Free All-in-One Solar Calculator

Solar Panel, Battery & Inverter Calculator

One calculator for every solar question: plan a home system from your appliances or bill, see what a 1 to 20 kW system will run, or size solar for a factory, shop or farm. Panels, battery, inverter, kWh per day, space and payback for the US, UK, Canada, Australia, Germany and every other country.

1 Where will the panels be?

Sun hours for every country →
Off-grid systems should use the worst month
h/day
Edit if you know your exact value
°
Negative in the southern hemisphere

2 How much electricity do you use?

Start from:
ApplianceWattsHours/dayQtyWh/day

Fridges, freezers and ACs cycle on and off; their daily energy already accounts for that.

3 What kind of system?

W
h
W
%
Add your local prices for cost and payback (optional)

Prices vary widely by country and change often, so we never guess them. Use quotes from local installers.

Your solar system plan

Includes 20% system losses, 10% inverter loss, 85% usable LiFePO4 / 50% lead-acid. Planning estimates: have a licensed installer confirm the final design.

What SolarWattPro is for

SolarWattPro is a free, independent planning site for solar power. It is built for homeowners, businesses and farmers in the United States, the United Kingdom, Canada, Australia and Germany: how much solar, battery and inverter they need, how many kWh a system will produce, what it will run, and whether it pays for itself under current local incentives, before they speak to an installer. The calculators work for every other country too.

We do not sell panels, batteries or installations, and no manufacturer pays to be recommended. The site is funded by advertising, which never influences a calculation or a guide. Every formula is shown, so you can check the numbers yourself.

How the calculator works

The calculator at the top of this page has three modes. Choose the one that matches your question.

1. Plan my system

You enter: your country, then either a list of appliances, your monthly electricity bill or your daily kWh, and the type of system (grid-tied, hybrid or off-grid).

You get: the solar array size and number of panels, yearly production, roof area, battery bank in kWh and amp-hours, charge controller, inverter with surge rating, best panel angle, and cost and payback from your own prices.

2. What will my system run?

You enter: a system size such as 1, 3, 5 or 10 kW, your country and, optionally, a battery and your price per unit.

You get: kWh per day, month and year, the worst month, what it runs (lighting, kitchen and laundry appliances, AC or heat pump, water heating and EV charging), how many of each appliance alone, and battery backup hours.

3. Business, industry and farms

You enter: one of 17 business types (factory, mill, office, shop, school, hospital, hotel, cold storage, farm irrigation, poultry or dairy farm, and more), a size from 10 kW to 1 MW, working days and, optionally, your bill and prices.

You get: production, the machines and equipment it runs, panels, three-phase inverters, roof or ground space, CO₂ avoided, how much of your bill it covers, and payback.

How the numbers are calculated

All calculators and guides on the site use the same formulas, so their answers always agree:

Solar productionkWh per day = System kW × Peak sun hours × 0.80

The 0.80 covers real-world losses: hot panels, dust, wiring and the inverter.

Solar array sizeArray watts = Daily watt-hours ÷ (Peak sun hours × 0.80)
Battery bankBattery Ah = (Daily Wh × Days of backup) ÷ (Voltage × Depth of discharge)

LiFePO4 lithium uses 85% of its capacity; lead-acid, AGM and tubular batteries 50%.

Charge controllerController amps = (Array watts ÷ Battery voltage) × 1.25

Where our data comes from, and its limits

  • Peak sun hours for each country are rounded national averages for planning. Sun varies within a country: for an exact site, check the Global Solar Atlas, PVGIS or NREL PVWatts.
  • Appliance and equipment wattages are typical values. Your own appliance label or a plug-in energy meter is always more accurate.
  • We never guess prices. Costs, savings and payback only appear when you enter your own local prices.
  • Results are planning estimates. Electrical work must be designed and installed by a qualified, licensed installer following your local codes.

Find everything on the site

Who is behind SolarWattPro

Yasir Hafeez

SolarWattPro is founded and edited by Yasir Hafeez, who builds the calculators and edits and checks every guide. Our editorial policy explains exactly how guides are researched, drafted and reviewed, and they are updated when products, prices or rules change.

About us · Editorial policy · Electrical disclaimer · Contact us

Your privacy

Every calculator runs entirely in your browser. The numbers you type are never sent to us or stored anywhere. The site uses Google AdSense for advertising, which may use cookies. Read our privacy policy.

Frequently asked questions

Is SolarWattPro free to use?

Yes. Every calculator and guide is free, with no sign-up. The site is supported by advertising, which never affects the results.

Which countries does the calculator work for?

It is built for the United States, the United Kingdom, Canada, Australia and Germany, with a detailed guide for each, and it works for every other country too. Choose your country and the calculator uses its average and worst-month peak sun hours, or type the exact sun hours for your city.

How accurate are the results?

They are planning estimates using the same formulas professional installers start from, with 20% real-world losses included. A qualified installer should confirm the final design for your roof, wiring and local rules.

What is a peak sun hour?

One hour of sunshine at the full strength of 1,000 watts per square metre. A location with 5 peak sun hours gets the same solar energy in a day as 5 hours of full sun, even though the sun is up for longer.

How many kWh does 1 kW of solar produce?

About 4 kWh a day at 5 peak sun hours, or roughly 1,460 kWh a year. On average that is about 4.0 kWh a day in the United States, 4.8 in Australia, 3.0 in Canada, 2.6 in Germany and 2.3 in the United Kingdom.

Can I use it for a factory, shop or farm?

Yes. Choose the Business, industry and farms mode and pick from 17 types of business, from factories and offices to farm irrigation pumps, cold storage and poultry farms.

Do you store the numbers I enter?

No. The calculators run in your browser and nothing you type is sent to us or saved.