System sizing

Oversizing for a future battery

Adding storage later? Enter your planned battery's capacity and efficiency to see how many extra panels cover its round-trip losses.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your current home usage
Add up 12 months of bills before the battery arrives, or your most recent full year.
3 Your planned battery
Most residential batteries range from 10 to 20 kWh usable per unit. Check the spec sheet, not the gross capacity.
DC-coupled converts energy once, AC-coupled converts twice, which costs some efficiency.
4 Cycling assumption
Defaults to 7, a full daily cycle. Lower this if the battery won't fully cycle every day.
5 Your location and equipment
Check NREL's PVWatts for your address.
Most home panels quoted in late 2025 were 400 to 460 watts, according to EnergySage's marketplace data.
6 Derate factor

How it works

Three steps to your battery allowance

Step 1

Find the battery's real usable capacity

Usable capacity is smaller than the headline number on the box, since depth-of-discharge limits and inverter idle draw both eat into it. Check the spec sheet's usable kWh figure specifically.

Step 2

Apply the right round-trip efficiency for your coupling type

DC-coupled and AC-coupled batteries lose different amounts of energy in the charge-discharge cycle. Check the DC to AC ratio calculator if you're still deciding on inverter architecture alongside the battery.

Step 3

Decide on critical loads or whole-home backup

Sizing to critical loads keeps the battery far more affordable than sizing for full whole-home backup, and the panel allowance for losses stays small either way. Check the oversizing for a future EV calculator if an EV is also part of the plan.

Why a battery needs extra production, not extra usage

A battery adds losses, not a new load

Unlike an EV or a heat pump, a battery doesn't add a new load to your home. The energy it discharges in the evening was already part of your annual usage, so the panels you sized for that usage already count it. What a battery adds is round-trip loss: for every 100 kWh charged, a 90% efficient battery returns only 90, and the panels have to replace the difference. For a 13.5 kWh battery cycled every day at 90%, that is about 546 kWh a year, roughly one extra panel, a figure the battery round trip efficiency loss calculator works out from the efficiency side.

Round-trip efficiency sets the size of the allowance

A battery rated at 90% returns 9 kWh for every 10 kWh charged, so refilling 13.5 kWh takes about 15 kWh of production, and the 1.5 kWh difference is lost as heat. Over 364 full cycles a year, that comes to 546 kWh. The efficiency you choose moves the number: about 152 kWh a year at 97%, 546 kWh at 90%, and 867 kWh at 85%. NLR's cost and performance data for residential battery storage adopts 86% as the representative round-trip efficiency, based on testing that found 83 to 87%, so the conservative 85% option is a prudent planning choice.

Why DC-coupled and AC-coupled batteries need different allowances

As the Department of Energy's solar-plus-storage overview explains, a DC-coupled system uses a bidirectional inverter to connect the battery directly to the PV array, while an AC-coupled system needs a bidirectional inverter alongside the PV inverter. Solar energy headed for an AC-coupled battery is therefore converted to AC, back to DC for storage, and to AC again on discharge, which is why AC-coupled systems usually quote lower round-trip efficiency. That difference changes the loss allowance by a few hundred kWh a year. AC coupling is the common choice when adding a battery to existing solar, since the existing PV inverter stays in place.

Critical loads versus whole-home backup changes the battery, not the panels

An average US home uses around 29 kWh a day, but sizing battery capacity to critical loads, roughly 10 to 15 kWh for essentials like refrigeration, lighting, and communications, keeps the battery far more affordable. If that future battery will charge from solar, the inverter needs to handle both jobs, and the hybrid inverter sizing calculator sizes one that can. Whole-home backup for 24 hours commonly needs 30 to 50 kWh of storage, several times the critical-loads figure. The panel allowance barely moves with that choice, because losses scale with the energy cycled, about 11% of it at 90% efficiency.

When extra panels do make sense

Sizing for annual energy covers a battery's losses, but not the seasonal gap. In winter the same array produces far less, so a battery may not fully recharge on short, cloudy days. If you want more reliable recharging, or an outage-ready system, add that margin on purpose: the 110% offset calculator covers a general growth buffer, the net-zero home calculator sizes to a full offset, and the future EV oversizing calculator adds a vehicle on top.

Questions

Frequently asked

Only a little, in annual terms. A battery doesn't add load, because the energy it discharges was already part of your usage. What it adds is round-trip loss, about 550 kWh a year for a 13.5 kWh battery cycled daily at 90% efficiency, which is roughly one extra panel. More panels matter mainly if you want reliable winter recharging.

Round-trip efficiency measures how much of the energy you put into a battery you get back out. A battery with 90% round-trip efficiency returns 9 kWh for every 10 kWh charged, so refilling a 13.5 kWh battery takes about 15 kWh of production, and the 1.5 kWh difference is the loss the extra panels replace.

Yes, somewhat. DC-coupled batteries convert solar energy once on the way in, while AC-coupled batteries convert it twice, so AC-coupled systems usually quote lower round-trip efficiency. Manufacturer figures tend to run higher than independent testing, and NREL's cost and performance data adopts 86% as representative for residential batteries, from testing that found 83 to 87%.

Sizing storage to critical loads is the more affordable route, roughly 10 to 15 kWh for essentials like refrigeration, lighting, and communications, while a full day of whole-home backup commonly needs 30 to 50 kWh. The panel allowance for round-trip losses stays small either way.

Related tools

Keep sizing your system

Oversizing for a future EV Panel count for 110% offset Panel count for a net-zero home Solar panel calculator DC to AC ratio calculator Panel count for 90% offset Panel count for 100% offset All system sizing tools