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Calculation Methodology

Every SmartEnergyPH calculator uses simple, published formulas. This page documents them, the default values they start with, where those defaults come from, and what the calculators can’t tell you.

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General principles

  • Your inputs, your results. Defaults are starting points. Every value can be changed.
  • Calculated in your browser. Nothing you type into a calculator is sent to us.
  • Full precision, rounded for display. Results are calculated without intermediate rounding and shown in Philippine pesos (₱) to two decimals, or whole pesos for larger annual figures.
  • Validated inputs. Blank, non-numeric, negative and implausibly large values are rejected with a message instead of producing a misleading result. For example, hours per day must be between 0 and 24, and appliance power can’t exceed 100,000 W.
  • Months. Where a calculator asks for days per month, it uses your number. Solar generation uses an average month of 365 ÷ 12 ≈ 30.4 days. Annual values are monthly × 12.

The example electricity rate

Calculators start with ₱15.06/kWh, rounded from ₱15.0615/kWh — Meralco’s announced overall rate for a typical household using 200 kWh a month in October 2026, as reported by The Philippine Star on 14 October 2026. This is one utility’s average rate for one consumption level in one month. Your rate depends on your distribution utility or electric cooperative, your usage and the billing month. Divide the total on your own bill by its kWh for the best estimate.

Electricity cost and appliance inventory

kWh per day   = watts ÷ 1,000 × hours per day × quantity
kWh per month = kWh per day × days per month
Monthly cost  = kWh per month × rate
Share (%)     = appliance monthly kWh ÷ total monthly kWh × 100

Limitations: assumes each appliance draws its entered power whenever it is on; real appliances often draw less on average (thermostats, standby, keep-warm). One average rate is applied to every kWh. Used by the electricity cost calculator and the appliance inventory.

Aircon electricity cost

Average power (W) = power input (W) × duty cycle (%)
kWh per month     = average power ÷ 1,000 × hours per day × days per month
Monthly cost      = kWh per month × rate

The starting power input of 900 W is purely illustrative. The duty cycle defaults to 100%, which assumes the entered power is drawn the whole time; lower it if you entered a rated rather than measured value. The aircon type selection labels your scenario only — we don’t apply any efficiency assumption for you. Horsepower is not used, because it describes a cooling-capacity class rather than electrical input.

Solar system size

Daily energy to cover = monthly kWh × share to cover ÷ (365 ÷ 12)
Battery losses        = night-time kWh × (1 ÷ round-trip efficiency − 1)
System size (kWp)     = (daily energy + battery losses) ÷ (peak sun hours × performance ratio)
Panels                = system size × 1,000 ÷ panel rating, rounded up to a whole panel
Panel area (m²)       = installed watts ÷ (panel efficiency × 1,000 W/m²)
Battery (kWh)         = night-time kWh ÷ usable share

Defaults: 100% of consumption, 550 W panels (an editable example), 21% panel efficiency, 90% usable battery share and 90% round-trip efficiency, with the same sun-hour and performance-ratio defaults as the solar savings calculator. Night-time energy above the share being covered is capped. Limitations: one average month; roof orientation, shading, structure, inverter sizing and grid-connection limits are not assessed.

Solar savings

Monthly generation = kWp × peak sun hours × performance ratio × 365 ÷ 12
Self-consumed      = min(generation × self-consumption %, monthly consumption)
Exported           = generation − self-consumed
Monthly savings    = self-consumed × rate + exported × export credit rate
                     (capped at the pre-solar energy bill)
Simple payback     = system cost ÷ annual savings

Defaults: 3 kWp, 4.7 peak sun hours, 78% performance ratio, 70% self-consumption, and an export credit of ₱0 — exports earn nothing unless you enter a value, because export compensation depends on your utility, your net-metering status and current ERC rules. Export credit above the remaining energy bill is reported but not counted, because net-metering credits are generally carried forward rather than paid out.

Limitations: monthly averages hide seasonal and weather variation; shading, orientation and tilt are only captured through the performance ratio; batteries, fixed charges, tariff tiers and subsidies are not modelled.

Solar payback

Savings (year n)    = year-one savings × (1 − degradation)^(n−1) × (1 + price change)^(n−1)
Cash flow (year n)  = savings − maintenance − replacement costs in year n
Cumulative (year n) = − installation cost + Σ cash flow (years 1…n)

Breakeven is the point after which the cumulative position stays at or above zero until the end of the period, interpolated linearly within the year. If the position is negative at the end of the period, the calculator reports no breakeven. Values are nominal pesos, without discounting, financing costs or taxes. Defaults: 25 years, 0.5% annual degradation, 0% electricity price change.

Power station runtime

Usable energy (Wh)    = rated capacity × usable capacity %
Delivered energy (Wh) = usable energy × conversion efficiency %
Runtime (hours)       = delivered energy ÷ total load (W)

Defaults: 95% usable capacity and 90% efficiency. If you enter a continuous output rating, the calculator warns when the total load exceeds it. It does not model surge loads, idle inverter consumption, temperature effects or battery ageing.

Sources for default assumptions

Peak sun hours and performance ratio

World Bank / ESMAP country data prepared by Solargis put the Philippines’ average daily global horizontal irradiation at about 4.73 kWh/m² — roughly 4.7 peak sun hours — with most of the country between about 4.25 and 5.17 (10th–90th percentile). The calculator starts at 4.7 hours. Look up your own location on the Global Solar Atlas for a better figure. An IEA PVPS study of rooftop systems in tropical Singapore reports typical performance ratios of about 75–80%, so the calculator starts at 78%. Installers’ simulation reports usually state their own figure — ask for it.

Panel degradation

The payback calculator starts at 0.5% per year, the median annual degradation rate found in NREL’s review of nearly 2,000 reported rates (Jordan & Kurtz). Use the figure in your panel’s performance warranty if you have it.

Example installed cost

The payback calculator’s hypothetical starting cost uses about ₱60,000 per kWp, the residential rooftop price Ember used in its May 2026 payback analysis. Ember does not publish a breakdown of what that price includes; real quotes vary widely with equipment, roof and location.

Power station usable capacity and AC efficiency

Defaults of 95% usable capacity and 90% conversion efficiency (≈ 0.855 combined). Several manufacturers publish a combined factor of about 0.85 for AC loads; BLUETTI describes 80–95% usable depth of discharge for LiFePO4 and 85–95% inverter efficiency.

Aircon label consumption basis

Under the DOE’s 2021 labelling guidelines for air conditioners, the estimated monthly kWh on the label is calculated as rated power input × 9 hours × 30 days × a factor of 0.8 (fixed-speed) or 0.5 (variable-speed/inverter). Our aircon calculator lets you choose your own hours and duty cycle instead.

Testing

The calculation functions are covered by automated tests, including zero, negative, decimal and very large inputs, and the pages are checked in a real browser before each release. If you find a result that looks wrong, please tell us what you entered.

Disclaimer

Results are estimates for general information. They are not a quote, a bill forecast, or financial, engineering or legal advice.