Key takeaways Step 1: List every load Interactive solar load planner Step 2: Size solar panels Step 3: Size batteries Step 4: Size the inverter Step 5: Size the charge controller
Key takeaways
- Start with daily watt-hours used — everything else flows from that number.
- Panel size depends on peak sun hours at your location.
- Battery size depends on backup time and depth of discharge.
- Inverter size must handle continuous load + surge.
- Use the interactive load planner below to build your own numbers.
Step 1: List every load
Add up appliance wattage and daily hours of use. Multiply watts by hours to get watt-hours (Wh).
Example: 100 W × 5 hours = 500 Wh.
Use the planner below to estimate your total daily load. It will suggest panel, battery, inverter, and charge-controller sizes.
Interactive solar load planner
Step 2: Size solar panels
Divide daily Wh by average peak sun hours to estimate needed panel watts.
Use the solar panel output calculator →
Once your watt-hour math says how many watts of panel you need, this is the module to check the number against (per manufacturer spec) — the $1/Watt-class benchmark that turns a sizing worksheet into a shopping list. Not for: whole-home or grid-tied arrays — by the Step 2 math, a 2,000Wh/day cabin needs ~625W, and whole-home loads run well past that in multiples of 100W modules. The honest tradeoff: 100W per module means you buy and mount many units for bigger arrays.
Check price on Amazon (opens in a new tab) Price & availability shown on Amazon.com — we may earn a commission.Step 3: Size batteries
Choose a battery bank to cover the number of hours or days you want in reserve.
Use the battery capacity calculator →
Battery-bank sizing is usable-kWh multiplication — and this is the unit most DIY banks multiply by. Built-in 100A BMS and low-temp protection come standard in the arithmetic (per manufacturer spec). Not for: 48V banks or whole-home loads without series/parallel-matched wiring — this is a 12V unit, and the bank math here sizes to the system voltage you pick. The honest tradeoff: usable kWh per unit is small, so big banks mean many units and more wiring.
Check price on Amazon (opens in a new tab) Price & availability shown on Amazon.com — we may earn a commission.Step 4: Size the inverter
The inverter must handle the sum of all loads that can run at once, plus motor surge. A practical rule: add ~25% headroom to your simultaneous running watts, then verify the worst startup moment — motors typically need 2–3× running watts to start, hard-start loads like well pumps 3–7×. Run the numbers with the inverter sizing calculator.
Step 5: Size the charge controller
MPPT controllers are rated by output amps at battery voltage. A conservative rule:
Controller amps ≈ (Panel watts ÷ Battery voltage) × 1.25
Example: 4,000 W array ÷ 48 V × 1.25 ≈ 104 A. Round up to the next standard size.
Compare MPPT and PWM in MPPT vs PWM and see charge controller cost.
If your controller sizing lands in the 20–30A range, this is the default answer: 100V open-circuit ceiling, lithium presets, and Bluetooth so you can verify the sizing against real charge data (per manufacturer spec). Not for: the Step 5 example — a 4,000W / 48V array needs ~104A, and this is a 20–30A-class 12/24V controller. The honest tradeoff: the 100V input rail caps string voltage; bigger arrays need the 150V line.
Check price on Amazon (opens in a new tab) Price & availability shown on Amazon.com — we may earn a commission.Common sizing mistakes
- Undersizing winter production: summer sun is not a year-round baseline.
- Forgetting inverter efficiency: AC loads need ~10% more battery than the raw Wh.
- Ignoring surge: pumps and compressors can need 3–7× running watts for a few seconds.
- String voltage too low: a 12 V system for a 3,000 W house creates huge currents and thick cabling.
Next logical reads
12V vs 24V vs 48V solar systems How to choose solar system voltage Solar system cost breakdown Solar components guide MPPT vs PWM Solar panel angle calculator
Best MPPT charge controllers (2026 buyer guide)