How it is calculated
- Running load = watts × quantity, added up for every appliance.
- Inverter = running load × (1 + 25%) ÷ 1,000 kW; kVA = kW ÷ 0.8.
- Energy needed = watts × quantity × hours of backup (fridges and freezers at half the time).
- Battery = energy ÷ 93% efficiency ÷ usable depth of discharge (80% lithium, 50% lead-acid); Ah = Wh ÷ volts.
Example: 3 fans, 6 LED bulbs, a fridge, a TV and a laptop make a running load of 630 W. Over 4 hours they use 2,120 Wh. The inverter needs 0.79 kW, or 0.98 kVA, and the battery 2.85 kWh of lithium (119 Ah at 24 V) or 4.56 kWh of lead-acid (190 Ah at 24 V).
Battery for the same load by hours of backup
| Backup | Lithium (kWh) | Lead-acid (kWh) | Lead-acid at 12 V (Ah) |
|---|---|---|---|
| 2 hours | 1.42 | 2.28 | 190 |
| 4 hours | 2.85 | 4.56 | 380 |
| 6 hours | 4.27 | 6.84 | 570 |
| 8 hours | 5.7 | 9.12 | 760 |
A lead-acid battery gives less than its rated capacity when emptied quickly (about 80% to 85% over 5 hours, according to Victron), so allow a somewhat bigger battery for short, heavy backup. Check the datasheets of the actual equipment with your installer before buying.
Frequently asked questions
How is the inverter size worked out?
Add the watts of everything that can run at the same time, then add a margin (25% by default) for motor starting surges and for heat, since inverters give less power in hot weather (a 4000 W inverter at 25 °C gives about 3700 W at 40 °C). Divide kW by the power factor (0.8) for kVA, the figure printed on most Pakistani inverters.
How is the battery size worked out?
Energy needed = each appliance’s watts × quantity × hours (fridges at half the time, as their compressor cycles). Battery capacity = energy ÷ inverter efficiency ÷ usable depth of discharge. Divide the watt-hours by the battery voltage to get amp-hours (Ah).
Why does lead-acid need a much bigger battery?
A lead-acid or tubular battery lasts far longer if it is only half emptied each time. Victron’s datasheet gives about 600 cycles at 50% discharge but only 400 at 80%. LiFePO4 batteries are rated for thousands of cycles at 80% discharge, so you can use more of their capacity.
12 V, 24 V or 48 V?
The energy is the same; only the amp-hours change. Higher voltage means lower current, thinner cables and less loss, so larger systems usually use 48 V. Use the voltage your inverter is built for: two 12 V batteries in series make 24 V and four make 48 V.
Should an AC or water motor run on the battery?
They can, but they need a large inverter and battery. A water motor or a non-inverter AC draws several times its rated power for a moment when it starts, so check that the inverter’s peak (surge) rating covers it. Running an AC on batteries for hours needs a very large bank; most homes back up fans, lights, the fridge and the internet router instead.
Guides
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Sources
- Victron Energy, Lithium Battery Smart 12.8 V datasheet: cycle life 2500 at 80%, 5000 at 50% depth of discharge; round-trip efficiency 92%
- Victron Energy, Gel and AGM batteries datasheet: 400 cycles at 80%, 600 at 50%, 1500 at 30% discharge; capacity falls at high discharge rates
- Victron Energy, MultiPlus-II datasheet: maximum efficiency 93% to 96%; 4000 W per 5000 VA at 25 °C, 3700 W at 40 °C
- Appliance wattages: K-Electric Energy Consumption Calculator (shared with our appliance units calculator)
- Sizing margin and default backup appliances: PakHisaab assumptions, editable
This calculator gives an estimate for guidance. It is not official advice. Rates and rules change, so confirm important decisions with the relevant authority or a qualified professional. See how we calculate, or report a mistake.