A night outage is paid by the battery
Between 18:00 and 06:00 panels generate nothing. Every watt-hour has to be stored in advance, so the battery drives the budget.
Residential solar backup
When the grid drops,your house stays on.
Tell us how many hours you lose power and what you want to keep running. You get panel count, battery capacity and inverter size — with the arithmetic in plain view.
Free · No signup · Nothing to install
Sunlight availableGrid down
A day with outages from 17:00 to 23:00. The amber edge is the sun’s curve. The mint blocks are your hours off the grid: the ones your battery covers alone.
The part people skip
How much you use matters less than when you use it.
Two homes with identical consumption can need very different systems, and the difference is the clock. That is why this tool asks about your hours before your bill.
A midday outage is paid by the sun
Lose the grid at 11:00 and your panels can feed the house directly while they refill the bank. Same battery, considerably more runtime.
Peak draw picks the inverter, not consumption
A 750 W water pump asks for triple that for one second on startup. That instant sets the inverter you must buy, even if the pump runs ten minutes a day.
What you walk away with
Four numbers you can hand to an installer.
No vague ranges. Every figure shows the formula behind it and the assumption you can change.
3,480Wh/day
Energy to back up
Each appliance multiplied by the hours it actually works during the outage.
4.2kWh
Battery capacity
Your consumption plus a 20 % margin, so the bank never lands on empty.
2× 550 W
Panels
What it takes to refill the bank on an average day of sun.
1,200W
Inverter
Continuous rating and startup surge, computed from the largest motor on your list.
Example: a home with a fridge, six bulbs, two fans, a router and chargers, losing power for 6 h in the evening.
The method, no black box
These are the four calculations. That is all of them.
They are the same ones an installer runs in a spreadsheet. We publish them so you can check the result instead of trusting it.
- 01
Add up energy, not power
Wh/day = Σ (watts × hours of use)
A 150 W fridge does not burn 150 W every hour: its compressor cycles and works roughly 40 % of the time. Counting watts instead of watt-hours is what inflates quotes.
- 02
Size the battery with margin
Battery (Wh) = Wh/day × 1.2
That extra 20 % keeps you from bottoming out the bank every day. It is the difference between a battery that lasts eight years and one that lasts three.
- 03
Turn sunlight into panels
Array (W) = Battery ÷ (peak sun hours × 0.8)
Peak sun hours depend on where you live: from 2.5 in cloudy temperate climates to 6.5 in arid ones. The 0.8 is real loss: wiring, controller, conversion and heat. Skip it and the system falls short in the first bad month.
- 04
Pick the inverter by startup
Surge = Σ watts + (largest motor × 2)
Motors demand up to three times their rating to start. The inverter has to absorb that hit or it shuts down exactly when you need it.
Three routes
The calculator tells you which one is yours.
No system wins in the abstract. One matches your scale, and the threshold sits near 5 kWh.
All-in-one portable
Up to ~4 kWh and 1,800 W draw
- Arrives ready: plug it in and go
- No electrician needed
- Comes with you if you move
- Costs considerably more per kWh
- You hit the capacity ceiling fast
- Rarely runs air conditioning
Fixed hybrid
From ~10 kWh, or with air conditioning
- Cheapest kWh at scale
- Grows by adding batteries or panels
- Can feed the whole panel board
- Needs certified installation
- Real work: board, grounding, protections
- Stays with the house
Mixed
The middle band, 4 to 10 kWh
- Start with essentials and grow
- Spreads the spend over time
- A portable covers you while you decide
- Two systems to manage
- You pay for installation twice
- Less efficient than going straight to fixed
Guides
Worth understanding before you buy.
8 min read
How many solar panels do I need
The answer does not come from your electricity bill or the size of your house. It comes from four numbers, and you already know three of them. This guide walks through them in order and ends with a worked example from start to finish.
9 min read
Lithium or lead-acid: which one actually pays
A lead battery costs a third as much and a lithium one lasts eight times longer. Compared by sticker price there is no argument; compared by the cost of the energy they will deliver over their lifetime, the conclusion flips. Here is the arithmetic.
9 min read
Portable station or hybrid inverter
A portable station plugs in and works. A hybrid inverter feeds the whole house and, at scale, costs close to half as much per kWh. The decision does not hinge on which is better, but on one number of yours: how much energy you need to store.