efficiency.

Smaller mini-fridges average 60–80W and can push runtime past 20 hours on the same battery. Older non-inverter fridges with 200W+ average draw cut runtime in half.

For an appliance-specific breakdown, see Solar Battery Runtime for Refrigerator.

A CPAP runs on 30–60W without a humidifier, or 80–120W with one. That means a 500Wh portable battery (like a Jackery Explorer 500) delivers roughly:

• 8–12 hours without humidifier (2 full nights)
• 4–5 hours with humidifier (part of one night)

For a full 8-hour night with humidifier and heated hose, look for at least 800–1,000Wh. Many CPAPs have a 12V DC input that bypasses the inverter entirely — using that can add 30–40% more runtime.

Short answer: not long. A 1,500W space heater running full-tilt pulls 1,500Wh every single hour. A 2,000Wh battery with ~1,440Wh usable gives you under an hour of heat.

Depends enormously on the AC unit type:

Inverter-driven mini-splits are dramatically better than window units because they modulate compressor speed — actual draw averages 30–60% of rated after room is cool. Standard window ACs cycle on/off at full power.

AC also has a surge draw of 3× running watts. You want a soft-start kit on any AC if you're planning to run it off battery.

These are the easy, friendly loads — modern electronics sip power:

• Router + modem (15W combined) — a 1kWh battery runs these for 60+ hours.
• Laptop (45–65W when working) — same 1kWh battery runs a laptop for 13–20 hours.
• Phone charger (5–10W) — effectively unlimited; a portable battery charges a phone 50+ times.
• LED lighting (5–15W per bulb) — a 2kWh battery runs 6 LED bulbs for 20+ hours.

For connectivity and basic productivity during outages, even a small solar generator (~500Wh) is usually enough for a couple of days.

Depends on what "whole house" means. Average US home uses ~30 kWh per day, but that includes HVAC, electric water heat, and oven — the heavy hitters.

• Essentials-only (fridge, lights, wifi, fans, small electronics) → 3–6 kWh per day.
• Normal use minus heat/AC/oven → 8–15 kWh per day.
• Full home with HVAC → 25–40 kWh per day.

A typical 10–14 kWh home battery (Tesla Powerwall, Enphase IQ) covers essentials for roughly 24 hours. Full-home backup needs 20–30+ kWh of storage paired with solar recharge during daylight.

In order of impact, the four biggest factors are:

• Battery size — largest lever; doubling capacity doubles runtime.
• Appliance wattage — a 1,500W heater drains a bank 15× faster than a 100W TV.
• Inverter efficiency — cheap modified-sine inverters can be 75–85% efficient; quality pure-sine is 92–95%.
• Usable depth of discharge — LiFePO4 delivers 80–90% usable; lead-acid only 50%.

Temperature, battery age, and simultaneous appliance usage (especially surge currents) compound these losses. To understand how much power your devices use each day, check the Solar Power Consumption Calculator.

Lithium batteries lose about 10–15% capacity below 32°F (0°C), and LiFePO4 BMSes typically block charging below freezing to prevent permanent damage.

Lead-acid is worse — 30–50% capacity loss below freezing, and high internal resistance in the cold means surge currents drop sharply. An AGM bank that delivers 1,000W at 70°F may only deliver 500W at 20°F.

Peukert's law says lead-acid capacity drops as discharge rate increases. A 100Ah AGM rated at the C/20 rate (5A for 20 hours) may only deliver 80Ah if you discharge it at 50A — that's 20% lost simply because you're drawing fast.

Lithium chemistries are essentially immune to Peukert losses at normal discharge rates — a 100Ah LiFePO4 delivers close to its full 100Ah whether you draw it at 10A or 100A.

Parasitic or "phantom" loads are the devices drawing power 24/7 even when nothing is "on" — inverter idle, modem, router, DVR, smart-home hubs, wall warts. Typical parasitic draw is 10–30W continuous, which is 240–720Wh per day.

On a 2,000Wh battery, that's 12–36% of your total capacity going to standby before you turn anything on. Most inverters also have an idle current of 10–25W just for being energized — use the unit's "eco" or "search" mode if available.

Total energy consumed stays the same — running a 100W fan plus a 60W light for 10 hours uses the same Wh as running them one-at-a-time for 10 hours each. But concurrent loads reduce how long the battery lasts in real time and can trigger three secondary problems:

• Your inverter must be sized for the peak simultaneous load, not the average.
• Higher combined draw triggers Peukert losses on lead-acid banks.
• Surge events (AC compressor plus microwave starting together) can trip inverter overload protection.

Take two "100Ah at 12V" batteries — both nameplate 1,200Wh. Actual usable AC runtime at a 100W load:

• LiFePO4: 1,200 × 0.90 DoD × 0.92 inverter = ~994 Wh usable → 9.9 hours
• AGM: 1,200 × 0.50 DoD × 0.92 inverter × 0.90 Peukert = ~497 Wh usable → 4.97 hours

Same sticker capacity, but LiFePO4 delivers roughly 2× the real runtime thanks to higher DoD and zero Peukert penalty. That's before counting the 5× longer cycle life.

Yes — all batteries lose capacity over time and cycles. Typical real-world degradation:

• LiFePO4: ~80% of original capacity after 3,000 cycles (roughly 8–10 years at daily cycling).
• NMC lithium: ~80% after 1,500 cycles (4–5 years).
• AGM/flooded lead-acid: ~80% after 400–800 cycles (2–4 years at daily cycling).

A 5-year-old AGM bank may deliver only 60–70% of its original runtime. When runtime drops noticeably and resting voltage doesn't recover to spec, it's time to replace.

Five levers, ranked by impact:

• Cut peak loads — swap the 1,500W kettle for a 600W slow kettle; use a 45W laptop instead of a 250W desktop.
• Run DC direct when possible — fans, CPAP, lights, fridges all come in 12V DC versions that skip the inverter entirely, gaining 10% efficiency.
• Shift loads to solar daylight hours — run the dishwasher, power tools, or laundry while the sun is charging, not at night off the battery.
• Kill parasitic loads — unplug wall warts, put the inverter in eco mode, disconnect when not actively using the system.
• Upgrade inverter efficiency — moving from an 82% modified-sine to a 94% pure-sine adds 12–15% runtime to every single load.

Solar must replace what you use plus a buffer for poor weather and seasonal variation. Rule of thumb: your panel array in watts should equal your daily battery use (Wh) divided by 4–5 peak sun hours, then oversize 20–30%.

Flip the runtime formula around to solve for capacity instead of hours:

Add 30% for winter, another 30% for a day of autonomy (cloudy days), and you're looking at roughly a 10 kWh bank for a true overnight 400W load. Run those numbers against the Solar Battery Size Calculator for a full spec.