Most solar systems are sized by rule of thumb: "a 5 kW system for a house with two air conditioners". Some of those work. The ones that do not either never fill their batteries in December, or trip the inverter every afternoon, or export power they could have stored. The method below takes an hour with a bill and a solar atlas, and it is what our engineers do before quoting any system.
Step 1: how much energy, and when
Start from the electricity bill. The monthly units (kWh) divided by 30 is the daily energy. Take twelve months if you can: a Lahore house that uses 250 units in January and 900 in July is two different sizing problems, and the answer depends on what you want solar to do.
- Grid-tied with net metering or net billing: size for the annual average, since summer surplus is credited against winter shortfall (subject to the current NEPRA rules and your distribution company's terms, which have been changing; confirm the buy-back arrangement before relying on it).
- Backup during load-shedding: size the array for average use and the battery for the outage hours you actually get, at the loads you actually need during them.
- Off-grid: size for the worst month, on both the load and the sun.
Separate the peak load from the energy. A house may use 10 kWh a day but draw 4 kW at 3 pm when the air conditioner, the water pump and the iron coincide. The energy sizes the array and battery; the peak sizes the inverter.
Step 2: peak sun hours for your site
Peak sun hours (PSH) is the day's solar energy on the panel plane expressed as hours of full 1 000 W/m² sun. It is the same number as daily irradiation in kWh/m². The Global Solar Atlas and NASA POWER give it by month for any location. Across the populated plains of Pakistan the annual average on a tilted plane is about 5 kWh/m²/day, with the north and the monsoon months lower and Balochistan and interior Sindh higher; a December figure of 3.5–4 and a May figure of 6–7 are typical for Lahore.
Use the annual average for grid-tied, the worst month for off-grid. Tilt the array at roughly the latitude (about 30° for Lahore, 25° for Karachi) facing south for the best annual yield; a flatter tilt favours summer, a steeper one winter, which can be the right choice for a system that struggles in December.
Step 3: the losses
A "550 W" panel delivers 550 W at 25 °C cell temperature and 1 000 W/m², which is a laboratory condition. On a roof in June the cells run at 60–70 °C and output falls by about 0.35 % per degree above 25 °C, so 12–15 % is lost to temperature alone before dust, wiring, mismatch and the inverter take their share. Together the losses are usually 15–25 %; the fraction that survives is the performance ratio, and 0.80 is a reasonable planning figure for a clean, well-ventilated array.
The array size follows directly:
For 10 kWh a day, 5 peak sun hours and a performance ratio of 0.8: 2.5 kWp, which is five 550 W panels. The solar sizing calculator does this arithmetic and the battery and inverter steps below.
Step 4: the battery, if there is one
Decide how many days, or hours, the battery must carry the load without sun: a few evening hours for load-shedding backup, one to two days for off-grid. Then:
Depth of discharge is the fraction of the battery you allow yourself to use: 80–90 % for lithium iron phosphate, 50 % for lead-acid if it is to last. One day of 10 kWh at 80 % and 95 % is 13.2 kWh, or 274 Ah at 48 V. Choose 48 V for anything above about 2 kW: the currents at 12 V are unmanageable (a 3 kW load at 12 V is 250 A).
Then check power, not just energy: the bank must deliver the peak load. A 274 Ah lithium bank rated for 0.5 C discharge can supply 137 A, about 6.5 kW at 48 V, which is fine; a lead-acid bank of the same Ah would be unhappy above 0.2 C. The battery runtime calculator reports the C-rate.
Step 5: the inverter
The inverter's continuous rating must exceed the peak AC load with margin for motor starting, so 1.25 × peak is the usual minimum, and its surge rating must cover the largest motor. A 3 kW peak asks for a 4 or 5 kW hybrid inverter.
The array can be larger than the inverter. Because panels seldom reach their rating, an array 10–30 % bigger than the inverter's AC rating (a DC:AC ratio of 1.1 to 1.3) makes fuller use of it through the day at the cost of clipping a little energy at noon in spring. Stay within the inverter's maximum DC input voltage (check the panel open-circuit voltage at the coldest morning, when it is highest) and its maximum MPPT current.
Step 6: cables and protection
DC cabling is where cheap installations lose energy quietly. A 2.75 kWp array as one string of five panels runs at about 200 V and 13 A, so 4 mm² solar cable over a 20 m run drops a little over 1 %. A 12 V battery-to-inverter run at 250 A is a different animal: it needs 70 mm² or more over even 2 m, which is one more argument for 48 V. Size every DC run with the voltage drop calculator and keep battery cables as short as physically possible.
Fit DC isolators and fuses rated for DC (an AC breaker will not clear a DC arc), surge protection on the DC and AC sides in a lightning-prone area, and earth the array frames. For grid-tied systems the inverter must have anti-islanding protection and the connection must go through the distribution company's approval process.
The worked example, all together
A house using 300 units a month with a 3 kW peak, wanting to cover its usage and to ride through evening load-shedding:
- Energy: 10 kWh/day. Peak: 3 kW.
- Sun: 5 PSH annual average (Lahore, 30° tilt, south).
- Array: 10 ÷ (5 × 0.8) = 2.5 kWp → five 550 W panels, 2.75 kWp, about 11 kWh on an average day and 4 000 kWh a year.
- Battery: one day's autonomy at 80 % DoD → 13.2 kWh, 274 Ah at 48 V; two 48 V 150 Ah LiFePO4 units.
- Inverter: 5 kW hybrid, surge 10 kW; the array-to-inverter ratio of 0.55 leaves room for five more panels later.
- Cables: 4 mm² DC string cable, 35 mm² battery cables under 1.5 m, 6 mm² AC output.
Costed honestly, this is a modest system that will do what it is asked. The same house with two 1.5-ton fixed-speed air conditioners running through the night would need roughly three times the battery and a larger inverter, and that is the conversation to have before anyone climbs on the roof.
Mistakes we see most often
- Sizing on nameplate watts with no losses, then wondering why a 5 kW system produces 3.5 kW in June.
- Mixing panel models or orientations in one string; the string runs at the weakest panel's current.
- Lead-acid batteries cycled to 80 % daily; they last a year.
- Battery cables too thin or too long; the inverter trips on low voltage at every surge though the bank is full.
- No shade survey: a water tank or parapet shading one panel for two hours cuts a whole string.
- Assuming export credit at the retail rate; check the current net-metering or net-billing terms before building the business case.
References
- World Bank Group / Solargis, Global Solar Atlas — monthly irradiation and PV yield estimates by location
- NASA POWER, Prediction Of Worldwide Energy Resources — irradiation data by coordinates
- IEC 61724-1:2021, Photovoltaic system performance — Part 1: Monitoring — performance ratio
- IEC 62548-1:2023, Photovoltaic (PV) arrays — Part 1: Design requirements
- NEPRA, Alternative & Renewable Energy Distributed Generation and Net Metering Regulations, 2015 (as amended)