How to Size a Solar Battery (Philippines)
TL;DR
Size a solar battery from the load you actually want to run after dark, not from your total monthly bill. List those appliances, multiply watts by hours to get evening kWh, add about 20% headroom, and that is your usable capacity target. Most Philippine homes land between 5kWh and 10kWh.
Size a solar battery from the load you want to run after dark, not from your monthly bill. That single distinction is the difference between a 5kWh pack and a 12kWh one, and at roughly ₱13,000-23,000 per usable kWh installed it is worth ₱100,000 or more of decisions.
Your bill covers a full 24 hours. The battery only ever needs to cover the hours when the sun is not producing and you are still using power. Everything the panels generate and you consume during the day never touches the battery at all.
The formula
Usable kWh needed = (evening watts × hours) ÷ 1,000 × 1.2
Three steps, in order:
- List only what you want backed up after dark. Not everything in the house.
- Multiply each appliance’s watts by the hours you run it, add them together, and divide by 1,000 to get kWh.
- Add about 20% headroom for capacity fade, the appliance you forgot, and the fact that draining to empty nightly shortens the pack’s life.
Whatever number comes out is your target in usable kWh. Insist that quotes state usable capacity, not nameplate — see how long do solar batteries last for why those two numbers diverge.
A worked example
A typical Philippine household wanting the essentials plus a bit of comfort through the night:
| Appliance | Watts | Hours after dark | kWh |
|---|---|---|---|
| Refrigerator (cycling) | ~150 avg | 12 | 1.8 |
| LED lights (6 rooms) | ~60 total | 5 | 0.3 |
| Electric fans (2) | ~120 total | 8 | 1.0 |
| TV | ~100 | 4 | 0.4 |
| Router, phones, misc | ~40 | 12 | 0.5 |
| Subtotal | 4.0 | ||
| +20% headroom | 4.8 |
So roughly a 5kWh usable pack — the most common residential size in the Philippines, and no accident.
Now add one 1.0 HP inverter aircon at around 800W for four hours: that is another 3.2kWh, pushing the requirement past 8kWh and, with headroom, to about 10kWh. Aircon is almost always the appliance that decides whether you buy a 5kWh pack or double it. Our appliance electricity cost guide has watt figures for the rest of a normal Philippine home.
Depth of discharge changes everything
Two packs advertised at 10kWh are not the same battery:
| Chemistry | Safe depth of discharge | Usable from a 10kWh nameplate |
|---|---|---|
| LiFePO4 | ~90-100% | ~9-10 kWh |
| Lead-acid | ~50% | ~5 kWh |
Lead-acid looks cheaper per nameplate kWh and stops looking cheap the moment you size on usable capacity. This is one of the main reasons LiFePO4 took over residential storage, covered in our battery types guide.
Can your array actually refill it?
A battery is only useful if it recharges. Check that your panels produce enough surplus — generation left over after daytime consumption — to refill the pack on a normal day.
A rough sanity check: a 5kW array in the Philippines produces on the order of 600 kWh a month, or about 20 kWh a day, of which a good chunk is consumed live by the house. If daytime use eats 12 kWh, the surplus is around 8 kWh. A 10kWh pack will not fully refill from that, and during the rainy season it will do considerably worse.
If the battery is much bigger than the daily surplus, you have bought capacity that will spend most of its life partly empty. Either size the pack down or size the array up — how many solar panels do I need covers the array side.
Mistakes that oversize a pack
- Sizing to the whole bill. Covered above, and comfortably the most expensive error.
- Backing up things that do not need it. The electric kettle, the iron, the water heater. High-wattage, short-duration appliances blow up a battery calculation and can simply wait for morning.
- Planning for the worst brownout you can imagine. Sizing for three days off grid rather than one evening multiplies the cost several times over for an event that may not happen. A generator is cheaper insurance for genuine multi-day outages — see solar vs generator.
- Ignoring depth of discharge when comparing two quotes.
What if I mostly want brownout backup?
Then size for the length of a typical outage in your area, not for a full night. Many homes are well served by a smaller pack that carries lights, fans, the router, and the ref through a few hours, with aircon deliberately excluded from the backup circuit.
That decision — which circuits the battery feeds — is worth having explicitly with your installer. A well-chosen essential-loads panel lets a modest battery deliver most of the practical benefit of a much larger one. Our typhoon and brownout backup guide covers how to set that up, and solar battery price shows what each step up in capacity costs.
Frequently asked questions
How many kWh of battery do I need?
Add up only what you want running after dark. Multiply each appliance's watts by the hours you run it, divide by 1,000 for kWh, then add roughly 20% headroom. Most Philippine homes wanting overnight essentials plus a few hours of aircon land somewhere between 5kWh and 10kWh usable.
Should I size the battery to my whole electric bill?
No, and this is the most common and most expensive mistake. Your bill covers 24 hours, but the battery only needs to cover the hours the sun is not producing and you are still using power. Sizing to the full bill routinely doubles the pack and the price for capacity you never discharge.
What is depth of discharge and why does it change the size?
Depth of discharge is how much of a pack you can safely use. LiFePO4 allows most of its nameplate capacity, so usable is close to rated. Lead-acid wants to stay above 50%, so a nominal 10kWh lead-acid bank gives about 5kWh usable. Always size on usable kWh.
How much headroom should I add?
Around 20% is a sensible default. It covers the appliance you forgot, gradual capacity fade as the pack ages, and the fact that running a battery to absolutely empty every night shortens its life. Much more than 20% and you are paying for capacity that mostly sits idle.
Does the battery need to match my solar array size?
Not directly, but the array has to be able to refill it. If the pack is much larger than the daily surplus your panels produce after covering daytime use, it will not fully recharge on a normal day, and it certainly will not during the rainy season.
What size battery runs an aircon overnight?
That depends on the unit, and it is usually the appliance that decides your pack size. A 1.0 HP inverter aircon drawing roughly 700-900W while running needs several kWh for a few hours, which is why homes wanting overnight aircon jump to 10kWh or more while homes wanting fans and a ref stay near 5kWh.