What size solar system do I need?
Array watts, battery kWh, and inverter/charge-controller pairing — sized for off-grid reality or grid-tie economics.
The whole system, not just panels
A solar system is four numbers, and they're sized in this order:
- Daily energy need (Wh/day) — list each load: watts × hours per day. This drives everything else.
- Array size (W) = daily Wh ÷ worst-month sun hours ÷ 0.80 efficiency. See the panel-count math for the breakdown.
- Battery bank (kWh) = daily Wh × autonomy days ÷ usable depth of discharge. Lithium: 80–90% usable; lead-acid: 50%. For 2 days autonomy on 5 kWh/day with lithium: 5 × 2 ÷ 0.85 ≈ 12 kWh.
- Inverter (W) — your largest simultaneous load, not your daily total. Well pump + fridge + microwave might spike 4 kW even in a tiny home.
Grid-tie skips the battery row entirely: array = annual kWh ÷ 365 ÷ sun hours ÷ 0.80, and the grid is your "battery."
Worked example: full-time off-grid cabin, 6 kWh/day, 4.0 winter sun hours
| Component | Math | Result |
|---|---|---|
| Array | 6,000 ÷ 4.0 ÷ 0.80 | ~1,900 W (5×400 W) |
| Battery bank | 6,000 × 2 days ÷ 0.85 (LiFePO4) | ~14 kWh (48 V × 300 Ah class) |
| Inverter | largest simultaneous load (well pump + fridge + tools) | 3,000–4,000 W |
| Charge controller | 1,900 ÷ 48 ÷ 0.85 | ~47 A → 60 A MPPT |
The same house grid-tied: that annual total averages back to 6,000 Wh/day; ÷ 4.8 average sun hours ÷ 0.80 ≈ 1,560 W of array, no battery, no inverter beyond a grid-tie unit — about 4 panels. Off-grid costs more because it must survive December; grid-tie only needs to win the year.
Off-grid vs grid-tie: what changes
- Sizing target: off-grid = worst month; grid-tie = annual average or bill offset.
- Battery: off-grid needs autonomy days (2 typical, 3 for cloudy climates); grid-tie hybrid adds a smaller bank for outages only.
- Inverter: off-grid units must handle surge (motor starts = 2–3× running watts); grid-tie just needs to match array output.
- Backup: off-grid systems should plan a generator input for long clouds — see GeneratorSizer for that half of the equation.
Inverter & charge-controller pairing
Three limits to check on every controller: max input voltage (your series string voltage cold — panel Voc rises ~0.3%/°C below 25°C), max input current, and max output amps to the bank. Output rule of thumb: controller A = array W ÷ bank V ÷ 0.85, rounded up. On the inverter side, keep array watts within the unit's max PV input, and size continuous AC rating to your biggest overlapping load stack — undersizing the inverter trips it every time the well pump starts, even with a full battery.
Common sizing mistakes
- Using annual-average sun hours for an off-grid build (December eats the difference).
- Sizing the inverter to daily kWh instead of simultaneous watts.
- Forgetting inverter idle draw — a 3,000 W unit can pull 20–40 W around the clock, nearly 1 kWh/day itself.
- Buying panels before the battery bank is sized; the bank sets charge current limits the array must respect.