Why your backup time is never a fixed number

The most common question we hear is: *"My battery is 5 kWh — how many hours of backup will I get on my air conditioner?"*

The honest answer is: it depends on your load at that moment, and with an air conditioner the load is never steady. An AC does not pull a fixed amount of power. Its consumption changes with:

  • The room temperature at that time
  • The temperature you set the AC to
  • The outdoor temperature
  • Whether the compressor is running hard or just cycling to hold the temperature

So an AC that pulls 1.2 kW in a cool room at night can easily pull 2 kW or more in the afternoon heat. That is why nobody can promise you "2 hours on AC" — the same battery might give 3 hours one day and 1.5 hours another. The battery is fine. The load changed.

The good news: you don't have to guess. You can measure your battery's real performance in about 30 minutes with a simple test anyone can do.

First, the one idea you must understand: kW vs kWh

These two look similar but mean very different things:

  • kW (kilowatt) = how *fast* you are using power right now. This is the load number your inverter screen shows.
  • kWh (kilowatt-hour) = how much energy is *stored* in your battery. This is the size of the tank.

Think of it like a water tank:

  • kWh is how many litres the tank holds (battery size: 5, 10, 15, 16.3 kWh…).
  • kW is how fast water is flowing out of the tap (your load right now).
  • Backup time = tank size ÷ how fast you're draining it.

Once you understand this, the whole test becomes easy.

The simple formula

Here is the master formula. It works for any battery and any load:

Battery used (%) = ( Load in kW ÷ Battery size in kWh ) × Hours × 100

You can also do it in two easy steps:

  • Step 1 — Energy used = Load (kW) × Time (hours)
  • Step 2 — Percentage used = ( Energy used ÷ Battery size ) × 100

That percentage is how much your battery charge *should* drop during the test. Then you simply compare it to how much it *actually* dropped.

Important — it works from ANY starting charge. You do not have to start at 90% or at any special number. The *drop* is what matters, and the drop is the same whether you begin at 90%, 72%, or 60%. So the full check is just:

Expected ending reading = Starting reading − Percentage used

Take whatever percentage your inverter shows at the moment you begin (that is your "starting reading"), calculate the expected drop with the formula, subtract it, and that is the number your battery should show when the test ends.

*Quick example:* if you begin the test at 60% on a 5 kWh battery at 1.4 kW load, after 30 minutes it should drop 14% and read about 46% (60 − 14). Begin at 72%, and it should read about 58%. The starting point can be anything — only the size of the drop tells you if the battery is healthy.

Do the test yourself — step by step

Set up these conditions first (this is important, or the test won't be accurate):

  • 1. Start as close to 100% as you can — ideally 90–100%. Even though the formula works from any starting reading (as we showed above), starting high matters for a practical reason: many installers set the inverter to cut off heavy loads at a low battery level — often 50%, sometimes 40%. This is called low-battery load-shedding, and it protects the battery. But it means that if you start your test too low, the inverter may switch your load off part-way through, and then your "constant load" condition is broken — the reading stops making sense. Starting near full gives you plenty of room to run the test before any cutoff kicks in. (Not sure where your cutoff is set? Ask your installer, or simply start near 100% to be safe.)
  • 2. Turn OFF all charging — no solar, no grid charging, no generator. The load must run 100% on the battery so nothing is refilling the tank while you measure.
  • 3. Keep a steady, constant load — turn on appliances that draw a stable amount (fans, lights, a water pump, a fridge). Avoid the AC for this test, because its load keeps jumping around. If the load suddenly drops to zero on its own during the test, you have probably hit the inverter's low-battery cutoff — recharge and restart the test from a higher level.

Now run the test:

  • 4. Read the load on your inverter screen and write it down — for example 1.4 kW. Try to keep it roughly constant.
  • 5. Write down the starting battery percentage (e.g. 90%) and note the start time.
  • 6. Wait 30 minutes (or 1 hour for an even more accurate result). Keep an eye that the load stays roughly the same.
  • 7. Write down the ending battery percentage.
  • 8. Compare the actual drop with what the formula predicted. If they are close, your battery and its calibration are healthy.

Worked example — a 5 kWh battery at 1.4 kW load

Let's use real numbers.

  • Battery size = 5 kWh
  • Load on inverter = 1.4 kW
  • Starting charge = 90%, no charging, load fully on battery.

How much should it drop in 1 hour?

  • Energy used in 1 hour = 1.4 kW × 1 hour = 1.4 kWh
  • Percentage used = ( 1.4 ÷ 5 ) × 100 = 28%

So one full hour at 1.4 kW should use about 28% of a 5 kWh battery.

How much in 30 minutes?

  • Energy used = 1.4 kW × 0.5 hour = 0.7 kWh
  • Percentage used = ( 0.7 ÷ 5 ) × 100 = 14%

So after 30 minutes you expect the charge to fall by about 14%.

The result: you started at 90%. After 30 minutes it should read around 76% (90 − 14).

If your battery shows 76% or close to it, your battery is fine and its charge calibration is accurate. A small difference of a percent or two is completely normal — it's just from small changes in the load during the test.

Quick reference — how fast different batteries drain

At a steady 1.4 kW load (the example above), here is roughly how much each battery drops:

  • 5 kWh: about 28% per hour (≈ 14% per 30 min) — full backup ≈ 3.5 hours
  • 10 kWh: about 14% per hour (≈ 7% per 30 min) — full backup ≈ 7 hours
  • 15 kWh: about 9.3% per hour (≈ 4.7% per 30 min) — full backup ≈ 10.7 hours
  • 16.3 kWh: about 8.6% per hour (≈ 4.3% per 30 min) — full backup ≈ 11.6 hours

The generalized trick — works for ANY load

Your load won't always be 1.4 kW, so here is the shortcut that lets you handle any load in your head.

First find your battery's "% per hour for each 1 kW of load":

% per hour per 1 kW = 100 ÷ Battery size (kWh)

  • 5 kWh → 20% per hour for every 1 kW of load
  • 10 kWh → 10% per hour for every 1 kW of load
  • 15 kWh → 6.7% per hour for every 1 kW of load
  • 16.3 kWh → 6.1% per hour for every 1 kW of load

Then just multiply by your actual load in kW.

*Example:* a 5 kWh battery at 1.4 kW → 20 × 1.4 = 28% per hour. Same answer as before, done in one step.

*Another example:* a 10 kWh battery running a 2 kW load → 10 × 2 = 20% per hour, so it drops about 10% in 30 minutes.

How to read your result — pass or fail

After your test, compare the actual drop to the expected drop:

  • Actual drop is about the same as expected (within ~2–3%): ✅ Your battery is healthy and its charge percentage is calibrated correctly.
  • Actual drop is a lot MORE than expected (e.g. it fell 25% when you expected 14%): ⚠️ This usually means the battery's real capacity has weakened with age, or the percentage reading is over-optimistic. Worth getting it checked.
  • Actual drop is a lot LESS than expected: ⚠️ Usually a calibration/reading lag. Repeat the test with a longer window (1 hour) to confirm.

A note on real-world losses: in practice your charge may drop a few percent more than the pure formula predicts. That is normal — the inverter loses a little energy converting DC to AC, and the battery management system uses a small amount too. A slightly larger drop of 2–4% is not a fault. Only a big gap (like double the expected drop) points to a real problem.

Reverse it — estimate your backup BEFORE an outage

The same formula tells you roughly how long you'll last on any load:

Backup hours = ( Battery size × usable share ) ÷ Load in kW

Using about 90% as usable (most lithium systems let you use most of the pack):

  • 5 kWh running a 1 kW load → (5 × 0.9) ÷ 1 = ~4.5 hours
  • 10 kWh running a 2 kW load → (10 × 0.9) ÷ 2 = ~4.5 hours
  • 16.3 kWh running a 1.5 kW load → (16.3 × 0.9) ÷ 1.5 = ~9.8 hours

Change the load number to match whatever you actually see on your inverter, and you get a realistic estimate — not a guess.

Common mistakes that ruin the test

  • Charging left on. If solar or grid is topping up the battery, your drop will look far too small. Charging must be OFF.
  • Using the AC as the test load. Its load jumps around, so your numbers won't match. Use steady loads for the test.
  • Starting at a low charge. Two problems: batteries can behave differently near empty, and you may hit the inverter's low-battery load cutoff (often set at 40–50%) mid-test — which shuts your load off and breaks the constant-load condition. Start near 100%.
  • Too short a test. 10 minutes is easy to misread. 30 minutes is good; 1 hour is best.
  • Not writing the load down. The whole calculation depends on the kW you saw — note it at the start.

The bottom line

Your battery's backup time is not a fixed number — it moves with your load, and AC load especially swings with the weather and your thermostat. But your battery's capacity is measurable. With one high charge, charging turned off, a steady load, and 30 minutes, you can prove whether your battery is delivering its rated kWh and whether its charge percentage is honest.

If you run this test and your numbers are far off — or you'd like help choosing a genuine, safe lithium battery — the team at Al Syed Solar is here to help. Our PowerStone SunKor lithium (LiFePO4) batteries are built with A-grade, latest-manufactured 2026 cells, a smart BMS, and built-in internal fire protection — so the capacity you pay for is the capacity you actually get.

Have questions about your battery's performance? Contact us or explore our solar batteries in Peshawar.