List what you actually run, pick your country, and get the array, inverter and battery you need — with the arithmetic shown, so you can check every number in the quote you're given.
Monthly output appears once the system is sized.
Assumes you use 80% of what you generate. Install cost estimated from typical local pricing.
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Every figure on this page comes from four steps. None of them is proprietary — you can check all of it on paper, and you should, because a calculator that won't show its arithmetic is asking you to trust it for no reason.
Each appliance contributes wattage × quantity × hours per day. Fridges and freezers are the exception: their compressors cycle rather than run continuously, so SunWatt applies a 35% duty factor. A 150 W fridge quoted at 24 hours contributes about 1.26 kWh a day, not 3.6 kWh. Calculators that skip this routinely oversize systems by a third.
Peak sun hours is the number of hours a day your location receives the equivalent of full 1,000 W/m² sunlight. Multiply it by a performance ratio of 0.80 — covering heat derating, dust, wiring resistance, mismatch and inverter conversion — and you have effective sun hours. Divide your daily consumption by that.
The inverter has to survive the worst instant, not the average hour. That instant is every running load already on, plus the inrush of the single largest motor starting. Motors are assumed not to start simultaneously, which is realistic in a home and conservative enough in practice. Ten percent headroom is added, then the result is rounded up to a size that is actually sold.
Array kWp divided by inverter kW is the DC-to-AC ratio. Between 1.0 and 1.3 is normal and mild oversizing genuinely improves morning, evening and winter yield. Above roughly 1.3 the inverter clips midday output and several manufacturers treat it as outside warranty. SunWatt flags both ends of that band.
Peak sun hour figures are national or major-city annual averages, consistent with published irradiance datasets such as the World Bank and ESMAP Global Solar Atlas, the European Commission's PVGIS, and NREL's NSRDB. Monthly shape factors follow the seasonal irradiance profile for each region, which is why a British system shows a summer peak roughly three times its December output while a Gulf system stays comparatively flat.
The 0.80 performance ratio is a widely used industry default for a well-installed, unshaded system. Installed cost per kW reflects typical residential pricing bands by market and moves faster than any published table — treat it as an order of magnitude, not a quote. Grid emission factors are national averages, so a coal-heavy grid shows far larger CO₂ savings per kWh than a nuclear or hydro one.
What a calculator cannot see: shade from a neighbouring building or a single tree, roof pitch and azimuth, available roof area, structural load capacity, local export and net-metering rules, permitting, and the condition of your existing wiring. Any of these can move the real answer by more than the calculation itself. Expect to land within 10–15% of a competent site survey on an unshaded roof at a sensible tilt, and further out than that if any of the above applies. This tool is for arriving at an installer meeting informed. It is not a design document.
Two ceiling fans, six LED bulbs, a fridge, a 1.5 ton inverter air conditioner running eight hours. Running load about 1,750 W; daily consumption about 14.6 kWh. At Pakistan's 5.2 peak sun hours that is 14.6 ÷ 4.16 ≈ 3.5 kWp, or six 600 W panels. The air conditioner surges to roughly 4,900 W on start, so the inverter is sized on 1,750 + 3,500 = 5,250 W plus headroom: a 6 kW unit, not the 4 kW the array size alone would suggest.
Three air conditioners, two fridges, a water pump, standard lighting and electronics. Around 5,900 W running and 47 kWh a day. At 6.2 peak sun hours: 47 ÷ 4.96 ≈ 9.5 kWp, sixteen 600 W panels. Pairing that with a 10 kW inverter gives a 0.95:1 ratio — comfortably inside the band, and with room for a fourth air conditioner later.
No air conditioning, a fridge freezer, lighting, a washing machine, a television and a laptop. About 8.5 kWh a day. The UK's 2.8 peak sun hours give an effective 2.24, so 8.5 ÷ 2.24 ≈ 3.8 kWp — seven 550 W panels and a 4 kW inverter. The catch is seasonal: that array produces roughly 17 kWh a day in June and under 5 kWh in December, which is why a UK battery is sized for evening shifting rather than winter autonomy.
If SunWatt reports 1.42:1, the array will produce more DC power at midday than the inverter can convert, and the excess is simply lost. If it reports 0.55:1, you have paid for inverter capacity you will never use. Both are common in quotes — the first because panels are cheap and inverters are not, the second because an installer standardised on one inverter model.
Typical figures used by this calculator. Nameplate ratings vary by brand, age and efficiency class, so check the label on your own unit where you can — every row here is editable in the tool above.
| Appliance | Running W | Surge × | Peak W | Typical h/day | kWh/day |
|---|---|---|---|---|---|
| Air conditioner, 1.0 ton inverter | 1,000 | 3.5 | 3,500 | 8 | 8.0 |
| Air conditioner, 1.5 ton inverter | 1,400 | 3.5 | 4,900 | 8 | 11.2 |
| Air conditioner, 2.0 ton inverter | 1,800 | 3.5 | 6,300 | 8 | 14.4 |
| Refrigerator, 200–350 L | 150 | 2.5 | 375 | 24 | 1.3 |
| Deep freezer | 200 | 2.5 | 500 | 24 | 1.7 |
| Ceiling fan | 75 | 1.2 | 90 | 12 | 0.9 |
| Pedestal fan | 55 | 1.2 | 66 | 8 | 0.4 |
| LED bulb | 10 | 1.0 | 10 | 6 | 0.06 |
| Fluorescent tube | 40 | 1.5 | 60 | 6 | 0.24 |
| Television, 40–55 inch LED | 120 | 1.1 | 132 | 5 | 0.6 |
| Television, 65 inch and above | 200 | 1.1 | 220 | 5 | 1.0 |
| Laptop | 65 | 1.0 | 65 | 6 | 0.4 |
| Desktop computer | 250 | 1.5 | 375 | 6 | 1.5 |
| Router and modem | 20 | 1.0 | 20 | 24 | 0.5 |
| Water pump, 0.5 HP | 375 | 4.0 | 1,500 | 2 | 0.8 |
| Water pump, 1 HP | 750 | 4.0 | 3,000 | 2 | 1.5 |
| Water pump, 2 HP | 1,500 | 4.0 | 6,000 | 2 | 3.0 |
| Microwave oven | 1,000 | 1.2 | 1,200 | 0.5 | 0.5 |
| Electric kettle | 1,500 | 1.1 | 1,650 | 0.5 | 0.75 |
| Clothes iron | 1,000 | 1.1 | 1,100 | 1 | 1.0 |
| Washing machine | 500 | 3.0 | 1,500 | 1 | 0.5 |
| Dishwasher | 1,200 | 2.0 | 2,400 | 1 | 1.2 |
| Water heater or geyser | 2,000 | 1.2 | 2,400 | 1 | 2.0 |
| Electric oven | 2,000 | 1.2 | 2,400 | 1 | 2.0 |
| EV charger, Level 1 | 1,440 | 1.2 | 1,728 | 6 | 8.6 |
| EV charger, Level 2 | 7,200 | 1.2 | 8,640 | 4 | 28.8 |
| Server or NAS | 350 | 2.0 | 700 | 24 | 8.4 |
| Point of sale terminal | 100 | 1.2 | 120 | 10 | 1.0 |
| Security cameras, set of four | 40 | 1.0 | 40 | 24 | 1.0 |
| Treadmill | 600 | 2.5 | 1,500 | 1 | 0.6 |
Annual daily averages. Multiply by 0.80 to get the effective sun hours used in sizing, then divide your daily kWh by that to get array size in kWp.
| Country | Region | Peak sun h/day | Effective h/day | kWh/yr per kWp |
|---|
Divide your daily electricity use in kWh by your location's effective sun hours — peak sun hours multiplied by about 0.80 — to get array size in kW, then divide by your panel wattage for the count. A home using 25 kWh a day at 5.2 peak sun hours needs roughly 6 kW, which is ten 600 W panels. Start from your last twelve electricity bills if you have them; they are more reliable than any appliance list.
The inverter must carry your continuous running load plus the startup surge of the single largest motor, because motors rarely start together. Add the two, add 10% headroom, then round up to a size sold in your market. A 3,000 W running load with a 1.5 ton air conditioner surging an extra 3,500 W needs roughly 7 kW — well above what the array size alone would suggest.
DC array size in kWp divided by inverter AC rating in kW. Most manufacturers permit 1.0:1 through 1.3:1, and slight oversizing genuinely raises morning, evening and winter output. Past about 1.3:1 the inverter clips midday peaks and many warranties stop applying, so check the datasheet for your specific model rather than relying on the general rule.
Multiply nameplate capacity by usable depth of discharge — roughly 90% for lithium iron phosphate, 50% for sealed lead acid, 40% for flooded — then divide by your night load in kW. A 10 kWh lithium battery running a 400 W night load gives about 22 hours. Add the air conditioner and that same battery gives under seven.
A 5 kW array is a peak DC rating measured under laboratory conditions at 25°C. Heat, dust, wiring resistance and inverter conversion typically remove about 20%, and panels only approach peak output for a few hours around midday. Continuous load should stay near 80% of the inverter rating, which is a different and usually smaller number than the panel rating.
Installed cost per kW varies enormously by market: roughly 650–750 US dollars per kW in Pakistan and India, 850–1,050 across the Gulf and much of Africa, and 1,300–2,100 in North America, Europe, Australia and Japan, where labour, permitting and compliance dominate. Batteries are normally quoted separately and frequently cost as much as the array.
Only if you need power when the grid is down or the sun isn't up. Where net metering credits exports at a fair rate, a grid-tied system without a battery usually pays back faster. Where outages are routine or export credit is poor, the battery is what makes the system useful rather than merely economic — a different question from whether it pays for itself.
On an unshaded roof at a sensible tilt, a calculation using national average irradiance and an 0.80 performance ratio typically lands within 10–15% of a professional assessment. It cannot see shade from a neighbouring building, roof orientation, structural limits or local export rules, any of which can move the answer further than the arithmetic itself. Use it to check a quote, not to replace a survey.