About SmartSolarSizer

Most solar calculators ask for one number — your monthly bill — and hand back a system size with no way to see how it was arrived at. That's fine if all you have is a bill. But if you're starting from "I want to run my AC and fridge on solar," working backward from a bill figure you don't have yet isn't very useful.

SmartSolarSizer starts from wherever you actually are. Know exactly what's running in your house? Add it appliance by appliance and get an inverter and array size built from real running and surge loads. Only have your electricity bill? Use that instead. Either way, you get the same depth of result: panel count, inverter rating, battery size, upfront cost, and a real year-by-year payback projection — not just a single ballpark figure.

Who we are

SmartSolarSizer is an independent project run by the SmartSolarSizer team. We're self-taught researchers, not installers or engineers. We studied how installers size panels, inverters and batteries, and how payback is worked out, then built a calculator that shows every step so you can check it yourself.

The guides on this site are written by the same team. They explain the formulas the calculator uses, in plain English.

How the calculator works

Every result comes from a short, standard formula. These are the main ones:

• Solar array size. Your daily energy use divided by your peak sun hours, then by 0.8 to allow about 20% for panel, wiring and inverter losses.array kW = daily kWh ÷ sun hours ÷ 0.8
• Inverter size. From appliances: the start-up (surge) load plus 25% headroom. From a bill: the array size plus 15%.
• Battery size. The energy you need for your chosen backup hours, divided by the share of the battery you can safely use and its efficiency (90% and 90% for lithium iron phosphate, 50% and 80% for lead-acid).battery kWh = (daily kWh ÷ 24 × backup hours) ÷ (usable share × efficiency)
• Payback period. Savings are worked out year by year for 25 years. Output drops each year for panel ageing (0.5% by default) and the value of each unit rises with electricity prices (5% a year by default). Payback is the first year your total savings cover the system cost. You can change both rates.

Where our figures come from

We use official data first. Where no official figure exists, we use reputable industry or news sources and say so. Each source is listed in the calculator next to the number it sets, with the date we checked it.

• Sun hours: PVGIS, the European Commission's solar data service. It covers most of the world, and we use it to set sun hours for the visitor's country.
• Appliance energy: ENERGY STAR certified product data for fridges, freezers, dishwashers, dryers, pool pumps and heat pumps, plus US Department of Energy, university extension and UK energy charity guides for running and start-up watts.
• Electricity prices: national statistics and regulators, such as Eurostat for Europe, the US Energy Information Administration and Ofgem in the UK.
• Installed system cost: published cost studies, such as Berkeley Lab in the US and the UK government's installation data, and market price reports where no study exists.

These are starting values. Your own bill, roof and installer quote will always be more accurate, so every input in the calculator can be changed.

Our editorial standards

• Independent. No company pays to be mentioned or ranked in our calculator or guides.
• Clearly labelled. The site may show ads to stay free. If we ever earn a commission from a product link, we'll label that link.
• Dated. Guides show the date they were published, and the date they were updated if we change them.
• Corrected. If you find a wrong figure or a broken formula, tell us. We check every report and fix confirmed errors.

What we're not

We're not a solar installer, and we don't sell equipment. SmartSolarSizer doesn't install anything or pass your details to installers. Every result on this site is a planning-stage estimate, not an engineering design; see our Terms of Service for the full disclaimer.