How big a system do I actually need?

Each appliance: quantity × watts × hours × how much of an "on" hour it actually draws. A fridge is rated 200 W but its compressor runs about 40% of the time, so it uses about 1.4 units a day, not 4.8. Sun hours are 9 am to 4 pm; everything else — early morning, evening, night — is battery time.

An inverter AC is the exception: it runs flat out until the room is cool, then settles to roughly 45% of its rating. We count the first hour at full power and the rest at 45%, so an eight-hour night on a 1.5 ton AC is about 5.4 units, not 10.

Lights, fans, TV, fridge and the like run together, so they're added up. Heavy loads are split: the AC and the induction hob coincide (evening); the pump, geyser, washing machine, iron and microwave usually don't, so we take the largest of them rather than all of them. The inverter must still run the pump with the lights on — so the peak is never less than light loads plus your single biggest heavy load.

Switch to everything at once if you really do run it all together. It's usually a size bigger, and it costs idle draw every day.

Continuous rating ≥ peak × 1.25, then rounded up to a PCU we fit: 3 kVA (2.4 kW continuous), 5.2 kVA (4.2 kW), 6 kVA (4.8 kW), 10 kVA (8 kW), or two 10 kVA units in parallel (16 kW). A kVA label is not watts — a 10 kVA unit runs 8 kW. Motors start at two to three times their running load, so we also check that the biggest start-up — on top of everything else that's on — fits inside the inverter's short surge, which is about twice its rating.

An inverter burns roughly 7 W per kW of rating just being on. A 10 kVA unit idles at about 1.3 units a day — the real cost of oversizing.

Recommended, with KSEB: the units you use between 6 pm and midnight ÷ (60% usable × 89% path efficiency). Usable is 60% because the inverter hands over to KSEB at 40% — good for the battery's life, and you never notice. Past midnight you are on KSEB anyway; the panels refill the battery next morning, and you pay KSEB only for the night units you actually drew. A bigger battery than this only buys you cover for a cut that lasts all night. Without KSEB: two full nights ÷ (80% × 89%).

Bare minimum: the first three hours after sunset at 80% depth. It gets you through dinner on the evening of a cut and nothing more.

One more rule: LiFePO4 is usually rated to discharge at about its own capacity per hour, so an 8 kW inverter needs at least 8 kWh behind it or it can't deliver full power. Shown as kWh and as Ah at the bank voltage that PCU uses — 24 V for 3 kVA, 48 V for 5.2 and 6 kVA, 96 V for 10 kVA.

Daily need = sun-hour units + night units ÷ 0.89 (they go through the battery) + the inverter's idle draw. Divide by 4 units per kW per day — the yearly average our own roofs make — and round up to 550 W panels.

Tick rainy months for 20% more: the monsoon average is nearer 3.3 units per kW. On the darkest days a roof makes about half, and that is what KSEB, or the two-night reserve, is for. Panels are the cheapest part of the system to oversize; the battery is the dearest.

How long your AC compressor really runs. Whether your roof fits the panels. How hot the room is where the battery will live. Three dark days in a row. Three-phase motors. All of those are the survey's job — this page gets you to the right size range and the right conversation.