How to Size a Small DIY Solar Backup for Brownouts in the Philippines

To size a small DIY solar backup, work in four steps: add up your daily loads in watt-hours, size the battery to cover them, size the panels to refill the battery, then pick a charge controller that can safely take those panels. In our worked example, about 794 Wh a day (including losses) points to a 12.8V 100Ah LiFePO4 battery, three PowMr 100W flexible panels in series, and a PowMr Keeper 30A MPPT controller. That pairing is listed as a fit in our compatibility data.

The numbers in the example are example assumptions, not measurements of your home. The method is what matters: redo each step with your own appliances and you’ll get a system sized for your household, not for a seller’s kit.

Why sizing order matters

Most failed small solar setups have the same story. Someone buys a “kit” because it was on sale, then finds the battery is flat by 9 p.m., or the panels can’t refill it after a cloudy day, or the controller shuts down because the panels push too much voltage into it. Each part depends on the one before:

1. Loads decide how much energy you need each day.
2. Battery size depends on those loads and how long you want to ride through a brownout.
3. Panels must refill the battery from the daily sun you actually get.
4. Charge controller must handle the panels’ voltage and wattage at your battery voltage.

The inverter (which turns battery DC into 230V AC for normal plugs) sits alongside the battery. It has to handle the biggest load you’ll run at once, and its own idle draw counts as a load too.

If you’d rather skip the wiring entirely, a portable power station does the battery, inverter and charge controller in one box. We cover that route near the end.

Step 1: List your loads in watt-hours per day

A watt-hour (Wh) is one watt used for one hour. A 45 W fan running for 6 hours uses 45 × 6 = 270 Wh.

For each appliance you want to keep running in a brownout:

• Find the wattage on its label or adapter. If the label shows only volts and amps, watts ≈ volts × amps.
• Estimate the hours per day you’d actually use it during an outage.
• Multiply. Then add everything up.

Our appliance consumption calculator does this for you, and the guide to backup power for brownouts helps you decide what’s essential.

Worked example: a typical brownout essentials list

Notice what’s missing: no aircon, no rice cooker, no flat iron, no water heater. A small DIY system can’t run these for long. Heating appliances and aircon draw hundreds or thousands of watts, which would need a far bigger battery and inverter. Keep the list to essentials.

Add losses

Power is lost turning DC into AC, and a little more in the wiring. As an example assumption, add 15%:

Daily energy needed ≈ 675 Wh ÷ 0.85 ≈ 794 Wh

The real figure depends on your inverter’s efficiency at your load level. For reference, PowMr’s catalogue lists a 94% peak efficiency for its hybrid inverters; efficiency is usually lower at light loads, which is why we use a more cautious example figure.

Don’t forget the inverter’s idle draw

An inverter uses power just by being on, even with nothing plugged in. This is listed as “no-load” consumption. PowMr’s 2024 catalogue lists its 4.5kW hybrid inverter at under 35 W no-load and its 6.5kW at under 50 W.

Do the sum: 35 W × 24 hours = up to 840 Wh a day. That’s more than our whole example load list. A multi-kilowatt inverter is the wrong tool for a router, a fan and some bulbs. For a small backup, choose a small inverter sized to your largest load, and look for its no-load figure in the manual. Then add that figure × the hours it will be switched on to your daily total.

Step 2: Size the battery

Battery capacity is usually written in amp-hours (Ah) at a nominal voltage. Multiply to get watt-hours:

Battery Wh = nominal volts × amp-hours
12.8V × 100Ah = 1,280 Wh

You shouldn’t plan to drain a battery to zero every night. As an example assumption, plan to use 80% of the nominal capacity. Check your own battery’s manual for its recommended limit, since this varies by chemistry and maker.

Nominal battery needed ≈ daily energy ÷ usable fraction
≈ 794 Wh ÷ 0.8 ≈ 993 Wh
At 12.8V: 993 Wh ÷ 12.8V ≈ 78 Ah

The next common size up is 100Ah, which gives 1,280 Wh nominal and about 1,024 Wh usable at our 80% example. That covers one example day (794 Wh) with roughly a quarter to spare.

How many days of backup?

One day of storage covers a typical evening and overnight brownout. Typhoon outages can last longer, and cloudy rainy-season days refill the battery slowly. If you want to cover two days without sun, double the battery. That’s a cost decision: battery is usually the most expensive part of a small system, so trimming the load list is often cheaper than adding storage.

12V or 24V?

For a system with a few hundred watts of panels, either works. Two things change at 24V:

• Current halves for the same power, so cables run cooler and can be thinner (your electrician sizes them).
• The same controller takes twice the panel wattage. The PowMr Keeper 20A, for example, is rated for 260 W of panels on a 12V battery and 520 W on a 24V battery.

Your inverter must match whichever battery voltage you choose. The Keeper controllers auto-detect 12V or 24V and accept a battery voltage range of 8V to 32V.

Step 3: Size the solar panels

Panels need to put back what you used. How much a panel produces each day depends on sunlight, panel temperature, dust, shading, wiring losses and charging losses.

We use two example assumptions:

• 4.5 peak sun hours per day. This is an illustration, not a measured figure for your town. Rainy-season days and shaded sites can be far lower.
• 75% system yield, to cover heat, dust, wiring, controller and battery charging losses.

Panel watts needed ≈ daily energy ÷ (peak sun hours × system yield)
≈ 794 Wh ÷ (4.5 × 0.75)
≈ 794 ÷ 3.375 ≈ 235 W

Round up. Three 100 W panels give 300 W:

Example daily output ≈ 300 W × 4.5 h × 0.75 ≈ 1,012 Wh

That’s about 27% more than our 794 Wh day, which helps on hazy days. Our solar system size calculator and the guide to how many solar panels you need explain the same sum for bigger systems.

Which panels we can actually pair

Pairing a panel with a controller needs the panel’s open-circuit voltage (Voc). Without it, nobody can check that a series string stays under the controller’s limit. Of the panels in our product list, these have verified figures:

The PowMr 100W and 200W rigid panels have no published Voc or Isc, so we don’t pair them with any controller. PowMr doesn’t publish a connector type for the flexible panels either. Check the listing photos or ask the seller whether you’ll need MC4 connectors.

For a deeper explanation of Voc, Isc and string maths, see how to match solar panels to a charge controller.

Step 4: Choose the charge controller

The charge controller sits between the panels and the battery. It stops the battery overcharging and, in an MPPT controller, converts the panels’ higher voltage into charging current. Our guide to MPPT vs PWM controllers explains why we only recommend MPPT for this kind of setup.

Two checks decide the controller size:

1. Wattage: total panel watts must not exceed the controller’s maximum PV input at your battery voltage.
2. Voltage: the string’s total Voc must stay at or below 90% of the controller’s maximum PV Voc. We use this 10% safety margin because panel voltage rises in cooler conditions and these panels don’t publish temperature coefficients.

PowMr’s Keeper manual gives these limits:

Worked example: 300 W on a 12V battery

• Keeper 20A: 300 W is above its 260 W limit at 12V. Too small. Its verified maximum with the 100W flexible panel at 12V is 2 in series (200 W).
• Keeper 30A: 3 × 21.6 V = 64.8 V, under the 67.5 V margin limit. 300 W is under 390 W. Fits. This is the compatibility row: 3 panels in series, 1 string.
• Keeper 40A: 3 × 21.6 V = 64.8 V, well under 90 V. Its verified maximum with the 100W flexible at 12V is 4 in series (86.4 V, 400 W), so it leaves room to add a fourth panel later.

A rough current check: 300 W ÷ 12.8 V ≈ 23 A of charging current, under the 30A rating.

Other verified fits for a small backup

These are the maximum verified configurations from our compatibility data; fewer panels is fine as long as you respect the 24V minimum below. On a 24V battery, always use at least 2 of these panels in series. One 12V-class panel (Vmp about 18 V) is below a 24V battery’s charging voltage.

Step 5: The inverter and the battery

We don’t currently list a standalone battery or a small inverter with verified specs, so we can’t name a pairing. When you and your electrician choose them, check:

• Battery voltage matches the inverter’s DC input and the controller setting.
• Battery type is supported by the controller. The Keeper manual lists LiFePO4 presets; set the system voltage before choosing a lithium type.
• Inverter output is 230V AC pure sine wave at 60Hz, the Philippine grid frequency. Some listings say “50/60Hz” while the maker’s catalogue says 50Hz only, so read the manual.
• Inverter rating covers your biggest simultaneous load, with headroom for motor start-up (fans, pumps).
• No-load draw is small, for the reason shown in Step 1.
• Protection: fuses or breakers between battery, controller and inverter, sized by your electrician.

A simpler route: a portable power station

If your goal is a fan, Wi-Fi and lights through a brownout, a portable power station can do the job without any wiring. The battery, inverter and charger are built in. You charge it from a wall outlet before the outage, and it plugs straight into your appliances.

Two examples from EcoFlow’s Philippine store specs:

• EcoFlow DELTA 3 1500: 1,536 Wh LFP battery, 1,800 W AC output, up to 500 W solar input. Its capacity is larger than our example 794 Wh day, before allowing for inverter losses. See our DELTA 3 1500 buyer guide.
• EcoFlow RIVER 2 Max: 512 Wh, 500 W AC output, up to 220 W solar input. You’d need to trim the example list (for instance, fewer fan hours) to fit. See our RIVER 2 Max buyer guide.

Use the power station runtime calculator to test your own load list. To add solar charging to a station, use only the panel pairings in our solar for power stations guides. Portable panels plugged into a power station can be set up by the owner following the manual.

Safety and permits

Plug-and-play portable panels and power stations can be set up by the owner following the manual. Anything wired to the house, a breaker panel, the roof or the grid needs a licensed electrician or Professional Electrical Engineer (PEE) and an LGU electrical permit. A battery-and-controller system connected to your home’s wiring is in that second group. Our guide to whether DIY solar is legal in the Philippines explains who may do what under RA 7920. If you ever want to export power to the grid, that needs your electric company’s process: see net metering.

A few practical points for brownout setups:

• The Keeper controllers are rated IP30, so keep them indoors in a dry, ventilated spot, above possible flood levels.
• Panels produce voltage whenever light hits them. Cover them or disconnect them before working on the wiring.
• Batteries store a lot of energy. A short circuit can cause fire. Fuse the battery cable close to the battery.

What to buy

For the worked example (12V battery, 300 W of panels):

• Charge controller: PowMr Keeper 30A MPPT on Lazada (or the Keeper 40A on Lazada if you may add a fourth panel)
• Panels: 3 × PowMr 100W flexible panel on Lazada, wired in series, or 2 × PowMr 150W flexible panel on Lazada in series
• For a 24V battery instead: PowMr Keeper 20A MPPT on Lazada with 2 × 150W in series, or 2 strings of 2 × 100W
• Battery and inverter: chosen with your electrician using the checklist in Step 5
• Fuses, breakers and cable: sized and installed by your electrician
• No-wiring alternative: EcoFlow DELTA 3 1500 on Shopee or on Lazada; EcoFlow RIVER 2 Max on Shopee or on Lazada

Prices change daily, so check today’s price on each listing.

Bottom line

Start with what you actually need during a brownout, in watt-hours, and size everything else from that. In our example, about 794 Wh a day leads to a 12.8V 100Ah battery, 300 W of panels and a PowMr Keeper 30A MPPT controller with three 100W flexible panels in series. On a 24V battery, the smaller Keeper 20A can handle the same 300 W. Keep the inverter small, count its idle draw, and have a licensed electrician do any wiring that connects to your home.