Planning off-grid power: sizing your power station, solar panel and consumption
Whether it's a weekend in the van, a cabin without grid power or balcony solar with a battery, the question is always the same: how big do battery and solar panel need to be so the power lasts? This guide shows the order in which to plan, which guide values apply and where the typical mistakes are.
Step 1: Determine your daily consumption honestly
Everything starts with consumption, not with the panel. Every device uses power times time: a laptop at 65 W for three hours is 195 Wh, a compressor cooler at 45 W that cycles for ten hours a day is 450 Wh. What matters is the actual runtime – not the time the device is switched on.
| Device | Power | Typical runtime | Wh/day |
|---|---|---|---|
| Compressor cooler | 45 W | 8–12 h (cycling) | 360–540 |
| Laptop | 65 W | 3 h | 195 |
| LED lights (2 lamps) | 10 W | 4 h | 40 |
| Smartphone charging (2×) | 10 W | 2 h | 40 |
| Mobile router | 10 W | 8 h | 80 |
| Diesel heater (fan) | 30 W | 6 h | 180 |
| Coffee maker | 1,200 W | 0.15 h (9 min) | 180 |
| Electric kettle | 1,500 W | 0.1 h (6 min) | 150 |
Frugal campers get by with 300–500 Wh per day, with a cooler and laptop it's 600–900 Wh, with a coffee maker, induction cooktop or Starlink quickly 1,000–1,500 Wh. A tiny house with fridge, lights, router and pump is at 1,000–2,000 Wh.
Step 2: Choose storage by off-grid days
The battery bridges the time without sun: the night and, when in doubt, two to three overcast days. The rule of thumb:
storage [Wh] = daily consumption [Wh] × off-grid days ÷ usable fractionThe usable fraction matters: a power station rated at “1,000 Wh” delivers 850–920 Wh at its outlets because the battery management keeps a reserve and the inverter and DC converters have losses. With a lead-acid battery (AGM, gel) only 50% is usable, with LiFePO4 solar batteries 80–90%.
- Weekend with cooler and laptop: 1,000–1,500 Wh – enough for one night plus one overcast day.
- A week of boondocking: 2,000–3,000 Wh with solar, so two rainy days are no problem.
- Tiny house / cabin: 3,000–5,000 Wh, depending on the fridge and the heater fan in winter.
- Balcony solar battery: 1,000–2,000 Wh cover a household's nighttime base load (router, fridge, standby).
Step 3: Size the solar panel by sun hours
A panel only delivers its rated power under lab conditions. For planning you use peak sun hours and a system efficiency of about 75%:
solar output [Wh/day] = panel power [W] × sun hours [h] × 0.75
required panel power [W] = daily consumption ÷ (sun hours × 0.75)| Season (continental US) | Peak sun hours/day | 200 W panel delivers |
|---|---|---|
| December/January (North) | 2–3 h | 300–450 Wh |
| March/October | 3.5–4.5 h | 525–675 Wh |
| April–September | 5–6 h | 750–900 Wh |
| Clear summer day (Southwest) | 6–7 h | 900–1,050 Wh |
| Overcast day (any season) | 10–25% of that | 50–250 Wh |
Plan 20–30% reserve above the calculated requirement, and check the solar input of the power station: many units accept only 100–400 W and limit the input voltage – a 400 W residential module with 40 V open-circuit voltage doesn't fit every input.
Step 4: Check the balance and estimate charge time
If the solar output exceeds consumption, the balance is positive – the battery fills up every day and the system runs indefinitely as long as the sun cooperates. If it's negative, the battery drains by the deficit each day; usable energy divided by deficit gives the number of days until empty.
The charge time of an empty battery from solar is capacity divided by effective panel power: 1,000 Wh ÷ (200 W × 0.75) = 6.7 sun hours – at 4 sun hours per day just under two days. If loads run at the same time, only the surplus counts.
Solar & off-grid calculator – calculates consumption, solar output, balance, off-grid days and charge time in one step.
Typical mistakes
- Daylight hours instead of peak sun hours: ten hours of sun in the sky is five to six peak sun hours in summer, two in winter.
- Planning for the best day: three overcast days in a row are normal. The “without sun” value is the honest reserve.
- Overlooking small continuous loads: a router at 10 W over 24 hours is 240 Wh – more than a coffee maker per day.
- Seeing surplus as reserve: once the battery is full at noon, the remaining solar output is wasted.
- Inverter always on: 8–15 W of idle draw all day costs 200–360 Wh.
Sources
- NREL – PVWatts Calculator and solar resource maps
- PVGIS – Photovoltaic Geographical Information System (European Commission, JRC)
- Manufacturer data sheets of power stations (usable capacity, solar input) and solar modules (STC rated power)
Published on 09/22/2026.