How to size a battery backup for a Filipino home without guessing

A battery's sticker tells you two things: voltage and amp-hours. A common one reads 12V 100Ah. Multiply those together and you get 1,200 watt-hours of stored energy. That number looks generous — until you learn that you cannot use most of it.

This is where nearly every first-time buyer goes wrong. Your real backup time depends on three figures, and only one of them is printed on the battery.

The three numbers that decide your runtime

1. Depth of discharge: how much you're allowed to use

Draining a battery to empty damages it. How far you can safely go depends entirely on the chemistry inside:

Read that table again, because it explains a lot of disappointment. A 100Ah lead-acid battery and a 100Ah lithium battery are advertised as the same size, but the lithium one gives you roughly 60–80% more actual power. It also survives several times more charge cycles. That is why the higher sticker price is often the cheaper option per year of service.

2. Conversion losses: the tax you pay to get 220V

Your battery stores DC power. Your appliances run on AC. The inverter that converts between them is not free — expect to lose 10–20% of your stored energy as heat in the inverter and the wiring. Cheap inverters and undersized cable sit at the bad end of that range.

Use 15% as a working estimate. If you measured 1,200Wh of stored energy and can safely use half of it, you have 600Wh; after losses you are really working with about 510Wh.

3. Your actual load, not your imagined load

Add up the wattage of everything that will be running at the same time. A typical brownout survival kit for a Filipino household looks like this:

Putting it together

The formula is simply usable energy divided by load:

Runtime (hours) = (Volts × Amp-hours × Depth of discharge × Efficiency) ÷ Watts

For our example with a 12V 100Ah deep-cycle lead-acid battery running 150W:

12 × 100 × 0.5 × 0.85 ÷ 150 = 3.4 hours

Swap in a LiFePO4 battery of the same 100Ah rating and the same load runs for about 5.4 hours — nearly two extra hours from an identically labelled battery.

Working backwards: sizing for a target

Most people arrive with a goal rather than a battery — “I want six hours of fan, lights and WiFi.” Flip the formula around:

Amp-hours needed = (Watts × Hours) ÷ (Volts × Depth of discharge × Efficiency)

For 150W over 6 hours on a 12V lead-acid system: (150 × 6) ÷ (12 × 0.5 × 0.85) = 176Ah. In practice that means two 100Ah batteries in parallel. With LiFePO4 the same goal needs about 110Ah — a single battery.

When to move up from 12V

Voltage is not just a spec, it decides how thick your cables must be. Power equals volts times amps, so at a fixed wattage, halving the voltage doubles the current — and current is what forces you into expensive copper.

A concrete example: 1,200W at 12V draws 100 amps, which needs roughly 50mm² copper on a 3-metre run. The same 1,200W at 48V draws 25 amps and runs comfortably on 6mm². The cable saving alone can pay for the higher-voltage inverter.

Four mistakes that shorten battery life

1. Mixing old and new batteries. In a parallel bank, the weakest battery drags the others down to its level. Always replace the whole bank together.
2. Mixing chemistries. Lead-acid and lithium charge at different voltages. Wiring them together damages both.
3. Charging too fast. Lead-acid prefers 10–20% of its Ah rating as charge current; LiFePO4 tolerates 20–50%. Exceeding it cooks the battery.
4. No fuse at the battery. The short stretch of cable between the battery terminal and the fuse is the most dangerous wiring in the whole system. A DC-rated fuse belongs within 18cm of the positive terminal.

What to check before you pay

• The Ah rating and chemistry printed on the battery, not just what the seller says.
• Whether the price includes a charger matched to the chemistry. A lead-acid charger will not properly fill a LiFePO4 battery.
• For lithium, whether a BMS (battery management system) is built in. Without one, a single cell going out of balance can destroy the pack.
• The warranty in writing, and what voids it.

Run your own numbers through the calculator before you go shopping. Walking into a store knowing you need “about 180Ah at 12V, or 110Ah if it's lithium” changes the entire conversation.