Free Home & Utilities Calculators

These calculators are for homeowners and self-builders sizing the systems in a house: solar and batteries, heating, water supply and ventilation. Enter your own figures to check capacity, payback or quantities before you buy. Everything runs in your browser and nothing is uploaded.

17 free tools 5 groups Runs in your browser Nothing uploaded

Featured Home & Utilities tools

Solar Panel Sizing CalculatorTurns appliance wattage and hours into panel size and battery bank capacity.
Heat Pump Payback CalculatorShows break-even years and annual savings from your current heating bill.
Home Battery Backup CalculatorShows how many hours of backup your appliances get from a given battery.
Rainwater Harvesting Tank SizerSizes a tank from roof area and local rainfall, with first-flush volume.

Solar & Batteries

4 tools

Panels, strings, backup hours and the cost of each battery cycle.

Van Builders / Off-Grid
Solar Panel Sizing Calculator
Add appliances with wattage and daily hours to get daily consumption, recommended panel wattage, and battery bank capacity. Days of autonomy slider.
Panel Wh + battery Ah + days autonomy
Solar Installers / DIY Solar
PV String & Array Sizing Calculator
Enter panel and inverter specs to get safe series string length, parallel strings, and temperature-corrected cold-day Voc check.
String length + Voc check + MPPT window + array kWp
Solar + Storage Homeowners
Home Battery Backup Calculator
Select your appliances, enter battery capacity, and see exactly how many hours of backup you get - plus brand comparison across Powerwall, Franklin WH, Generac, and Enphase.
Backup hours + brand comparison
Home Battery Owners
Home Battery Cost Per Cycle Calculator
Enter battery price, rated cycle life and warranty length to see the real cost per charge cycle and per warranty year.
Cost per cycle + warranty-year + usage-adjusted cost

Heating, Cooling & Air

5 tools

Heat pump and baseboard sizing, sauna heaters and ventilation.

Homeowners Switching from Gas / Oil
Heat Pump Payback Calculator
Enter your current heating bill, heat pump COP, electricity rate, and install cost to see break-even years, annual savings, and CO2 reduction. IRA credit included.
Break-even years + savings chart + CO2
Homeowners Planning a Heat Pump
Heat Pump Room Sizing Calculator
Enter each room's size, insulation, and windows to get BTU/hr, kW load, and recommended mini-split unit size per room.
Per-room BTU/kW load + mini-split sizing
Heating Pros & Retrofitters
Fin-Tube Baseboard Length Calculator
Enter a room heat load and an average water temperature and get BTU per linear foot from the manufacturer's published rating chart, the feet of element needed, whether it fits your wall, and what dropping to 140F costs in extra footage.
BTU per foot + feet needed + wall fit
Home Sauna Builders
Sauna Heater Sizing Calculator
Find the right heater size in kW or BTU for your sauna room, whether you're going electric, wood-burning, or far-infrared panels.
kW & BTU sizing · Electric / wood / IR · Warm-up time
Classrooms / Home Offices / Bedrooms
Indoor CO2 Ventilation Calculator
Enter room size, occupants, and ventilation level to estimate steady-state CO2 ppm. ASHRAE-style math made simple.
Room CO2 ppm + ventilation guidance

Water

4 tools

Rainwater, greywater, well output and plumbing pressure surges.

Homeowners & Self-Builders
Rainwater Harvesting Tank Sizer
Enter your roof area, select your city for rainfall data, choose your water uses, and get tank size, first-flush volume, and annual savings.
Tank size + monthly chart + annual savings
Water-Conscious Homeowners
Greywater System Sizing Calculator
Calculate how much greywater your household produces, the system capacity you need, and how fast a greywater reuse system pays for itself.
Daily volume · System capacity · Payback time
Well Owners / Home Buyers
Well Recovery Rate Calculator
Enter the gallons pumped out and the time your well took to recover to get GPM and GPH, a verdict against the 3 and 5 GPM residential standards, casing storage, and how long a shower can run.
GPM + yield verdict + fixture run time
Plumbers & Homeowners
Water Hammer Arrestor Sizing Calculator
Count the fixtures on a branch line and get the total fixture units, the PDI-WH 201 size letter from A to F, whether high pressure bumps it up a size, how many arrestors the run needs, and where to put them.
Fixture units + PDI size letter + placement

Kitchen Circuits & Fire Safety

2 tools

Circuit sizing for an induction cooktop and a defensible space score for a property.

Kitchen Renovators / Electricians
Induction Cooktop Amperage Calculator
Enter the cooktop's kW rating and voltage to estimate amperage, recommended breaker size, and copper wire gauge.
Breaker size + wire gauge estimator
Homeowners in Fire-Prone Areas
Wildfire Defensible Space Scorer
Score your home's wildfire defensible space across Zone 0 to Zone 2, with a ranked priority action list and CAL FIRE / IBHS compliance flags.
0-100 score · Ranked priority actions · CAL FIRE / IBHS flags

Decorating

2 tools

Paint and wallpaper quantities for a room.

Home Painters
Interior Paint Calculator
Enter wall sizes, subtract doors and windows, pick coats and finish to get gallons, liters, cans to buy, and cost. Imperial or metric.
Gallons + liters + cans + cost
Home Renovators
Wallpaper Calculator
Enter room size, doors and windows, roll dimensions and pattern repeat to get the exact number of rolls. Handles straight, drop and free match. Metric or imperial.
rolls + drops + pattern repeat + cost

Which tool do I need?

Size solar panels and a battery bank for an off-grid setupSolar Panel Sizing Calculator
Check how many panels fit in a string on an inverterPV String & Array Sizing Calculator
See how long a battery runs your homeHome Battery Backup Calculator
Work out the cost of each battery cycleHome Battery Cost Per Cycle Calculator
See whether a heat pump pays for itselfHeat Pump Payback Calculator
Choose a mini-split size for each roomHeat Pump Room Sizing Calculator
Measure how fast a well recoversWell Recovery Rate Calculator
Size a water hammer arrestorWater Hammer Arrestor Sizing Calculator
Estimate CO2 levels in a bedroom or officeIndoor CO2 Ventilation Calculator
Check the breaker and wire for an induction cooktopInduction Cooktop Amperage Calculator
Buy the right amount of paintInterior Paint Calculator
Count wallpaper rolls with a pattern repeatWallpaper Calculator

Home & Utilities FAQ

How do I size solar panels and batteries?

List each appliance with its wattage and daily hours in the solar panel sizing calculator. It returns daily consumption, a recommended panel wattage and a battery bank capacity, with a days-of-autonomy slider.

How long will a home battery last during an outage?

Select your appliances and enter the battery capacity in the home battery backup calculator. It shows the hours of backup and compares brands including Powerwall, Franklin WH, Generac and Enphase.

How do I check what a heat pump will save?

Enter your current heating bill, the heat pump COP, your electricity rate and the install cost. The payback calculator returns break-even years, annual savings and CO2 reduction.

How do I measure well recovery rate?

Enter the gallons pumped out and the time the well took to recover. The calculator returns GPM and GPH and compares the result with the 3 and 5 GPM residential standards.

How much wallpaper do I need?

Enter the room size, doors and windows, roll dimensions and pattern repeat in the wallpaper calculator. It returns the number of rolls and handles straight, drop and free match patterns.

Can these calculators replace an installer's survey?

No. They estimate from the figures you enter. A qualified installer or electrician should confirm sizing and safety for your own property.

Last updated: September 30, 2026