Where the number comes from

Your daily usage (monthly ÷ 30) divided by peak sun hours gives the "ideal" system size with zero losses. Real systems lose energy to inverter conversion, wiring resistance, heat and dust — typically about 20% — so the system needs to be bigger than the ideal number to actually deliver what you use. That's the number this tool solves for, then rounds up to a whole number of panels.

Getting your peak sun hours right

This is the single input that changes the answer the most, and it's genuinely regional — sunny desert climates can see 6-7 peak sun hours, while cloudier northern regions might see 3-4. The default here is a conservative middle estimate; check your utility company or a local installer for your actual number before treating this as final.

Nameplate capacity is not usable capacity

A battery you can't fully drain and can't charge and discharge without loss doesn't give you its full rated capacity in practice. Depth of discharge (how far down you can safely drain it — often 80% for lithium) and round-trip efficiency (energy lost converting to and from storage — often around 90%) both shrink the usable number. This tool works backward from the energy you actually need to the installed capacity that delivers it.

The wall doesn't equal the battery

Charging isn't perfectly efficient — some energy is lost as heat in the charger and battery during the process, typically around 10%. That means the electricity you actually pay for is a bit more than what the car's efficiency rating alone would suggest, which is the gap this calculator accounts for.

Wattage × hours is the whole formula

Every electrical device's running cost comes down to the same arithmetic: watts tell you the rate of energy use, multiply by hours to get energy, multiply by your rate to get cost. The wattage is usually printed on the device itself or its power supply — look for a label with "W" on it.

A single bill number tells you nothing on its own

$150 a month means something completely different for a small efficient apartment than for a large drafty house. Dividing your annual usage by your home's floor area gives a number you can actually compare against typical homes of the same general efficiency category — that comparison is what tells you whether there's real room to save, or whether you're already doing fine.

COP is the whole story

A heat pump's Coefficient of Performance (COP) tells you how many units of heat it delivers per unit of electricity — a COP of 3 means 3x more heat than a plain electric resistance heater for the same electricity. Comparing against gas or oil means first converting your fuel spend into the actual heat energy it delivered, using each fuel's real energy content and typical furnace efficiency, so the comparison is apples to apples.

EER converts BTU into watts

An air conditioner's Energy Efficiency Ratio (EER) is BTU of cooling per watt of electricity — a higher EER means less electricity for the same cooling output. Dividing the unit's BTU rating by its EER gives the actual watts it draws while running, which is the number the rest of the cost calculation is built on.

Two paybacks, and the honest one is shorter

The simple payback — system cost divided by the first year's savings — is the number most people quote, but it quietly assumes electricity never gets more expensive. It almost always does. Accounting for a modest, realistic annual price increase (electricity has historically outpaced general inflation in most markets) means each future year saves you more than the last, which pulls the true break-even point earlier.

Grid intensity is the number that varies most

Every kWh of solar you produce avoids generating that same kWh from your local grid mix — and grids vary enormously in how carbon-intensive they are, from coal-heavy grids well above 0.6 kg CO2/kWh to hydro- or nuclear-heavy grids under 0.1. The default here is a rough global average; your local grid operator or national energy agency usually publishes the real figure for your area.

Where "annual production" comes from

If you've already sized a system with the Solar Panel Calculator, its annual production number plugs directly in here.

The core formulas

Constants used

A note on accuracy

These are standard, published formulas and typical figures — not a substitute for a proper site survey, a licensed electrician, or a professional energy audit. Peak sun hours, grid carbon intensity, and local energy prices vary enormously by location, and every default here is editable so you can substitute your own numbers.