Winter Load Shedding: How Many Hours Will Your Battery Bank Really Last?

Every winter in Pakistan, the same conversation plays out on WhatsApp groups and in dealer showrooms: “Bhai, outage lag gayi hai, battery kitna chalegi?” The honest answer is almost never a round number like “8 hours” or “all night.” It depends on exactly how many amp-hours sit in your battery bank, how deeply you are allowed to discharge that chemistry, how efficient your inverter is at converting DC to AC, and — most importantly — what you are actually running during the outage. Most people only find out the real number the hard way, when the lights die at 2 a.m. in the middle of a long winter loadshedding spell.

This post walks through the actual math so you can calculate your own backup time instead of guessing, compares lead-acid and lithium battery banks side by side, and shows what happens to that number the moment you add a room heater or geyser into the mix (we will stay on the “how long does it last” question here — if you want the full heater and geyser load-planning guide, that is a separate, deeper topic). If you already know how many batteries you need for capacity but have never actually sat down and calculated runtime hours, this is the piece that fills that gap.

Why “How Many Batteries” and “How Many Hours” Are Different Questions

A lot of buyers conflate two separate decisions. One is “how many batteries do I need to run my chosen inverter at its rated capacity” — essentially a hardware compatibility question tied to your inverter’s battery voltage (12V, 24V, or 48V configurations) and how many strings it can support. We have covered that sizing question in detail in our guide on how many batteries you need for a 6kW inverter, which is about matching battery count to inverter capacity, not runtime.

The second question — the one this article answers — is “once I have that battery bank installed, how many hours of actual backup do I get during a loadshedding outage, given my real household load in winter.” These are related but genuinely different calculations, and confusing them is why so many households end up with a battery bank that is technically “correctly sized” for the inverter but still runs out of charge halfway through a long outage.

The Core Formula: How Backup Hours Are Actually Calculated

Every backup-hour estimate, whether for lead-acid or lithium, comes down to one relationship:

Backup hours = (Battery capacity in Wh × usable Depth of Discharge × inverter efficiency) ÷ Your connected load in watts

Breaking that down piece by piece:

Battery capacity in Wh (watt-hours): calculated as battery voltage × amp-hour (Ah) rating. A single 12V, 150Ah lead-acid battery stores roughly 1,800Wh nominally (12 × 150). A 48V, 100Ah lithium battery stores roughly 4,800Wh (48 × 100). Multiply by however many batteries are wired into the bank to get total nominal capacity.

Usable Depth of Discharge (DoD): this is the part most people skip, and it is the single biggest reason lead-acid and lithium banks of similar nominal capacity behave so differently in practice. Deep-cycle lead-acid batteries are generally only discharged to about 50% of their rated capacity before voltage sags and cycle life starts dropping sharply — go deeper regularly and you shorten the battery’s lifespan significantly. Lithium (LiFePO4) batteries, by contrast, are commonly rated for 80-90% usable depth of discharge without the same cycle-life penalty, because the chemistry tolerates deeper, more frequent discharge far better. This single difference is why a lithium bank often delivers noticeably more usable runtime than a lead-acid bank of the same nominal Wh rating, even before you factor in anything else. We go into this comparison in more depth in our dedicated lithium vs lead-acid battery guide, which is worth reading before you commit to either chemistry for a new installation.

Inverter efficiency: converting stored DC battery energy into usable AC power is never 100% lossless. Typical modern hybrid and off-grid inverters run somewhere in the 85-93% efficiency range depending on load level and inverter quality, with losses shed as heat. This is a real number, not a rounding error — on a long winter night it genuinely changes your usable hours by several percentage points.

Connected load in watts: this is the number households get most wrong, usually by underestimating. It needs to include every appliance actually drawing power during the outage, not just the obvious ones — LED bulbs and tube lights, ceiling fans (many people assume fans are a summer-only load and forget they still run through winter evenings for ventilation or for the elderly who keep a fan on at low speed), Wi-Fi router and modem, LED or LCD TV, and the refrigerator compressor, which doesn’t run continuously but still contributes a real average wattage over an hour because of its on/off duty cycle. If you have not actually totaled this up room by room, our room-by-room inverter sizing calculator guide walks through exactly how to do it properly rather than guessing a round number like “I think we use about 500 watts.”

Worked Example: Essentials-Only Load, Lead-Acid vs Lithium

Let’s run the formula with realistic numbers for a typical urban household running only winter essentials during an outage: a handful of LED lights, one or two fans on low, a Wi-Fi router, a TV, and a refrigerator’s average draw. A reasonable essentials-only load for a mid-sized household in winter sits around 250-300 watts once averaged over an hour (fridge compressor cycling included). We will use 280W for this example.

Lead-acid bank example: two 12V, 150Ah deep-cycle batteries wired in series for a 24V bank. Nominal capacity: 24V × 150Ah = 3,600Wh. At 50% usable DoD, that is 1,800Wh available. At 87% inverter efficiency, usable output is roughly 1,566Wh. Divided by a 280W essentials load, that gives approximately 5.6 hours of backup.

Lithium bank example: a single 51.2V, 100Ah LiFePO4 battery. Nominal capacity: 51.2V × 100Ah = 5,120Wh. At 85% usable DoD (a conservative middle figure within the commonly cited 80-90% range), that is 4,352Wh available. At 92% inverter efficiency (lithium setups often pair with newer, more efficient hybrid inverters), usable output is roughly 4,004Wh. Divided by the same 280W load, that gives approximately 14.3 hours of backup.

That gap — roughly 5.6 hours versus over 14 hours — is not a marketing exaggeration, it is simple arithmetic driven almost entirely by the DoD difference plus a smaller efficiency gain. It is also why lithium has become the preferred chemistry for households that regularly face longer winter outages, despite the higher upfront cost. For the current pricing on lithium battery banks sized to your specific load, WhatsApp us now for the latest price of the battery at 03192070022 — pricing varies by capacity and brand, so it is best discussed directly rather than quoted as a flat figure here.

What Happens When You Add a Heater: The Same Math, a Very Different Answer

Resistive heating loads — room heaters, geysers, immersion rods — are brutal on backup time because they draw continuous high wattage for as long as they are switched on, with none of the intermittent cycling that a fridge or the gentler draw of lighting gives you. A single 1,000W room heater running continuously is, by itself, often a bigger load than the rest of the entire house combined.

Take the same lithium example above — 4,004Wh of usable output — and add just one 1,000W room heater to the 280W essentials load, for a combined 1,280W draw. Backup time drops from 14.3 hours to roughly 3.1 hours. Add a second heater or run a geyser element at the same time and you can be looking at well under two hours of backup, even on a battery bank that comfortably carries the house through an all-essentials winter night.

This is the single most common reason people feel their “new battery isn’t lasting like it used to” — the battery hasn’t degraded, the load profile has simply changed because winter added resistive heating appliances that were never part of the original backup-hours calculation. If you want the full breakdown of running a geyser and room heaters off your inverter specifically, including safe combined-load tables, that deserves its own dedicated guide — but the short version for backup-hours purposes is: every 1,000W of continuous heating load you add can cut your battery runtime by more than half, depending on your base load.

How Long Do Winter Outages Actually Run in Punjab and Sindh?

This is the question all of the above math is ultimately in service of — but it’s also the hardest one to answer with a confident, fixed number, and we want to be upfront about that rather than inventing a figure. Loadshedding duration in Pakistan varies significantly by province, by DISCO (LESCO, FESCO, MEPCO, GEPCO, K-Electric, and so on), by feeder/area within a city, and by how the national supply-demand gap is running in a given week — and schedules are frequently revised with little notice. Reports through 2026 have described outages in parts of Punjab stretching to two to three hours at a stretch during periods of higher shortfall, while other areas on “protected” feeders see far less. We are not going to state a single nationwide winter-outage-hours figure as settled fact here, because the honest picture is that it changes by location and by week, and the only reliable way to know your own exposure is to check your specific DISCO’s current published schedule and your own recent experience on your feeder.

What this means practically: rather than sizing your battery bank around an assumed outage length you read somewhere online, size it around the actual pattern you have personally observed in your area over the last month or two, with a safety margin on top. If outages on your feeder have been running 2-3 hours but occasionally stretch to 4-5, size your bank to comfortably cover the longer end, not the average.

A Quick Reference Table: Essentials-Only Backup by Battery Bank Size

To make this easier to apply without redoing the algebra every time, here is an illustrative table using the same 280W essentials load and the same efficiency assumptions as above. Treat these as worked examples to calibrate your own numbers against, not exact guarantees — your real-world figures will shift with your specific load, battery age, and ambient temperature (cold weather itself reduces usable lead-acid capacity somewhat, which is a further reason lead-acid banks underperform their rated numbers in winter specifically).

Single 12V/150Ah lead-acid battery (1,800Wh nominal, 50% DoD, 87% efficiency): roughly 2.8 hours on a 280W essentials load.

Two 12V/150Ah lead-acid batteries, 24V bank (3,600Wh nominal): roughly 5.6 hours, as calculated above.

Four 12V/150Ah lead-acid batteries, 48V bank (7,200Wh nominal): roughly 11.2 hours.

Single 51.2V/100Ah lithium battery (5,120Wh nominal, 85% DoD, 92% efficiency): roughly 14.3 hours, as calculated above.

Two 51.2V/100Ah lithium batteries in parallel (10,240Wh nominal): roughly 28.6 hours — effectively covering even a very long multi-outage winter day on essentials alone.

For help matching one of these bank sizes to your specific inverter and household, or for current availability and WhatsApp us now for the latest price of the battery at 03192070022, our team can walk through the numbers with you directly rather than you having to get the arithmetic exactly right yourself.

Why Your Battery Might Be Underperforming Its “Rated” Backup Time

If your real-world backup hours are consistently coming in under even the conservative estimates above, a few common culprits are worth checking before assuming the battery itself is at fault. Partial charging is the biggest one — in winter, shorter daylight hours and weaker solar irradiance mean a solar-assisted system may not be fully topping up the battery bank before the next outage hits, so you are starting each outage from, say, 80% state of charge instead of 100%, which proportionally shortens every hour of backup calculated above. If you suspect your battery genuinely isn’t reaching full charge between outages, our guide on diagnosing an inverter that isn’t charging the battery properly covers the common causes, from charging-current settings to wiring and sensor issues.

Battery age also matters more than most people expect — a lead-acid battery’s usable capacity declines meaningfully after 2-3 years of regular cycling even with good maintenance, so a 150Ah battery that is three winters old may realistically be delivering closer to 70-80% of its original rated capacity. And finally, cold ambient temperature itself measurably reduces the effective capacity lead-acid chemistry can deliver at any given moment, which is another reason winter backup numbers for lead-acid banks can run shorter than the same battery delivers in summer testing.

Matching the Inverter to the Battery Strategy

The backup-hours math above assumes a reasonably efficient inverter doing the DC-to-AC conversion, but inverter choice itself affects the numbers meaningfully, particularly for households planning to lean on lithium for the efficiency and DoD gains. Pairing a lithium bank with an inverter that properly supports lithium battery communication protocols (so the inverter can read real state-of-charge and adjust charging behavior accordingly, rather than treating the lithium bank with generic lead-acid charging logic) matters for getting the full benefit of the chemistry. Our guide to the best hybrid inverters available in Pakistan right now covers which models handle this well if you are planning a lithium upgrade alongside a new or replacement inverter this winter.

Buy Inverter Shop premium inverters in Pakistan. High-performance home and industrial inverters with ISO certification, nationwide delivery, and a team that can talk you through exactly which battery bank size fits your real winter outage pattern rather than a generic recommendation.

A Simple Checklist Before You Trust Any Backup-Hours Number

Before relying on any backup-hours figure — ours, a dealer’s, or a spec sheet’s — run through this short checklist: have you totaled your actual connected load including fans, router, and fridge average draw, not just the obvious items; have you confirmed your battery’s real usable DoD rather than its full nominal capacity; have you accounted for your specific inverter’s efficiency rather than assuming 100% conversion; have you factored in whether any resistive heating load (heater, geyser) will be running during the outage; and have you based your expected outage length on your own feeder’s recent pattern rather than a generic assumption. Getting all five right is the difference between a battery bank that quietly gets you through winter and one that leaves you in the dark an hour before the power comes back.