In this guide
How to Size a Home UPS and Inverter Battery: Load, Backup Time and Battery Capacity
A power cut arrives, the inverter beeps, and the house runs on battery — until it does not. The most common reason a home backup disappoints is not a faulty battery. It is a sizing error made at purchase time: the load was never added up, watts were confused with VA, or the battery was sized for the inverter's sticker rating rather than for the hours the family actually needs.
This guide sizes a home UPS system in the order that prevents those errors: load first, inverter rating second, battery capacity last. Every formula is shown with a worked example, so you can repeat the same arithmetic for your own home.
The problem: three numbers that look interchangeable but are not
Three ratings dominate every UPS conversation, and mixing them up is the root of most sizing mistakes:
- Load, in watts (W). What your appliances actually draw. This is the only number that decides how long a battery lasts.
- Inverter rating, in volt-amperes (VA). What the inverter can deliver. VA is always equal to or larger than watts for the same load, because of power factor.
- Battery capacity, in ampere-hours (Ah). How much charge the battery stores, at a stated voltage. Ah alone says nothing about backup time until you combine it with voltage, usable depth of discharge, and inverter efficiency.
A shop recommendation of "800 VA with a 150 Ah battery" is meaningless until you know the load it must carry and the hours it must carry it for. Two homes with the same inverter can get wildly different backup times because their loads differ.
Step 1: audit the load you actually want on backup
Walk through the house during a typical evening cut and list only what the backup must run. Do not size for everything in the house — size for what matters during a cut. Fans, lights, a router, and perhaps a television are the usual list. A refrigerator, water heater, air conditioner, or washing machine changes the answer completely, and is treated separately below.
Read the wattage off each appliance's rating plate or manual. Where only a current rating is given, watts are approximately volts times amps for resistive loads. For a fan, use its rated wattage rather than a guess: a typical ceiling fan draws around 70 to 75 W, not the 40 W many people assume.
Here is the example load used through the rest of this guide:
| Appliance | Quantity | Each | Total |
|---|---|---|---|
| Ceiling fan | 2 | 75 W | 150 W |
| LED lamp | 5 | 10 W | 50 W |
| Television | 1 | 80 W | 80 W |
| Wi-Fi router | 1 | 20 W | 20 W |
| Total | 300 W |
Add a margin of about 20 percent for loads you forgot and for appliances drawing slightly more than their plates claim. That takes the design load to 360 W. The worked example below keeps the round 300 W figure and notes where the margin changes the result, so the arithmetic stays easy to follow.
A small script makes the audit repeatable. Add or remove rows and the totals update:
loads = [("Ceiling fan", 2, 75), ("LED lamp", 5, 10),
("Television", 1, 80), ("Wi-Fi router", 1, 20)]
total = sum(qty * watts for _, qty, watts in loads)
print("Total load:", total, "W")
print("With 20% margin:", round(total * 1.2), "W")
Step 2: convert watts to VA for the inverter rating
Inverters are rated in VA, not watts, because they must supply both the real power (watts) and the reactive power that inductive loads such as fan motors demand. The link between them is the power factor:
VA = watts / power factor
For a mixed home load, a power factor of 0.8 is a reasonable planning figure unless the inverter's manual states its own output power factor. For the example load:
VA = 300 / 0.8 = 375 VA
A 600 VA inverter therefore covers the arithmetic, and an 800 VA unit is the common comfortable choice: it leaves headroom for the brief starting surge when a fan motor starts, and for the 20 percent margin added in Step 1 (360 W / 0.8 = 450 VA, still inside 600 VA, but closer to its limit than is comfortable). Do not buy far above the need either — a hugely oversized inverter idles inefficiently and costs more for no extra backup time, because backup time comes from the battery, not the inverter.
Step 3: work out the battery capacity from backup time
This is the step most buyers skip, and it is the one that decides whether the lights stay on for one hour or three. Work backwards from the energy you need:
- Energy delivered to the load: load in watts times backup hours.
- Energy the battery must supply: divide by inverter efficiency, typically around 0.85 for a home inverter. The inverter loses some energy as heat, so the battery must hold more than the load receives.
- Nominal stored energy: divide again by the battery's usable fraction. A lead-acid battery should not be regularly drained to empty; planning on about 50 percent usable depth of discharge keeps its life reasonable.
- Ampere-hours: divide the nominal energy in watt-hours by the battery voltage, usually 12 V for a single-battery home system.
For the example — 300 W for 3 hours:
Energy to load = 300 W x 3 h = 900 Wh
Battery must supply = 900 / 0.85 = about 1,059 Wh
Nominal stored = 1,059 / 0.50 = about 2,118 Wh
Capacity = 2,118 / 12 V = about 177 Ah
The next common size up is a 200 Ah battery. Check it backwards: 200 Ah at 12 V stores 2,400 Wh nominally; half is usable, giving 1,200 Wh; after 0.85 inverter efficiency, 1,020 Wh reaches the load; at 300 W that is 3.4 hours. The sizing closes: a 200 Ah battery honestly delivers the 3 hours asked for, with a little spare.
The same script extends the audit to a capacity check:
load_w, hours = 300, 3
energy = load_w * hours / 0.85 / 0.50
capacity_ah = energy / 12
print("Required capacity:", round(capacity_ah), "Ah")
If you add the 20 percent margin (360 W), the requirement rises to about 212 Ah — which is the point where a second battery in parallel, or a 24 V two-battery system, becomes the honest answer rather than a single 200 Ah unit. That is exactly the kind of decision the arithmetic exists to force.
Step 4: choose the battery type honestly
For home backup, the practical choice is usually between tubular lead-acid and lithium iron phosphate (LiFePO4):
| Feature | Tubular lead-acid | LiFePO4 |
|---|---|---|
| Usable depth of discharge | About 50 percent for decent life | About 80 to 90 percent |
| Upfront cost | Lower | Higher, roughly several times the lead-acid price |
| Life | A few years of regular cycling | Many more cycles, often outlasting the inverter |
| Maintenance | Needs water top-ups and ventilation | Sealed, no top-ups |
| Weight and size | Heavy and bulky | Lighter and smaller for the same usable energy |
The depth-of-discharge difference changes the maths in Step 3 directly. A lithium battery sized at 80 percent usable needs only about 1,324 Wh nominal for the same example (900 / 0.85 / 0.80), or about 110 Ah at 12 V — roughly half the lead-acid Ah for the same backup. Comparing a 150 Ah lead-acid battery with a 100 Ah lithium battery on Ah alone is therefore misleading; compare usable watt-hours, not the headline number.
Whatever the chemistry, match the battery voltage to the inverter: a 12 V inverter takes one 12 V battery (or several in parallel for capacity), while a 24 V inverter takes two 12 V batteries in series. Mixing an old battery with a new one in parallel is poor practice — the weaker battery drags the pair down and both age faster.
Step 5: waveform, charging, and changeover
Three inverter details matter more than brand names:
- Waveform. A pure sine-wave output suits every home load, including fan motors, which hum and run hot on the stepped waveform of cheaper modified sine-wave units. For a home with fans, pure sine-wave is the safe default.
- Charging current. A common rule of thumb is a charging current near a tenth of the battery's Ah rating — about 15 to 20 A for a 150 to 200 Ah battery — but the battery manufacturer's recommendation overrides any rule of thumb. Too small a charger means the battery never fully recovers between cuts; too large shortens its life.
- Changeover time. A home UPS switches from mains to battery in roughly 10 to 20 milliseconds. Computers may reboot on a slow changeover, which is why a dedicated computer UPS, with a much faster switch, is a different product from a home inverter. Do not expect a home inverter to hold up a desktop PC through every cut.
Loads that break the simple maths
The method above assumes a steady, modest load. Three cases need separate treatment:
- Refrigerators and air conditioners. Their compressors draw several times their running wattage for a second at start-up. Size the inverter for the surge, not the running watts, and expect backup time to shrink sharply. Many installers simply leave these off the backup circuit, which is often the right call.
- Long cuts and daily cycling. A battery sized for an occasional 2-hour cut will age fast if it is cycled deeply every day. If cuts are daily, size for the worst usual cut, not the average, and favour the deeper usable capacity of lithium or a larger lead-acid bank.
- Solar later. If rooftop solar is planned, the inverter and battery choice changes — charge controllers, solar-compatible inverters, and daytime charging all enter the design. Buy the solar-ready variant now rather than replacing the inverter later.
Installation and safety notes
A battery stores a large amount of energy, and lead-acid batteries release hydrogen gas while charging. Place the battery and inverter in a ventilated spot, away from flames and sparks, on a level surface that can carry the weight. Keep terminals clean and tight — a loose terminal heats up and is a common cause of melted connectors. Never connect inverter output back into the mains wiring without a proper changeover switch; back-feeding the grid endangers line workers and is illegal in most jurisdictions. Mains-side wiring and changeover installation belong to a licensed electrician, not a do-it-yourself weekend job.
Common sizing mistakes
- Buying VA as if it were watts. A 600 VA inverter does not deliver 600 W; at 0.8 power factor it delivers about 480 W.
- Sizing the battery to the inverter instead of the load. The inverter's rating does not set backup time — load and battery capacity do.
- Using full Ah as usable energy. A 150 Ah lead-acid battery at 12 V does not deliver 1,800 Wh to your home; after usable depth of discharge and inverter losses, roughly half that reaches the load.
- Forgetting the fan surge and the forgotten load. The phone charger, the set-top box, and the second fan all count. Audit the real evening load, not the remembered one.
- Adding a second-hand battery to a new bank. Parallel batteries should match in age, type, and capacity, or the weakest one limits the whole bank.
Frequently asked questions
How long will a 150 Ah battery run my home?
It depends entirely on the load. At 300 W, a 150 Ah lead-acid battery delivers roughly 2.5 hours by the Step 3 arithmetic (150 x 12 x 0.50 x 0.85 / 300). At 150 W it delivers about twice that. Any answer that does not ask for your load is a guess.
Is a bigger inverter better?
Only up to a point. The inverter must cover the load's VA plus surge headroom. Beyond that, extra VA buys no extra backup time and wastes money and standby efficiency. Spend the difference on battery capacity instead.
Can I run a refrigerator on a home inverter?
Sometimes, if the inverter is sized for the compressor's starting surge and the battery bank is large — but backup time will be short and battery wear will be high. Most homes leave refrigeration off the backup circuit and size for lights, fans, and connectivity.
How often should I top up a tubular battery?
Check the electrolyte level every few months and top up with distilled water only, to the marked level, when the plates are covered but the level is low. Never use tap water, and never overfill. Sealed and lithium batteries need no top-ups.
Why does my backup time keep shrinking?
Usually one of three causes: the load has grown since installation, the battery has aged and lost capacity, or the battery is chronically undercharged because the charger is too small or cuts are too frequent for full recharge between them. Re-audit the load first; it is the cheapest check.
Does a higher Ah battery charge slower?
Yes, with the same charger. A 200 Ah battery takes roughly a third longer to recharge than a 150 Ah one. If cuts are frequent, charger current matters as much as capacity.
Limitations
This guide sizes a conventional single-phase home backup system. It does not cover three-phase homes, whole-house automatic transfer installations, or solar-hybrid design, each of which needs a site-specific assessment. The efficiency (0.85), power factor (0.8), and usable depth-of-discharge (50 percent) figures are planning values for the worked example — your equipment's manuals give the exact numbers, and they override the values used here. Battery prices, sizes, and model availability change often; always confirm the current catalogue before ordering, and have mains-side work done by a licensed electrician.