"How many watts do I need?" is the most common question about solar generators, and it actually hides three questions: how big a battery, how powerful an inverter, and how many watts of solar panels. Each has a straightforward answer once you know what you want to run. This guide walks through the arithmetic with a worked example you can adapt.
Key terms. Watts (W) measure power at a moment. Watt-hours (Wh) measure energy over time: 100 W for 3 hours is 300 Wh. 1 kWh = 1,000 Wh. A power station's battery is rated in Wh; its inverter and solar input are rated in W.
Step 1: List your loads
Make a list of everything you'd like to run during an outage or off-grid trip. For each item, write down:
- Its running watts, from the label, the manual, or a plug-in watt meter.
- Its surge or start-up watts, if it has a motor or compressor.
- How many hours per day you'd actually run it.
Be honest about hours. Lights at night, a router all day, a laptop for a few hours. For appliances that cycle, like refrigerators and freezers, a watt meter left plugged in for 24 hours gives the most accurate daily energy figure. The EnergyGuide label's yearly kWh divided by 365 is a reasonable estimate.
Step 2: Calculate daily watt-hours
Multiply watts by hours for each load and add them up. Here's an example for a modest outage plan. The numbers are illustrative; use your own.
| Load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| Wi-Fi router + modem | 20 | 12 | 240 |
| LED lights (3) | 30 total | 5 | 150 |
| Phones (3) | 15 each | 2 each | 90 |
| Laptop | 50 | 3 | 150 |
| Refrigerator (measured) | varies | cycles | 1,200 |
| Total | 1,830 | ||
Notice how the refrigerator dominates. Running the fridge only part of the day, and keeping the door shut, is the single biggest lever on your system size.
Step 3: Size the battery
Power stations lose some energy converting stored DC power into AC, typically around 10 to 20 percent. Divide your daily total by an efficiency factor of about 0.85:
1,830 Wh ÷ 0.85 ≈ 2,150 Wh
That's the battery you'd need to cover one full day with no recharging. From here, decide how much autonomy you want:
- Bridge short outages without solar: size for the expected outage length.
- Multi-day outages with solar: size the battery for roughly one day, and size the solar to refill it daily (Step 5).
- Cut the list: dropping the fridge from this example brings the need down to about 740 Wh, within reach of many mid-size stations.
Step 4: Size the inverter
The inverter's continuous rating must exceed the total watts of everything running at the same time, and its surge rating must cover the biggest start-up spike. In the example, the worst case might be the fridge compressor starting while the router, lights and laptop run. If the fridge runs at about 150 W but surges much higher on start-up, you need a surge rating that covers that spike plus everything else. Most 1 to 2 kWh stations offer 1,500 W or more continuous, which handles this example comfortably. Large loads like microwaves, hair dryers and space heaters are what push inverter requirements up; they also drain batteries quickly.
Step 5: Size the solar panels
Solar output depends on your location's peak sun hours, the equivalent number of hours per day of full-strength sunlight. It varies by region and season. The National Renewable Energy Laboratory publishes solar resource data; for outage planning, use a conservative figure for the season you're most worried about.
Panels also deliver less than their rated watts in the real world because of heat, sun angle, dust, wiring and charge-controller losses. A planning derate of about 0.75 is reasonable for portable panels in good conditions.
The formula:
Panel watts = daily Wh needed ÷ (peak sun hours × derate)
For the example, refilling 2,150 Wh in a location with 4 peak sun hours:
2,150 ÷ (4 × 0.75) ≈ 717 W of panels
Without the fridge (about 740 Wh needed after losses):
740 ÷ (4 × 0.75) ≈ 247 W of panels
That's the difference between a single 200 to 300 W panel setup and a much larger array, all because of one appliance.
Step 6: Check the station's solar limits
Your panel plan must fit the power station's solar input. Check:
- Maximum solar input watts. Extra panel watts beyond this are wasted at peak, though they help on cloudy days and at low sun angles.
- Voltage range. Panels in series add voltage; the total must stay within the station's limit, including the higher voltage panels produce in cold weather.
- Current limit. Panels in parallel add current; check the maximum amps.
- Connectors. Make sure you have the right cables and adapters.
Some stations have two solar inputs, which can let you connect more panels.
Quick reference
| Goal | Battery (rough) | Solar (rough, at 4 sun hours) |
|---|---|---|
| Phones, lights, router for a day | 500–800 Wh | 200–300 W |
| Above plus CPAP overnight | 800–1,200 Wh | 300–400 W |
| Above plus a refrigerator | 1,500–2,500 Wh | 600–900 W |
These ranges are rough illustrations; your own load list is what counts.
Common mistakes
- Sizing from rated panel watts with no derate, then wondering why the battery never fills.
- Ignoring surge and choosing a station that trips when the fridge starts.
- Planning with summer sun hours for a winter storm.
- Forgetting inverter losses.
- Buying more panel watts than the station can accept.
The bottom line
Size a solar generator in order: list loads, total the watt-hours, divide by about 0.85 for battery size, confirm continuous and surge watts for the inverter, then size panels using conservative sun hours and a derate of about 0.75. Trim big loads first; it's always cheaper than buying a bigger system. See our power station picks and the panel picks below.
