Solar Calculator Philippines

How to use this calculator

1. Use the all-in tariff, not the generation chargeYour bill separates generation, transmission, distribution and taxes. The rate that matters for solar savings is the total divided by kWh, because solar displaces all of the volumetric charges, not just generation.
2. Average across a year if you canConsumption swings with the seasons, and sizing off a single hot month oversizes the array. Take twelve months of bills and average them.
3. Leave the derate factor alone unless you know betterEighty percent is a realistic default covering inverter losses, heat, dust, wiring and ageing. Installers quoting figures that imply 95% are quoting laboratory conditions.
4. Treat the payback as indicativeIt uses today's tariff and assumes it holds. Tariffs have historically risen, which shortens real payback, but the figure is a comparison tool rather than a forecast.

The derate factor is doing most of the work

A panel rated at 550 W produces 550 W under Standard Test Conditions: 1000 W/m² of irradiance, a cell temperature of 25°C, and a specified light spectrum. Those conditions occur on a Philippine roof approximately never.

Cell temperature is the largest departure. Panels in tropical sun routinely run at 55 to 65°C, and output falls by roughly 0.3 to 0.4% for every degree above 25°C. At 60°C that is a loss of around 12% before anything else.

Inverter conversion costs another 3 to 5%. DC wiring and connections cost 2 to 3%. Dust and soiling cost 2 to 5% depending on how often it rains and how often the panels are cleaned — and in dry months with heavy dust, considerably more. Panels degrade around 0.5% a year, so a system is producing about 10% less at year twenty than at year one.

Multiply those together and 80% is a fair working figure for a well-installed system. Some installers quote 85%, which is optimistic but defensible for a cool, clean, well-ventilated array. A quote implying 90% or more is describing a laboratory.

The reason this matters commercially is that the derate factor sits in the denominator. Sizing at 90% rather than 80% produces an array 11% smaller — which looks cheaper on the quotation and underdelivers for twenty-five years. When comparing quotes, the derate assumption is the number to ask about, and an installer who cannot state theirs is not one to buy from.

Worked example: a ₱6,000 monthly bill

At an all-in tariff of ₱12 per kWh, a ₱6,000 bill is 500 kWh a month.

Daily consumption: 500 ÷ 30.44 = 16.43 kWh.

Array size: 16.43 ÷ (4.8 × 0.80) = 16.43 ÷ 3.84 = 4.28 kWp.

At 550 W per panel: 4,280 ÷ 550 = 7.8 panels, which rounds up to 8. That gives an installed array of 4.4 kWp.

Roof area at 2.6 m² per panel: 8 × 2.6 = 20.8 m². In practice allow 25 to 30 m² for spacing, walkways and edge setbacks.

Expected generation: 4.4 × 4.8 × 0.80 = 16.9 kWh a day, which is 514 kWh a month and about 6,170 kWh a year. That slightly exceeds the 500 kWh consumption because of the rounding up to eight panels.

Value of that generation at ₱12/kWh: about ₱6,170 a month.

At ₱55,000 per installed kWp, the system costs around ₱242,000. Simple payback: 242,000 ÷ (6,170 × 12) = 3.3 years.

That payback figure is the optimistic reading, and it assumes every kilowatt-hour generated displaces one you would otherwise have bought. It only holds if you either use the power as it is generated or can export it at the same rate you pay — which is the assumption most worth interrogating before signing anything.

System size against consumption

Roof area figures are panel area only. Allow 25% to 40% more for spacing, access walkways, edge setbacks and any obstruction. A roof that measures large enough on paper frequently does not fit the array once shading from a water tank, an aerial or a neighbouring building is accounted for.

Net metering, batteries and what actually limits the saving

Solar generates during the day. Most households use most of their electricity in the evening. That mismatch, not the array size, is what determines whether a system delivers the saving the arithmetic promises.

Net metering is the mechanism that bridges it. Under the Philippine net metering scheme for systems up to 100 kW, exported energy is credited against your bill — but at the utility's blended generation rate, not at the retail rate you pay. Since generation is only part of a retail tariff, an exported kilowatt-hour is worth substantially less than one you consume directly. The payback calculation above quietly assumes full retail value for everything generated, which overstates the saving for a household that exports heavily.

The practical response is to shift consumption into daylight. Running the washing machine, the water heater and the pool pump at midday, and pre-cooling the house in the afternoon, converts export credits into direct offset at full retail value. Households that do this get materially better returns from the same array than households that do not.

Batteries solve the mismatch directly by storing daytime generation for evening use, and they also provide backup during outages. They also roughly double the system cost and have a service life shorter than the panels, so on pure economics they rarely pay for themselves at current prices. Where outages are frequent, the backup value may justify them on grounds that have nothing to do with payback.

Oversizing beyond your consumption is usually poor economics under net metering, because the excess earns the lower export rate. Sizing to around 80% to 100% of annual consumption is the common recommendation.

Shading deserves more attention than it gets. A single shaded panel can drag down a whole string on a system without optimisers or microinverters. Assess shading across the day and across the year, remembering that the sun sits lower in December than in June.

Assumptions and limitations

• Sizes a grid-tied array against annual consumption — battery systems need a different method
• Peak sun hours vary by location and season; the default is a national mid-range figure
• Assumes unshaded panels at a reasonable orientation and tilt
• The payback assumes every kilowatt-hour displaces one at full retail rate, which net metering does not deliver for exports
• Assumes today's tariff holds, and does not model tariff escalation
• Excludes maintenance, inverter replacement at around year twelve, and insurance
• Not financial advice — obtain site-specific quotations before committing

Questions people ask about this in Philippines