1. Start with energy, not panel count
When a user starts from a monthly bill, CalcSolarPH converts the bill to estimated monthly energy using the electricity rate entered or selected in the calculator. Daily energy is the monthly estimate divided by 30. When an appliance profile is supplied, that load profile can take precedence over the bill-derived daily energy.
Daily kWh = Monthly kWh ÷ 30
2. Use location-aware solar-resource data
The calculator uses the regional dataset published on our Philippine Solar Resource Data page. It is based on PVGIS-ERA5 modelling with a stated 14% PV-system loss assumption. These are modelled planning inputs, not measurements from a particular roof.
After the energy-derived base size, bill-based recommendations apply a declared panel sizing allowance for practical headroom: +50% up to 4 kWp base size, +45% from above 4 to 8 kWp, and +40% above 8 kWp. This is deliberately shown in the calculator result; it is not an additional hidden system-loss factor.
The calculator's target bill-energy offset (default 80%) is a real bill-based sizing input. It scales the energy target before the declared practical panel allowance is applied. For example, an 80% target asks the calculator to size for roughly 80% of the estimated energy use, while the panel allowance then adds practical headroom. The target is a planning scenario, not a guarantee that the utility bill will fall by exactly the same percentage.
Shade, roof orientation and tilt, module temperature, soiling, inverter clipping, downtime and weather can move actual production away from the model.
3. Inverter sizing is separate from panel sizing, but they must agree
CalcSolarPH sizes the inverter from the higher of the practical recommended PV array and the relevant load requirement, then applies the calculator's stated load/headroom rule and available inverter tiers. Whole-panel rounding alone should not automatically force a larger inverter tier, but a deliberately larger panel array should be treated as the actual installed array. Motors, pumps and refrigeration loads need extra care because startup demand can be much higher than running demand.
The result is a planning recommendation, not a final equipment approval. Final selection must verify continuous AC output, surge capability, battery voltage/current, MPPT voltage/current, PV DC limits, phase requirements, grid-interconnection rules and the exact manufacturer's documentation.
4. Battery sizing follows the night-load requirement
Battery storage defaults to a 1-night minimum backup target for hybrid/off-grid planning. The visitor can increase this to 2 nights or explicitly choose no battery backup for a grid-tied/no-backup design. For bill-based planning, the calculator estimates the overnight energy share (currently 48% of average daily consumption); for appliance-based planning, selected night appliances can replace that assumption. Once backup duration is selected, required battery energy is adjusted for usable depth of discharge and the battery/inverter path-efficiency assumption.
This produces a planning target. It is not a substitute for checking the inverter's voltage/current limits, battery BMS limits, temperature requirements, wiring and protection.
5. Cost and payback are estimates
The calculator combines current planning assumptions for panels, inverter, battery, balance-of-system hardware and installation labour. The inverter figure specifically is anchored to this site's own product catalog: each inverter size tier the calculator can recommend is priced from the average of currently-listed marketplace prices for that tier where a verified listing exists, rather than a single flat rate per kW. Real inverter pricing shows economies of scale: a 6.5kW unit does not cost roughly twice a 3kW unit despite having roughly twice the capacity, so a flat per-kW rate meaningfully overstated mid-to-large systems in earlier versions of this calculator. A small number of larger, less commonly stocked tiers still use an estimated figure where the catalog does not yet carry a fully seller-verified listing; these are marked as such in the underlying pricing table and are revisited as new listings are added. Prices vary by brand, seller, location, freight, roof complexity and scope, and marketplace prices change over time. Treat the result as a current planning estimate, not a quote.
Simple payback uses a bill-offset assumption. The longer-term view includes assumptions such as electricity-rate growth and panel degradation. These outputs are planning scenarios, not financial promises.
6. Our evidence standard
We distinguish between source data, calculator assumptions, editorial interpretation, and real-world measurements. We do not label a modelled number as a field measurement. When a regulation is proposed rather than in force, we label it that way.
- Source data is named where practical.
- Material assumptions are shown instead of hidden.
- Prices and regulatory claims are treated as time-sensitive.
- Safety-sensitive advice includes limits and professional handoff points.
Method version
CalcSolarPH Planning Method v1.3, reviewed October 13, 2026. This revision anchors inverter hardware cost to the site's own verified product catalog instead of a flat ₱/kW rate (see section 5), and updates the documented default target bill-energy offset from 60% to 80%. Material formula or assumption changes will be recorded here.