What size solar system do you need?
Free sizing calculators for solar and off-grid builds: system size, battery bank, DC cable and voltage drop, PV fuses and running costs. Built for rooftops, RVs, boats, cabins and tiny homes.
Sizing a system, in order
Every solar build answers the same three questions in the same sequence. Get the first one wrong and the other two are wrong with it.
How much energy do you use?
Everything downstream is scaled from your daily kilowatt-hours. Take it off a bill, or add up every appliance one at a time.
Add up your loads →How big is the array and the bank?
Peak sun hours and system losses turn that load into kilowatts of panel; days of autonomy turn it into amp-hours of battery.
Size the system → Size the bank →What cable and protection?
Low-voltage DC is unforgiving. Pick the controller and inverter, size the cable for the drop rather than the current, and fuse every leg.
Size the controller → Size the cable →Solar calculators
Single-purpose tools that give you a number, not a sales funnel. Metric and imperial throughout, with the formula and a worked example on every page.
Solar System Size Calculator
What size solar system you need in kW, from daily kWh use, peak sun hours and system losses
Open calculatorPlug-in Solar Savings Calculator
What a UK plug-in or balcony solar kit saves a year, including the generation you spill to the grid unpaid
Open calculatorUK Solar Map
How much sun every postcode gets, and what an 800 VA plug-in kit makes there on a roof or a balcony, from PVGIS satellite data
Open calculatorWorld Solar Map
Where the sun is right now, how much a panel makes anywhere on Earth, and which countries let you plug a kit into a socket
Open calculatorSun Path Calculator
The sun's track across your sky on any date, and exactly when a tree, chimney or neighbouring roof puts your panels in shade
Open calculatorOff-Grid Load Calculator
Add up every appliance to get daily watt-hours, amp-hours and the peak load your inverter has to carry
Open calculatorBattery Bank Size Calculator
Battery bank capacity in Ah and kWh from daily use, days of autonomy, depth of discharge and temperature
Open calculatorBattery Bank Runtime Calculator
How long a battery bank lasts from capacity, system voltage, depth of discharge and load
Open calculatorCharge Controller Sizing Calculator
Amp rating for an MPPT or PWM solar charge controller, with the array-watt limit at 12V, 24V and 48V
Open calculatorInverter Sizing Calculator
Continuous and surge inverter rating from your load, plus the DC input current the battery cable has to carry
Open calculatorSolar Panel Series vs Parallel Calculator
Array voltage and current for any series and parallel arrangement, including cold-weather Voc
Open calculatorSolar Panel Output Calculator
Daily, monthly and annual kWh from an array you already have, with cell temperature handled separately
Open calculatorSolar Payback Calculator
Payback period, lifetime saving and ROI from install cost, incentives, export rate and tariff inflation
Open calculatorDC Voltage Drop Calculator
Voltage drop on 12V, 24V and 48V DC runs by cable length, current and conductor size
Open calculatorSolar Cable Size Calculator
Smallest DC cable that carries the current and stays inside your voltage-drop limit (AWG/mm2)
Open calculatorAWG to mm2 Conversion Calculator
Convert AWG to mm2, circular mils, kcmil and diameter for PV and battery cable
Open calculatorPV Fuse and Breaker Sizing Calculator
Next standard fuse or breaker size for a PV string, controller or inverter circuit
Open calculatorEnergy Cost Calculator (kWh)
What an appliance or a whole load costs to run, and what solar offsets it saves
Open calculatorWatt Hour Calculator
Watts and hours to Wh and kWh, and amp-hours either way at 12V, 24V or 48V
Open calculatorWhy solar sizing is its own problem
Solar arithmetic looks like ordinary electrical work until you run the numbers. Three things make it different, and all three catch people out.
Your supply is a forecast, not a rating
A grid circuit delivers what it says on the breaker. An array delivers what the sky gives it, which varies by season, by latitude and by which way the roof faces. Peak sun hours is how that variability gets folded into one number, and the December figure matters far more than the annual average if there is no grid to fall back on.
Low voltage punishes long cable
A 3% voltage drop budget on a 240 V circuit is 7.2 volts. On a 12 V battery circuit it is 0.36 volts, and the same power needs twenty times the current. That is why van and boat wiring uses cable that looks absurd next to a house circuit, and why moving from 12 V to 48 V usually pays for itself in copper alone.
Storage is sized on the worst week, not the average day
An array sized to the annual average is fine on a grid-tied roof and useless off-grid. Off-grid banks are sized on days of autonomy and on depth of discharge, which is the real constraint on how much of a battery you actually own.
Solar Sizing Calculator
Free solar and off-grid sizing calculators
Tools
- Solar System Size Calculator
- Plug-in Solar Savings Calculator
- UK Solar Map
- World Solar Map
- Sun Path Calculator
- Off-Grid Load Calculator
- Battery Bank Size Calculator
- Battery Bank Runtime Calculator
- Charge Controller Sizing Calculator
- Inverter Sizing Calculator
- Solar Panel Series vs Parallel Calculator
- Solar Panel Output Calculator
- Solar Payback Calculator
- DC Voltage Drop Calculator
- Solar Cable Size Calculator
- AWG to mm2 Conversion Calculator
- PV Fuse and Breaker Sizing Calculator
- Energy Cost Calculator (kWh)
- Watt Hour Calculator
- All Tools
Guides
Resources
More Calculators
- Electrical Calculators AC wiring, voltage drop and cable sizing tools
- Flow Rate Calculator Pump, pipe and flow rate tools
© 2026 Solar Sizing Calculator. All rights reserved. A Nomogram Works site.