What size generator do you need? For outages, and as solar backup

Two different jobs

• Running things directly in an outage: the generator powers your appliances. Size it to the loads.
• Backing up a solar system: the generator charges the batteries during a dark week, and the inverter runs the house. Size it to the battery charger.

Running things directly

1. Add the running watts of everything that could be on at the same time.
2. Find the one appliance with the biggest starting surge, and add the extra it needs to start (its starting watts minus its running watts). Only the biggest one counts, since motors rarely start at the same moment.
3. Leave some room: a generator run near its limit all day works hard and burns more fuel.

Appliance	Running watts	Watts to start
Refrigerator	100–250 W	about 1,200 W
Chest freezer	60–100 W	about 800 W
Sump pump (⅓–½ hp)	300–800 W	about 2,000 W
Furnace fan	100–800 W	about 1,200 W
Well pump (½–1 hp)	800–1,500 W	about 3,500 W
Window AC	500–1,000 W	about 2,000 W
Microwave	1,000–1,500 W	about 1,500 W
Space heater	1,500 W on high	about 1,500 W

Outage examples

Essentials: fridge 250 W, chest freezer 100 W, sump pump 800 W, furnace fan 800 W (an older motor) and lights and chargers 200 W come to about 2,150 W running. The sump pump's start adds about 1,200 W more, so about 3,350 W at the worst moment. A 3,500–5,000 W generator covers it with room to spare.

Essentials plus a well pump: add 1,500 W running, and the pump becomes the biggest starter (about 2,000 W extra). That's about 3,650 W running and 5,650 W at start-up: a 6,000–7,500 W generator, with a 240V outlet if the pump needs it.

Backing up a solar system

Here the generator only has to feed the battery charger (plus whatever runs while it charges), so the charger's rating sets the size:

System	Charger	Generator
12V van or RV, Victron MultiPlus-II 12/3000	up to 120 A, about 1.7 kW	about 2,000–3,000 W
48V cabin or home, EG4 6000XP	up to 110 A from a generator, about 5.5–6 kW	EG4 recommends more than 10 kW per inverter; or set the generator charge current lower to match a smaller one

Two details from the manuals worth knowing before you buy:

• Clean power: EG4 asks for a generator with under 3% total harmonic distortion (THD). It's on most inverter generators' spec sheets; check before you buy.
• Turn the charger down to fit: both makers let you lower the charging current (Victron's AC input current limit, EG4's generator charge current setting), so a smaller, quieter generator works; it just charges more slowly.

In a dark-winter area, a small generator for the worst weeks usually costs less than doubling the batteries. Power station vs generator vs solar compares the options for outages, and the home backup guide sizes a battery system that runs the house silently.