How long will a 5 kWh battery last in a power cut?

Roughly, 5 kWh runs about 500 W of load for 10 hours, or 1 kW for 5 hours, before accounting for real-world losses. So a 5 kWh battery comfortably carries lights, fans, a router and a fridge overnight, but not heavy air-conditioning for long.

How long will a 16 kWh battery run an air-conditioner?

A typical inverter AC averages around 1 kW once the room is cool. Running one AC plus about 500 W of essentials draws roughly 1.5 kW, so a 16 kWh battery gives in the region of 8-10 hours before real-world losses. Two ACs roughly halves that.

How much does a home backup battery cost in Sri Lanka?

As of July 2026, EAST LiFePO4 batteries run from roughly Rs 404,000 for a 5 kWh rack unit to around Rs 755,000 for 10 kWh and Rs 972,000 for 16 kWh, before installation. A complete 6 kW backup system with 10 kWh of storage installed is around Rs 1.2 million as a package.

Should I size for the whole house or just essentials?

Most people back up essential circuits, not the whole house, because it is far cheaper and covers what matters in an outage. We separate essential loads during design so your backup lasts, and you can always add modules later.

Can I add more battery later?

Yes. EAST LiFePO4 storage is stackable, so you can start with what you need and add modules over time without changing the inverter. That makes it sensible to size for today and expand as your needs grow.

How Much Battery Backup Do I Need for Power Cuts?

Q: How much battery backup do I need for power cuts?

It depends on which loads you want to keep on and for how long. For lights, fans, TV, router and a fridge through a typical evening cut, many Sri Lankan homes are comfortable with 5-10 kWh. Add air-conditioning or a water pump and you move toward 10-20 kWh. Our tool sizes it from your actual loads.

July 16, 2026

The most common question we get is a simple one: how much battery do I actually need to get through a power cut? The honest answer is that it depends on which appliances you want to keep running and for how long. This guide gives you a practical way to estimate it — the same method our consultants use before a site survey.

Start with the loads, not the bill

Backup sizing is about power (kW) and energy (kWh), not your monthly rupees. List the things you want to keep on during an outage and roughly how much they draw:

LED lights — a few watts each; a whole house of them rarely exceeds 100 W.
Ceiling fan — about 60–75 W each.
Wi-Fi router — about 15 W.
TV — about 100 W.
Fridge — about 150 W on average, with brief higher draws when the compressor starts.
Laptop / phone charging — 60–100 W.
Inverter air-conditioner — roughly 900–1,500 W per unit while cooling, less once the room holds temperature.
Water pump — about 750 W running, with a hard starting surge.

Add up what would realistically run at the same time. A typical "essentials" evening — lights, three fans, router, TV, fridge — comes to roughly 500–600 W. Add one AC and you are near 1.5–2 kW. That number is your backup load.

A simple rule of thumb

Usable battery energy divided by your load gives you runtime. As a rough guide, 5 kWh runs about 500 W for 10 hours, or 1 kW for 5 hours, before real-world losses. Scaling that:

Essentials only (≈500 W) — a 5 kWh battery covers an evening cut with margin; 10 kWh carries you overnight comfortably.
Essentials plus one AC (≈1.5 kW) — 10 kWh gives around 6 hours; 16 kWh gives 8–10 hours.
Essentials plus two ACs or a pump (≈2.5–3 kW) — you want 16–20 kWh for an evening, more for overnight.
Whole-home, longer outages — 20 kWh and up. Whether that runs on a single-phase or three-phase system depends on your premises supply and connected loads, not on the battery size — a single-phase inverter can carry a large battery within the manufacturer's limits.

Real-world losses — inverter efficiency, battery depth-of-discharge limits, cable losses — typically take 10–15% off the paper number, which is why we size with headroom rather than to the last watt-hour.

What the common sizes cost

For planning purposes, as of July 2026 the EAST LiFePO4 range we install prices roughly as follows (battery only, before installation): a 5 kWh unit from around Rs 404,000 (rack-mounted) with wall-mounted and stacked formats a step above; 10 kWh around Rs 755,000; and 16 kWh around Rs 972,000. As a complete installed package, a 6 kW hybrid inverter with 10 kWh of storage — our backup starter — is around Rs 1.2 million. Browse the battery range and packages for current figures; prices move with exchange rates and stock.

Back up essentials, not everything

Backing up the whole house is expensive and rarely necessary. Most homes back up an essentials circuit — lights, fans, fridge, router, TV, phone charging, perhaps one AC in a bedroom — which is far cheaper and covers what matters when the grid drops. During installation we separate those loads onto the backed-up circuit so your battery isn't drained by a water heater or oven you forgot was on. That circuit design is as important as the kWh number: a well-designed 10 kWh system often outlasts a poorly designed 16 kWh one in a real cut.

Remember the inverter, not just the kWh

Two numbers matter: how much energy the battery holds (kWh, which sets runtime) and how much power the inverter can deliver at once (kW, which sets how many things run together). A big battery behind a small inverter still cannot start a heavy motor — pumps and older ACs draw a multiple of their running power for a moment at startup, and the inverter has to carry that surge. This is why we pair, for example, a 6 kW hybrid inverter with 10–16 kWh for essentials-plus-one-AC homes, and move to 10–12 kW single-phase or three-phase units when the backed-up load list grows. Our build-my-system tool pairs the two correctly for you.

Lead-acid vs LiFePO4 for backup

If you are comparing against a traditional lead-acid "UPS bank": lead-acid can only use about half its rated capacity without damaging itself, wears out after hundreds of deep cycles rather than thousands, and needs maintenance. LiFePO4 delivers nearly all its rated energy, cycles daily for years, and sits maintenance-free on a wall or floor. For a battery you may cycle every day — not just during cuts — the economics favour LiFePO4 decisively; our LiFePO4 vs lead-acid guide runs the numbers.

You can start small and grow

Because EAST LiFePO4 storage is stackable, you can size for today and add modules later without changing the inverter. An all-in-one system makes this especially simple — you just stack another module. And the same battery that carries you through outages also shifts your load off the expensive evening peak on a time-of-use tariff, so it earns its keep every day, not only during cuts. For the full picture, see how to beat the 2026 CEB tariff with solar and battery.

Build your system to size backup from your actual loads in a couple of minutes, or ask a consultant to confirm it on a site visit.

Written by the MyEnergy Solutions technical team. Last reviewed: July 2026. Sources: MyEnergy EAST price list (July 2026); appliance draw figures are typical manufacturer ratings. Runtime figures are indicative and depend on your real loads, battery state and system design; we confirm sizing on your quotation.

Back to blog